chromatic abberation s

66 c’t Digital Photography 9 (2012)

© Copyright by Heise Zeitschriften Verlag

Chromatic aberrations and other lens errors were just as prevalentin analog times as they are today, but it is the development ofdigital image processing technology that has given us the tools weneed to correct them effectively. This article takes a look at someof the specialized programs on offer and explains how you can usethem to get rid of those irritating RAW and JPEG color smears.

Sascha Steinhoff

Chromatic

Aberrations

67c’t Digital Photography 9 (2012)

Chromatic aberration is a widespreadphenomenon that occurs in both

contemporary and legacy lenses. The term isused to describe a number of different typesof focusing errors.

Spectral ColorsWhether you use high-end or cheapequipment, most lenses display one sort ofchromatic aberration or another. Regardlessof exactly how they are formed, chromaticaberrations are always caused by light ofdifferent wavelengths being refracted todiffering degrees. The individual elements ina lens not only direct the incident light butalso split it into its component colors the sameway that a prism does.

This article introduces various types ofchromatic aberrations and explains how todistinguish them from other types of opticalerrors such as sensor blooming or flare. Wealso test a range of dedicated software toolsdesigned to correct lens errors.

Axial and TransverseAberrationsGenerally, we distinguish betweenlongitudinal (axial) and transverse (lateral)aberrations as seen in relation to the opticalaxis of the lens. Both types can cause falsecolors and halo effects, but havefundamentally different characteristics.

FringingLateral color errors often occur at the edges ofimages because the images of red, green andblue spectra are reproduced at slightlydifferent sizes. This causes color fringing,which is especially obvious at high-contrastedges. The further a point is from the centerof the image, the more prone it will be to thistype of error, whereas fringing doesn’t occurat all in the center of the frame. Stopping theaperture down neither reduces nor increasesthe likelihood of this type of error occurring.

All non-radial high-contrast edges arelikely to suffer from fringing. Sure-fire sourcesof this type of artifact are subjects at the edgeof the frame that contain thin black linesagainst bright backgrounds, such as theleafless branches of a tree in the snow or achain link fence shot against a bright sky.

The color of a fringing artifact depends onwhether it occurs at a dark-to-light or alight-to-dark transition, seen relative to thecenter of the frame.

Lateral aberrations are relatively simple to correct, provided a couple of basicpreconditions are met. You need to know indetail about the specific optical characteristics


of your particular lens and you should performthe adjustments on a RAW image file. Thesepreconditions usually produce great resultswith automatic computer-based or in-cameracorrection tools. Lateral errors don’t produce‘hard’ double images the way axial errorsoften do, but it is still virtually impossible tocompletely erase edge softness and loss ofcontrast after shooting. It is neverthelessrelatively easy to filter out obvious colorfringes.

Double Images and False ColorsAxial aberrations are anomalies caused bythe different component colors focusing atdifferent points along the optical axis. Thisresults in multiple, single-color imagesinstead of a single, sharp, multi-color image.Like the sub-images produced by lateralaberrations, those produced by axialaberrations tend to appear at high-contrastedges. However, they neither vary in size nordepend on the direction of the incident light.

Axial aberrations cause fringing and imagesoftness across the entire frame and areparticularly obvious at wider apertures (i.e., inimages with shallow depth of field). Thecritical factor here is the focus setting ratherthan the position of an object within theframe. Lateral errors increase toward theedges of the frame.

Depending on the type of subject, it ispossible to reduce the incidence of axialaberrations by stopping down the lens.Smaller apertures increase depth of field andreduce the probability of a single colorchannel appearing unsharp in the final image.

The obvious reduction in sharpnesscaused by axial aberrations manifests itself inthe form of double images at high-contrast

edges. Axial aberrations can also producewhole areas of false colors that often tendtoward magenta and are particularly obviousin brighter image areas. This leads to colorcasts in image areas that lie outside the areaof sharp focus.

The spectral color of such areas changesaccording to whether the affected imagedetail lies in front of or behind the focalpoint. Axial aberrations seldom occur at thefocal point itself because its depth of field isusually sufficient to cover all three colorchannels.

Due to their predominant magenta color,axial aberration artifacts are often known as‘purple fringing’. However, this is not aparticularly precise term, as other types ofartifacts – such as image noise, blooming andstray ultraviolet or infrared light – can causesimilar looking effects. Even lateral

aberrations can be confused with axial onesunder certain circumstances.

Computer-based correction of axialaberrations is only possible to a limiteddegree, as no tool exists that can effectivelyadjust a single unsharp color channel.Fringing and color casts can be removed in ageneral sense, but this type of correctionalways results in an additional reduction inimage sharpness that tests the limits of eventhe smartest algorithms.

Other Types of Image Error

In addition to the types of chromaticaberrations we have mentioned, there arevarious other phenomena that can lead tofringing and color errors, including stray UVand infrared light. Compact cameras especiallyhave a high degree of automatic error


Chromatic Aberrations | Basics


Most of today’s image sensors are arranged according to the BayerPattern, which alternates rows of green/blue/green and red/green/red photoreceptors. In order to be displayed on a monitor or saved as an image file, the data captured by thephotoreceptors has to be converted to an RGBformat made up of pixels . Each pixel contains red,green and blue sub-pixels. This conversion processis known as interpolation and is performed using ademosaicing algorithm.

Algorithms designed for correcting chromaticaberrations are much more effective when applied to RAW imagedata than they are when used on pre-processed JPEG image files. Ifyou want or have to shoot in JPEG format, you should use a camera

that has built-in color correction functionality. All Nikon DSLRS sincethe D300 automatically correct lateral color aberrations at the edgesof the frame. This feature cannot be configured separately in the

camera’s firmware, but can be adjusted manuallyusing Nikon’s Capture NX software package.

Camera hardware, too, can influence the creationof chromatic aberrations. Microlenses positionedin front of the image sensor ensure that the lightentering the lens is distributed evenly across thesensor, but also have a reputation for fostering

chromatic aberrations. Even if they have identically sized sensorsand are fitted with the same lens, different camera models oftenproduce fringing artifacts with quite different characteristics.

In-camera Error Correction

Lateral aberrationsare caused when the

light entering the lensis split into its

component colors.The resulting rays,with their varying

frequencies, hit thelens at different

points on a plane thatis positioned at rightangles to the opticalaxis, causing colorfringing, reduced

contrast and a loss ofimage sharpness.

c’t Digital Photography 9 (2012)

Lateral aberrations: the lowerdetail image is taken from thecenter of the main image and does not display any chromaticaberrations. Green and pinkfringes occur more frequentlywithin the wire mesh toward theedges of the frame. This type oferror is easy to correct digitally.

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correction built into their firmware that oftencauses interpolation errors. Image noise canalso cause color errors, and brightly lit objectscan produce fringing due to oversaturation.

The image sensor itself can also be thesource of fringing effects, due to smearing orblooming. Blooming often occurs in oldercameras that are equipped with CCD sensors,especially if used to shoot a bright subject(such as the sun) using a wide aperture. Newercameras with CMOS sensors are less sensitiveto this type of error.

Color errors and ghosting can also occurif you photograph a brightly lit subject usinga poor-quality lens. It is often quite tricky todifferentiate between the exact causes ofthese types of effects.

Software Filters are a Proven Solution

Chromatic aberrations are some of the moreirritating image errors that can occur, and areobvious even to the untrained eye. They occurregularly in various forms, even if you haveinvested a lot in high-end equipment.

Software filters are a proven way to takethe edge off lateral aberrations, whereas axialaberrations are more difficult to correctdigitally. As for all other types of lens errors,chromatic aberrations cannot be completelyeliminated, even using the most sophisticatedtools, so it is essential to use the best lensesyou can and appropriate shooting techniquesto keep them to a minimum.



An accepted way to prevent axialaberrations from occurring is to usethe optimum aperture. This is thesmallest aperture at which your lensproduces its best sharpness. Theoptimum aperture depends on aseries of variables and can be quitecomplicated to determine. Generallyspeaking, it will be somewherebetween f8 and f11 for a conventionalDSLR, although the precise value willvary from lens to lens. Other systems,such as Micro Four Thirds, havegreater overall depth of field than DXor full-frame cameras and are thusless prone to chromatic aberrations.The only reliable way to determinethe optimum aperture for yourparticular setup is to subject it to acontrolled lens test.

Optimum Aperture

Typical axial aberrations: only thebranches in the foreground show amagenta cast and ghost-like ‘doubles’.The trees in the distant background arenot affected.

Image: Thomas Saur

Axial aberrationsoccur when the

images formed by theindividual color

channels focus atdifferent places alongthe optical axis of the

lens. The effect isparticularly evidentat wide apertures

and results infringing and

psychedelic-lookingdouble images.

Chromatic aberrations in photographicsystems are caused by the refraction of lightwithin the elements of the camera’s lens.Various types of specialized elementsprovide effective ways to combat theireffects.

The ‘Right’ Glass

Optical glass is available in a wide range of qualities. Nikon uses its patented ED (Extra-low Dispersion) and Super ED glass,while Canon uses its own fluorite, UD (Ultra-low Dispersion) and Super UD glass.

There is not enough natural fluorite in theworld to cater for all lens manufacturers’needs, so Canon began producing artificial

crystal fluorite in the 1960s. Third-party lensmanufacturers all produce various types oflenses with low-dispersion elements, such asTamron’s LD (Low Dispersion), XLD (Extra LowDispersion) and AD (Anomalous Dispersion),Sigma’s FLD (F Low Dispersion), ELD (ExtraLow Dispersion) and SLD (Special LowDispersion), as well as Tokina’s SD (Super LowDispersion) and HLD (High Refraction, Low Disper sion) models, to name just a few.

Low-refraction glass is most often used intelephoto lenses due to the particularsensitivity of long focal lengths to chromaticaberrations. However, some wide-anglelenses are also built using this type of element.

Photographic lenses are never madecompletely out of elements with low

refractive indexes, but instead use just one ortwo within a system of conventional glasselements. This helps to keep manufacturingprocesses simple and the cost of productiondown to an acceptable level.

As well as increasing costs, ED glass is alsosaid to scratch more easily than conventionaloptical glass, although this is not generally aproblem because ED glass is not usually usedfor front or rear lens elements. Fluorite is saidto be more sensitive to impact than normalglass and its refractive index actuallyfluctuates with changes in temperature.

Low refractive index elements arenowadays a standard component in mosthigh-end photographic lenses throughout therange of available focal lengths.

Spherical vs. Aspheric

To keep costs down, most optical systems usespherical lenses. With their spherical surfaces,these are cheap to produce but are inherentlyprone to significant chromatic aberration.

Chromatic Aberrationsin Camera Lenses


Chromatic Aberrations | Lenses

Every lens-based optical system has to deal with chromatic aberrations in one form or another. It is impossible to eliminate them completely, but specially constructed lenses can help to lessen their effects at source. The following sections explain what to look out for when purchasing cameras, lenses and optical accessories.

Aspheric lens

R

Spherical lens

R

The surface of a spherical lens is shaped likepart of the surface of a sphere. Aspheric lenseshave a more complex shape that is moredifficult to manufacture.

Conventional glass

ED glass

Secondary spectrum

Secondary spectrum

The low refractiveindex of ED glassreduces the degree ofdispersion of incidentlight, thus reducing (but notcompletelyeliminating) theresulting aberrations

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Aspheric lenses have better reproductioncharacteristics. Because they haveasymmetrically curved surfaces, they used tobe extremely expensive to produce and wereonly built into extreme high-end lenses whenthe technology was introduced in the 1970s.New production techniques have reduced thecost of aspheric elements to a point at whichsome manufacturers even build them intomid-range and budget lenses, such as thecurrent US$100 Nikkor 18-55mm basic zoom.The other 10 elements in this particular lensare spherical.

Most lenses are built using a combinationof spherical and aspheric elements. The moreaspheric elements a lens contains, the lighterand more compact the overall design can be.Aspheric elements can also help to reducedistortion and increase the opticalperformance of wide-aperture lenses.

Aspheric elements are divided intodifferent grades according to their shape.The ones used in cheaper lenses (like theNikkor mentioned above) are usually hybridmodels constructed from spherical glasslenses glued to additional plastic elementsthat give them their aspheric shape. Theintended aspherical effect is somewhatreduced by the fact that plastic disperseslight more than glass. Further up the pricescale are pressed glass aspheric lenses, andmost expensive of all are ground glass

aspheric elements, which also have the bestreproduction characteristics.

Whether high-end or budget, most lenselements are spherical, and individualaspheric elements are added to enhanceoptical performance.

Diffractive Elements

Canon introduced the then revolutionary EF400mm f/4 DO IS USM lens at Photokina 2000.The new Diffractive Optical (DO) elements inthe lens made it extremely compact andreduced chromatic aberration significantly.The technology hasn’t been developed muchfurther, and the lens since added to the rangeis the EF 70-300mm f/4.5-5.6 DO IS USM. Thisamazing lens weighs just 720grams (1.58 lb)and is only 10cm (3.9 inches) long. A directcomparison with the identically priced EF70-300mm f/4-5.6L IS USM (1,050grams,14.3cm) demonstrates the real advantage ofDO elements, in that they are significantlysmaller and lighter than their conventionalglass counterparts.

DO elements are built using a uniquetwo-piece construction that consists of twodiffraction gratings mounted between twoglass elements. DO elements designed forphotographic purposes have to include atleast two diffraction gratings in order to coverthe entire spectrum of visible light.

Diffraction gratings are difficult tomanufacture due to the variable distancesbetween the openings in the grating and theirextreme thinness of just a few micrometers.You can see the pattern of the grating if youhold the front element of a DO lens up to thelight. A DO element on its own has similardispersion characteristics to those of a normalglass element and produces its own chromaticaberrations. The all-important difference isthat a DO element disperses the componentcolors of the spectrum in the oppositesequence to that of a normal lens element.Combining a conventional element and a DOelement thus cancels out color deviations andeliminates chromatic aberrations.

The disadvantages of DO elements includetheir tendency to produce bokeh that mirrorsthe structure of the grating rather than theunobtrusive circles produced by conventionallenses. These DO circles of confusion tend tolook like a sliced onion and are similar to thering-shaped ones produced by mirrortelephoto lenses.

The precise shape of bokeh depends notonly on the lens but also on the nature of thesubject, and whether you find it attractivewhen it does occur is a matter of personaltaste. With just two lenses being introducedin 11 years, DO lens technology remains aniche product and has yet to break into themass market.



Diffraction grating

Glass elements

DO elements use custom diffractive gratings positionedbetween two lens elements to alter the path of the light raysentering the lens. This structure is visible to the naked eye ifyou look at a DO element head-on (shown on the left).

Aspheric element

UD element

This Canon EF 24mm f/1.4 lens has one aspheric and one UDelement. Aspheric elements are used to improve the qualityof high-end lenses and to make value lenses less expensive.

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Achromatic, Apochromatic andSuper Achromatic Lenses

Photographic lenses are classified according tothe type and quality of the color correctionfeatures they possess. Achromatic colorcorrection techniques focus just two colors(usually red and blue) in the same plane andhave been used since the 19th century. The firstphotographic lenses were achromaticallycorrected.

The color errors that achromatic lensesnevertheless produce, known as ‘secondaryspectrum’, are very difficult to eliminatecompletely. However, the effects ofsecondary spectrum on image quality aregenerally negligible if you use a good qualityachromatic lens.

Optical engineers are constantly searchingfor new ways to reduce or eliminatesecondary spectrum and thus improve thequality of photographic images.

Apochromatic lenses can correct threecolors. While achromatic lenses require twoelements made of different materials, anapochromatic lens requires at least threedifferent types of element. Apochromaticcorrection is designed to reduce or, ideally,eliminate secondary spectrum by focusing allred, green and blue rays to the same point onthe focal plane. The only way to do this usingcurrently available materials is if you are

prepared to accept other types of aberrationsas part of the deal. Most lenses thereforerepresent a compromise between faithful colorreproduction and a number of other opticalcharacteristics and anomalies.

Lens manufacturers build lenses to astandard that they themselves are happy tocall apochromatic. However, it is probablysafe to say that Sigma’s definition of‘apochromatic’ for a US$300 lens will differfrom the Leitz definition of the term in a lensthat costs 20 times as much.

The term ‘apochromatic’ is neitherstandardized nor patented, so you can onlyreally find out what various manufacturersmean by it by conducting a thorough lenstest. The incidence of chromatic aberrationsvaries enormously throughout the zoomrange, particularly in budget telephotozooms that nevertheless carry the ‘APO’label. Today’s market offers APO models inthe entire range from budget to high-end,while some manufacturers whose lensesactually earn the description don’t use it at all.

At the very top end of the quality (andprice) scale are ‘super achromatic’ lenses thatare capable of correcting four different colors.The Carl Zeiss Tele-Superachromat T* 5.6/350(for Hasselblad cameras) is one example of anextreme high-end lens with unbeatable colorcorrection.

Even if it is not as commonly used as‘apochromatic’, the term ‘super achromatic’ isnot precisely defined and is still usedaccording to the needs of marketing peoplerather than photographers. The first superachromatic lenses were made by Zeiss in the1960s and the ‘Superachromat’ label hasremained a Zeiss trademark ever since. Zeiss’direct competitor Leitz describes its equivalentlenses as having APO characteristics, althoughthe products themselves are directlycomparable. Once again, even though thedescription is based on definable technicalattributes, the presence or lack thereof doesn’taccurately describe the quality or performanceof a lens.

Close-up Lenses and TeleconvertersIf you interfere with a lens manufacturer’scarefully designed system of elements andgears, you run the risk of producing not onlyoptical anomalies but also additional chromaticaberrations.

Using a teleconverter is a simple way toincrease the focal length of a lens, but isalmost guaranteed to increase chromaticaberrations too. Teleconverters are normallyconstructed with quite a lot of internal spaceand some manufacturers use this to addadditional corrective elements.


Achromatic correction – a standard feature in mostphotographic lenses these days – corrects two colorsbut leaves the third uncorrected

The conventionalelement (top) andthe DO element(center) refractincoming light inopposite directions.Used together, thetwo elementscancel out eachother’s aberrations.

c’t Digital Photography 9 (2012)74


In contrast, most close-up lenses made forattachment to the filter thread on a lens havesingle-element designs and cannot becorrected using additional elements.

Some high-end close-up lenses (or‘achromats’, as they are sometimes called) areconstructed using twin-element achromaticdesigns and are appropriately corrected.Some achromats are designed for use with aspecific lens and offer appropriately highimage quality with relatively few chromaticaberrations. Twin-element achromats are,however, larger, heavier and more expensivethan their single-element counterparts. As faras we are aware, there are no three-elementapochromatic close-up lenses.

Many of the telephoto and wide-angleaccessory lenses available for compact camerasrepresent a serious change in the design of thecamera’s optical system and often produceobvious chromatic aberrations as a result. High-end accessory teleconverters like the NikonTC-E3ED use low-dispersion glass and are a lotmore expensive than the competition as a result.

Introduced in 2004, the Nikon TC-E3PFtelephoto converter is based on a PhaseFresnel lens that works much like the CanonDO lenses described above. In spite of its smallsize and light weight, this innovativetechnology didn’t gain a foothold in themarket and, along with the other planned PFlenses, has now been discontinued.


Chromatic aberrations are caused by therefraction of light in optical lenses, so thefewer lenses an optical system contains,the less likely it is to produce irritatingfringing effects. Mirror telephoto (or‘catadioptric’) lenses contain very fewelements and those they do contain don’trefract the light entering the lens to agreat degree. As a result, these lenseshave a reputation for producing little orno chromatic aberration. On the negativeside, mirror lenses produce donut-shapedbokeh, which is generally consideredunattractive.

There are still a number of third-partymanufacturers – mainly in Asia and easternEurope – producing this type of lens.These include Vivitar, Rokinon, Bower,Phoenix, Opteka, Walimex, Danubia,Maksutov and Samyang. Most of thelenses we have seen look extraordinarilysimilar to the models offered by Maksutovand Samyang, and we are fairly sure thatmany are simply rebranded lenses madeby one or other of the two.

Mirror telephotos are cheap, lightweightand compact. The disadvantages of this

type of design are their relatively small,non-adjustable apertures and theirvulnerability to capturing stray light.

In general, conventional lenses with glasselements produce better-quality images,relegating mirror lenses to the level of ahobbyist’s tool in most everyday photographic situations.

Mirror Telephoto Lenses

Mirror telephoto lenses are light andcompact in comparison to theconventional competition. The Rubinar500mm f/5.6 lens shown here weighsjust 1.6kg (3.5 lb) and is 23.5cm(9¼ inches) long.

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Achromatic lens Apochromatic lens

The structure of an apochromatic lens is more complex and therefore more expensivethan that of an achromatic lens

Apochromatic correction is, theoretically, capable of completelyeliminating secondary spectrum and requires the use of at leastthree elements made of three different optical materials




Chromatic Aberrations | Software Test


We checked out a range of imageviewers and RAW converters, fromACDSee to Lightroom andRawTherapee, to see just how usefultheir built-in filters are for combattingchromatic aberrations.

For our lateral aberration test, we used ablack, red and white checkerboard target thatmade it relatively easy to observe the increasein fringing toward the edge of the frame. Forthe axial aberration test, we used a photo ofa grid pattern taken at an angle that causedstrong color casts in front of and behind theplane of focus.

Chromatic aberrations in images ofhomogeneous patterns look very differentfrom the ones that occur in everyday subjects,so we tested each program (in RAW and JPEGformats) on four selected test photos too.

While comparing our results, we noticedthat the colors in our RAW samples variedquite strongly, so we ended up comparing theeffect of the filters themselves rather thanoverall image quality. In other words, weobserved how the program behaved with thefilter switched on and off. For the JPEG test,we converted all of our RAW source imagesusing RawTherapee.

Overall, our test revealed that lateralaberrations can be effectively removed usingautomatic tools and an appropriate lensprofile. Axial aberrations are much moredifficult to correct. Many programs don’t havebuilt-in tools for combating this type of errorand we didn’t find any really effectiveautomatic filters. We simply couldn’teliminate our focus-dependent color casts.Some of the better test programs managedto suppress isolated color patches and purplefringing to a tolerable degree.

Filters that are capable of removingblue/yellow fringes from out-of-focus areas inan irregularly shaped subject are few and farbetween. This is an important feature forphotographers who use long telephoto lensesat wide apertures – a setup that is prone toproducing artifacts, even if the lens carries aLeica or Zeiss label.

ACDSee Pro 4

Since the release of version 4, the ACDSee ProDetail menu contains the ChromaticAberration Correction tool, which has slidersfor correcting red/cyan and blue/yellowerrors. The Detail menu also contains theDefringe tool.

The chromatic aberration tool worked wellon the lateral errors in our RAW files, andproduced visible improvements. But take care:if you remove fringes at the top edge of theframe, you need to make sure that they havebeen effectively removed from the bottomedge as well. This aspect of the ACDSee tool’sfunctionality is slightly temperamental due toits lack of lens profile support.

There is no before/after view, but theoriginal image can be displayed quickly andeasily using a single mouse click. Nikon’sCapture NX 2, for example, is much moresluggish in this respect.

We were unable to produce anyimprovement in our axial aberration testimage and the program had trouble with thehigh-contrast edges in our portrait shot too.Our attempts to remove blue fringingproduced new halo-like double images, whichwasn’t the effect we were looking for!

On the other hand, the program scoredwell on the purple fringing in our dolphinimage. The Defringe tool effectivelysuppressed the obvious pink fireworks in the

water, although the tool’s limits are stillobvious in the 100% view. The tool requiresyou to juggle three separate sliders, and thereis no single setting that can effectively removeirregular fringing in a single subject.

Basically, the tool works very well if thefringes you want to remove are all the samecolor and you are not too critical of theresults. The Defringe tool reliably removedthe color cast in our tree image, although – aswith Capture NX 2 – the color of the skysuffered a little as a result. Our JPEG testscaused no surprises, and the results were verysimilar to those produced by our RAW run.

To conclude, the ACDSee tool is great forremoving lateral aberrations, but the lack ofa batch processing tool makes it less practicalfor use with multiple images. The Defringetool is fine for removing single-color fringingof consistent strength. ACDSee does notcurrently offer any usable options forremoving variable axial aberrations.

Apple Aperture

Apple’s own RAW converter started with abang but hasn’t managed to keep up with thecompetition since. It still doesn’t provide lensprofile support and it supports only a limitednumber of camera models. On the positiveside Aperture’s Chromatic Aberration toolincludes features that are superior to thoseoffered by much of the competition.

The bad news is that you can only applycorrections manually using the Red/Cyan andBlue/Yellow sliders, and the only way tocompare before/after versions of an image isto use a working copy of your image as a

Computer-based Chromatic Aberration Filters

OUR TEST PROGRAMS Manufacturer Program URL Version tested Win Mac Linux Price (US$)

ACD Systems ACDSee Pro 4 de.acdsee.com 4.0 yes yes no 74.99

Adobe Lightroom 3 www.adobe.com 3.5 yes yes no 299.99

Apple Aperture www.apple.com 3.2.1 no yes no 159.95

DxO DxO Optics Pro www.dxo.com 6.6.0.173 yes yes no 169.00

Nikon Capture NX2 www.nikon.com 2.2.8 yes yes no 179.95

Phase One Capture One www.phaseone.com 6.3.2 yes yes no 59.99-179.99

RawTherapee Team RawTherapee www.rawtherapee.com 4.0.4.2 yes yes yes free

Our test images and theirsource files are included onthis issue’s free DVD. You can use them to performthe same tests as we did onyour computer at home and decidefor yourself which are the best toolsfor your situation.

Free Test Images

Fr

ee on DVD

workaround. You can, however, use the M keyto toggle between views. The potentially coolloupe view took too long to refresh on our2011 2.3Ghz i5 Mac Mini, and it’s quicker touse a 100% (or larger) preview. The toolremoved the lateral aberration from our RAWtest image in fine style.

Apple’s brush HUD is a great bonus andcan be used to apply, strengthen or weakenthe effect selectively. This is a great aid whenyou are using generic filters that you onlywant to apply to part of an image. In suchcases you can also modify a filter’s effect onceyou have applied it.

We weren’t able to improve our axialaberration using the Apple tool, and it doesn’thave a dedicated defringing option. Our JPEG test turned out very similar to our RAWrun-through.

The brush functionality is great forperforming selective adjustments, but is farless practical than tools like Nikon’s CaptureNX 2 that support profile-based corrections.Many automatic correction tools nowadaysproduce results that are just as good asmanual corrections and are also a lot quickerto use. Aperture does not have a dedicatedfilter for correcting axial aberrations.

Capture One

Capture One has included lens correctionfilters for a while now, but the lens profilessupplied are few in number and are aimedsquarely at medium-format photographers. Itdoes enable you to define and save your ownpresets for individual lenses, but this is alaborious process, especially when dealingwith zoom lenses. This is because a preset’saperture, focal length and ISO value (amongstother settings) all have to match those of thephoto you want to adjust.

There is also a separate ChromaticAberration tool, which we tested using theprogram’s preset generic lens profile. All youhave to do is select the Analyze option in thetool’s drop-down menu and wait for a fewmoments while the program calculates theappropriate adjustments. The process worksvery well for lateral aberrations, but there areno user-configurable options. The Viewer’sZoom function enabled us to detect slightfringing in our target and skyscraper sample

images that we were unable to see in thestandard view. Generally, you can ignore suchslight errors.

The filter is obviously designed to correctonly lateral aberrations, and using it on ourimages with axial aberrations producedadditional flaws. The model in our test portraitacquired a red fringe on her hand, so we can’trecommend using it as a general chromaticaberration correction tool. On the plus side, theaxial errors in our images were less severe due



Original Corrected

The reflections on the surfaceof the water produce obviouspurple fringing that can beeffectively reduced using theACDSee Pro 4 Defringe tool

Aperture’s Chromatic Aberrationtool can be applied and removed

selectively using a brush


to Capture One’s advanced conversionfunctionality, which reduced the requireddegree of correction in the first place. Overall,the filter didn’t produce any visibleimprovements. The only really positive resultwe achieved was using the Purple Fringingoption on our dolphin image, where itnoticeably reduced the extraneous colors inthe reflections.

An attempt to correct a JPEG imageresulted in a disappointing ‘Unsupported file’warning and grayed out automatic ChromaticAberration and Purple Fringing options. Lenserror correction for JPEG images is simply notsupported.

Overall, Capture One made a mixedimpression. The automatic correction toolwas easy to use, but produced only averagequality results and lacks even the simplestuser-adjustable options. The lack of JPEGsupport isn’t a problem for most pro andsemi-pro photographers, but makes theprogram useless for hobby photographersand those wanting to process JPEGsequences shot using a DSLR.

DxO Optics Pro

Automatic correction tools are DxO’s trumpcard and, together with the countless dedicatedcamera and lens modules the manufactureroffers, make performing lens corrections a snapfor the owners of most currently availablecamera/lens combinations. Things are not quiteso simple for users of exotic gear or unusualcamera/lens combinations. In principle, OpticsProwas not really designed with manual profileadjustments in mind and users are dependenton the company providing a profile for eachspecific setup. There are currently about 5,000modules available, but many more arenecessary to completely cover all currentlyavailable cameras and lenses. There was nomodule available for three of our six testimages.

Optics Pro removed lateral aberrationsautomatically and effectively from our RAWimages. There is no way to performcorrections on images shot using a cameraand lens that are not covered by anappropriate module. This lack of manual

adjustment options is a serious weakness. Inour case, there was no profile available for ourolder 80-200mm f/2.8 Nikkor or our rare LeicaNoctilux 50mm f/1.0. On the other hand, ourcombination of a Zeiss 28mm f/2.0 and aCanon EOS 5DII is supported.

We were able to correct the axialaberration in our tree image and the ‘before’and ‘after’ views helped us to judge theresults. The program runs smoothly, althoughscrolling through a 100% preview of acorrected image was sometimes a littlesluggish. Optics Pro is, however, faster thanCapture NX 2 in every respect.

Optics Pro was no different from the otherprograms in our test and wasn’t able to correctcolor casts with varying colors in our axialaberration test image, although it producedgood results when correcting the axialaberration in our test portrait. It wasn’t reallyclear to us why we had to select the Purplefringing option to correct blue fringes, but itworked nonetheless. The same setting alsoeffectively corrected the reflections in ourdolphin image.




The Capture One Zoom tool makes it easy to detect chromatic aberration,even in the standard Viewer window

The program was not nearly as good atcorrecting lateral aberrations in JPEG images.DxO says that this is because our sampleimages were not OoC (Out of Camera) JPEGsproduced directly by the camera and claimsbetter results for OoC files. Our correctionattempts left obvious artifacts in our targetand skyscraper images, and the lack ofmanual adjustment options makes DxOunsuitable for everyday JPEG corrections.

Good automatic lateral corrections in RAWbut poor JPEG results and the absence ofprofiles and manual settings make Optics Proa mixed bag. Axial corrections wereconsistently good for RAW and JPEG images.The program is a recommend for RAW

photographers who use popular equipmentcovered by the its proprietary profile modules.

Adobe Lightroom

Lightroom uses the same profile-based ACR(Adobe Camera Raw) RAW conversion moduleas Photoshop. Our test delivered great resultsfor lateral aberrations in both Lightroom andACR, and the filter’s handling was exactly thesame in both programs. Unfortunately, bothwere unable to produce usable results for ouraxial aberrations, and failed to remove theblue fringe in the portrait, the purple fringe inthe tree image and our dolphin’s coloredreflections.

Lightroom’s automatic filter producedmarkedly better results when correcting RAWimages automatically using lens profiles,although a little manual tweaking improvedthings quite a bit for our lateral aberrationtest. The small number of Canon and Nikonlens profiles available in JPEG mode makesmanual correction unavoidable for manyimages. If you have the choice, always shootRAW images – the range of lens profiles formaking corrections later on is much broader.

In contrast to ACR, Lightroom allows you toselect a lens profile without first having toselect a camera model.

All in all, Lightroom is a great program, butstill has a lot of room for improvement when

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DxO Optics Proeffectively filteredout the axialaberration on themodel’s chin. Thebefore/after andmagnified viewsdisplay significantimprovements.


it comes to correcting chromatic aberrations.Filtering lateral errors in RAW mode is easyand effective, but the program does notprovide an effective solution for fringes andflare caused by axial lens errors.

Nikon Capture NX 2

Just like its browser/editor cousin ViewNX 2,Nikon’s RAW converter offers broad-basedoptions for correcting chromatic aberrations.For images shot using current Nikon DSLRs,the Auto Lateral Color Aberration tool isautomatically activated in the Camera & Lens

Corrections section of the program’s Developpanel. You cannot adjust this setting in thecamera, but you can undo it by uncheckingthe option in the software interface. There areno other adjustment options for theautomatic version, but a separate, manualLateral Color Aberration tool is available in theAdjust panel. This contains separate sliders forcorrecting red-cyan and blue-yellow artifacts,and an additional slider for adjusting theopacity of the effect.

The axial correction tool is not activated bydefault but has a single slider for adjusting thestrength of the effect it produces.

Capture NX 2 produced good results for ourlateral error target and skyscraper images inRAW mode. The Nikon automatic correctionfunction works well and it is only evident thateven Nikon lenses produce color errors whenyou switch it off.

Capture NX 2 wasn’t able to do much withthe variable color casts in front of and behindthe focal plane in our axial target image. It wasable to reduce the obvious fringing in thedolphin image, but some errors neverthelessremained visible after correction.

The lack of a before/after view is ahindrance if you want to compare corrected




Before/After views like the one shown here (in Adobe Lightroom)are perfect for comparing a corrected image with the original

Before After

and uncorrected image versions duringcomplex corrections, and the built-in Comparewith Original option is not at all easy to use.We were unable to try out RAW correction onour other test images because they weren’tcaptured in Nikon’s proprietary NEF format.

The program filtered lateral errors fairlyeffectively in JPEG mode. It produced minimalimprovements in our axial target, but didmanage to reduce the color cast caused by our

original RAW conversion using RawTherapee.Capturewasn’t able to fully correct the varyingcolor casts in JPEG mode, but did improvethings somewhat. It adjusted the purple castin the branches of our tree to much morenatural tones, but tended to adjust the overallcolors in an image while correcting axial errors.While it produced a visible improvement in ourcolorful portrait image, it did produce someslight edge artifacts in the process.

Purple fringing correction worked muchbetter for our JPEG test image than it did forRAW, and reduced the strong colored artifactsso much that they became virtually invisible.

Capture NX 2 offers solid, easy-to-use colorerror correction tools, but is restricted tocorrecting only Nikon RAW images. It can beused to correct JPEGs from any camera. Thesoftware’s known weaknesses regardingstability, sluggishness and rather odd user


Nikon’s Capture NX 2 is the only programwe found that has dedicated lateral andaxial aberration correction tools

Capture NX 2 removed purple fringing moreefficiently in our JPEG test image than it didin our RAW sample

The quality of the RAW correctionwas clearly not as good


interface haven’t noticeably improved in thecurrent release.

RawTherapee

This freeware RAW converter has, over theyears, developed into a comprehensive RAWconversion and editing package with a featurelist that is just as long as that of thecommercial competition (see our article onRawTherapee in Issue 7 of c’t DigitalPhotography). The program includes anumber of tools for correcting chromaticaberrations – the Detail tab includes theDefringe filter, the Lens/Geometry tabincludes the C/A Correction tool and the RAWtab has its own Chromatic Aberrations toolthat offers manual and automatic errorcorrection.

RawTherapee was able to open all of ourtest images, and the automatic correctiontool produced great results (although still notquite up to Nikon standards) for our lateralaberration test. Manual corrections are quitelaborious because the preview imagerefreshes very slowly after each slideradjustment. As recommended by thesoftware’s authors, we used a dual-core 64-bitsystem with plenty of RAM for our test, butwe still spent quite a lot of time waiting.

There is no before/after view, whichmakes comparing images difficult, and‘features’ like the heavily pixelated viewwhen shifting a detail in the preview windowoften reminded us of the program’s opensource heritage. This isn’t particularly nice tolook at but does make recognizingerroneously colored pixels quite simple.

In general, the program’s automaticcorrection tool produced passable results withlittle effort for everyday subjects like ourskyscraper. It is safe to say that the differencesin quality produced by all of the programs wetested are not particularly significant atnormal viewing distances. WhereRawTherapee couldn’t really compete wascorrecting axial errors, and we weren’t able toproduce any visible improvements using anyof the built-in tools.

Neither the automatic nor the manualChromatic Aberration tool worked at all forJPEG images, although the Defringe and C/ACorrection tools both produced usable resultsafter a bit of manual tweaking (automaticcorrection only works for RAW images). Careis required when correcting chromaticaberrations in RawTherapee. We often foundthat by the time we had perfected acorrection in one part of an image, newartifacts had appeared elsewhere. Just as inRAW mode, we weren’t able to produce anyconvincing JPEG results for our axial errors,and we searched in vain for settings thatgenuinely improved our target and sampleimages.

RawTherapee is fine for correcting lateralerrors in JPEG and RAW images, although itshandling in RAW mode is much moreuser-friendly. If you don’t want to correct anysignificant axial errors and you don’t mind abit of a fight with a somewhat clunkyinterface, RawTherapee offers a no-frills RAWconverter that is easy to use, and its freewarestatus makes it a real alternative to thecommercial competition, especially foroccasional users. (keh) c

RawTherapee‘s automatic filter reducedthe effect of the error, although themagnified view shows that the tool stillhas its limits

The unfiltered image shows very obviouslateral aberrations

Setting the correction values manuallyenabled us to completely remove thefringing effects

RawTherapee includesautomatic and manualchromatic aberration

correction tools


c’t Digital Photography 9 (2012)82


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