1. Fundamentals of Lenses Anindya Das 20th October 2012

2. Introduction Lens is one of the most important parts of the camera Lens is the eye of the camera and it helps to form animage on the sensor During lens design a designer have to keep thefollowing in mind:1. Weight2. Size3. Cost4. Amount of distortion 3. Important Lens Attributes Focal Length Speed Focusing Mechanism Lens Mount Image Stabilization 4. Focal Length Determines the size of the image that falls on the sensor Longer focal length larger size, Shorter focal length smallersize Measured in mm distance between the lens and the sensor(where image is sharply formed) when the subject is at infinity Photographic lens is a combination of atleast 4 elements whilecomplex ones can go to more than 20. Multiple elements needed to correct several optical defects foundin a single lens 5. Focal Length Images at different focal lengths 6. Focal Length Images at different focal lengths 7. Focal Length Angle of View (AOV)46deg32deg A lens is qualified telephoto or wide angle by its Angle ofView AOV depends on the focal length and the size of thesensor Cropped sensor gives the magnification of focal length in35mm times the cropping factor 8. Sensor conversion factor of various sizesSensor Size App. Conversion factorUsed Bywith reference to 35mm4/3 System 2.00Olympus, Kodak,Panasonic, LeciaAPS-C1.60All Cannon DSLRs except EOS 1D and aboveDX 1.50All Nikon DSLRs except D3 and aboveAPS-H1.30Cannon EOS 1D series, Leica M8Full Frame/35mm1.00Nikon D3 and Cannon EOS 1Dsfilm seriesFilm 6 x 4.5 cm0.58Pentax, Mamiya, Bronica, HasselbladFilm 6 x 6 cm0.51 Pentax, Mamiya, Bronica,Most of the super zoom equipped bridge digital cameras haveHasselbladvery small sensors and thus need only a short focal length toget a great telephoto effect 9. Lens Speed It doesnt have anything to do with the speed of thesubject to be photographed Speed is an indication of the light gathering capacityof the lens Referred to as the f number and is denoted as f/ f-number denoted by f/De.g. if the focal length is 16 mm and the entrance pupildiameter is 1 mm, the f-number is 16 and the apertureexpressed as f/16 Lens has a diaphragm to control amount of lightpassing through the lens. Its set of blades make anopening called aperture Aperture size can be controlled automatic or manually 10. Lens Speed Stops Stops are in app. multiples of 1.4 or 2. It is bcoz each stop doubles or halves the area of the aperture openingAll DSLRs giveautomatic diapragmswhich allow clear viewfrom the viewfinderwhile composingregardless of theshutter speed chosen 11. Fast Lens A heavy price to payLens SpeedWeight (gms) Looks Likef/5.6505f/4.0 1,440f/2.8 2,870f/2.0 7,545 12. Focusing Mechanism Since all the objects in a composition are notequidistant a lens needs to be focussed Heclical focusing mechanism is used. Lens needs tobe moved away from focal plane for near objects andcloser for far objects Manual focusing done with a focusing ring andchecking sharpness in the view finderTwo different autofocus methods: There is a motor in the camera which drives the lensthrough a mechanical coupling The body signals the motor inside the lens which doesthe needful. When the body detects the correct focusthe driving stops 13. Lens Mount System of mechanical and electrical linkages thatensures proper interface between camera and lensbody Propriety to each brand and cannot be changed afterpurchase Several independent lens manufacturers who producelenses in different camera mounts e.g. Sigma andTamronFour Thirds System Allows for interchange of lenses and bodies fromdifferent manufacturers. Standard created by Olympusand Kodak Sensor sizes are as small as crop factor of 2 (i.e. a 14. Image Stabilization Handshake adversely affects the sharpness of a picture.IS helps in improving the same. Also known as Vibration Reduction(VR) and OpticalStabilization(OR) Shake is detected by a gyroscope inside the lens andgroup of lens are moved to counteract this With this you can go 2-3 stops slower shutter speeds(e.g. if you needed 1/500s you can g upto 1/125s or1/60s and even get a sharp picture All lenses have a switch to activate/deactivate IS/VR 15. Types of LensesType Diagonal Focal Length Focal Length Type of Angle of for Full for CroppedPhotography View FrameSensors (AOV)DSLRsDSLRsUltra Wide 84 deg or24mm or less 16mm or less Landscapes,AngleMore Architecture,InteriorsModerate 62 84 deg 24 -35 mm16 -24 mmGeneral purpose,Wide AngleLandscapeNormal Around 45- 40 50 mm 25 - 35 mm General Purpose, 50 deg Candid, Low LightShort to 12 28 deg 85 200 mm 60 150 mmPortraits, Candid,MediumStreetsTelephotoSuper8 deg or 300mm or 200mm or Sports, Racing,Telephotoless greatergreaterWildlife, Birds 16. Normal Lens Focal Length equal to the diagonal of the sensor frame Generally they are the fastest lens around and are bestsuited for low light photography Provide a view that is closest to the human eye With this you can go 2-3 stops slower shutter speeds(e.g. if you needed 1/500s you can g upto 1/125s or1/60s and even get a sharp picture All lenses have a switch to activate/deactivate IS/VR 17. Telephoto Lens Lenses with long focal length High Magnification and narrow angle of view Most application in wildlife, birds and sports photography Have an effect of compressing distance between nearand far looks less than reality Due to high magnification tendency for cameramovement induced shake to show up very easily Use shutter speed of at least 1/(35mm focal lengthequivalent).E.g. If you hold a 200mm lens on acropped sensor (APS sized) then minimum speed shouldbe 1/300s 18. Wide Angle Lens Shorter focal length and give wide angle of view They can decompress or expand the relative distancebetween objects Can cause some problems from perspective point of view Best suited for landscapes, interiors etc Should not be used for portraits where it will make you goclose and you end up with some exaggerated featurese.g. nose Lens will be very close to the camera body At very short focal lengths the lens is so close to the bodythat back of lens interferes with mirror movement Retro-focus used in wide angle lens which providessufficient physical length but keeping the focal length 19. Zoom Lens and Prime LensZoom Lens Lens having a range of focal length for the user to work on Most popular among the DSLR users Wider the range harder it is to keep image quality high acrossall focal lengths. Sweet spot in zoom lens sharpest and clearestPrime Lens Lens whose focal length cannot be changed Much faster than the zoom lens (e.g. 50mm f1.8). Helpful inlow light photography Optics in prime lens are of way higher quality Much better in terms of sharpness, color reproduction,distortion etc. 20. Difference in composition at different focallengths 21. Zoom Ratio Number you get by dividing the longer focal length of azoom lens with the shorter one is called zoom ratio E.g. for a 18-72mm zoom lens the zoom ratio is 4 Very high ratio exist (in excess of 20). Generally suchhigh ratio lenses compromise on image quality High zoom ratios are easy to make in small digitalcameras but not possible for full frame sensors e.g. 5-90mm on bridge cameras would correspond to 28-500mm on full frame cameras. This will be a huge lensand technically challenging 22. Focusing and Aperture in Zoom Lens Parfocal Lens: Zoom lens which hold on to the focuswhen the focal length is changed. Difficult to design andcostly to produce Varifocal Lens: Focus changes when you zoom. Thusneed to focus (automatic / manual) after you zoom. Mostcommonly used technique Focusing is done with turning the ring on the lens wherethe focal lengths are marked. For P&S two way rocker switch is used to adjust focallength Lens speed changes when you zoom. 18-70mm f/3.5 f/5.6 is a variable aperture zoom lens. 23. Macro Lens Allows to take pictures with high magnification Special helical focusing mechanism which extends lensaway from focal plane and focus very close to the subject Macro lens gives magnifications of >= 1:1 Normal zoom lens produces an image as big as 1:4 onthe sensor Available for range of focal lengths 50mm to 200mm Higher focal length macro lenses gives a greater workingdistance 1. 50 mm lens 2. 300 mm mirror lens 3. 300 mm lens 24. Fish Eye Lenses Extremely wide angle lens with very short focal lengths Circular Fish-eye: Produces 180 deg coverage all round and creates a circular image within the frame Full Frame: Diagonal AOV of 180deg and produces an image that covers the frame fully 25. Tilt and Shift Lenses Ability to tilt or shift a part of the lens Allows tremendous control over the Depth of Field Normally tilting a camera to include a building makes itlook tilting backwards. Can be corrected with this lens Widely used in table top, architectural and landscapephotography Expensive lenses. Also requires skill and understanding 26. Tele-Converters Added between the lens and the body to give longer focallength Cannot be individually used with the body Generally available with multiplication factors of 1.4X, 2X and3X Increase the focal length by that much factor Decrease the lens speed by that much factor Results in loss in quality as one moves up the multiplicationfactor Problems with autofocus. For a max aperture range of f/5.6tele convertors cannot be used Lens with max aperture of f/4.0 convertor 1.4X can be usedwhich will increase aperture to f/5.6. Hence autofocus ispossible 27. So which lens is suitable for you Lot depends on the type of photography you do and the typeof subjects For P&S zoom lens with range from 28mm to 105mm (35mmequivalent) will be fine in most situations Bridge cameras give a lot more telephoto reach For cropped DSLRs two lenses will be fine : Zoom of 18-55mm and 55-200mm . This can be covered in 18-200mm butwill be expensive and lead to loss of image quality For beyond 200mm lenses image stabilization is a must Telephoto lenses (more than 300mm) are for very specificphotography Image quality greatly depends on the lens and less on thebody 28. Happy Clicking