Mirror vs Optical lens+

LED lighting test

M Fitton, P Loveridge, J O’DellRAL

High Power Targets GroupMike Fitton

Mirror LensSamyang 800mm MC IF f/8

Optical lensTamron SP 150-600mm F/5-6.3 Di VC USD+ Sigma 1.4X Teleconverter EX APO DG

Fixed focal length = 800mmFixed aperture = f8Weight = 870g

Focal length = 210-840mm (with 1.4x)Aperture = f8 - f32 (with 1.4x)Weight = 1951g

High Power Targets GroupMike Fitton

Advantages of the optical lens• Better resolution• More contrast• Variable focal length (Can fine tune image size)• Variable aperture (Control depth of field, improved sharpness)• No “donut” out of focus artefacts

• Longer• Heavier• More expensive

Disadvantages of the optical lens

High Power Targets GroupMike Fitton

Test setup

Lens hood Tamron 150-600mm

Sigma 1.4x teleconverter

C-mount to Nikon adaptor (with aperture control)

Megaspeed x7 Highspeed camera1920 x 1080 pixelsSensor 13.2 x 8.8 mm

485mm (longest zoom)

Distance from inner box window to camera sensor = 9.50m

386mm (longest zoom)

High Power Targets GroupMike Fitton

Test setup

A mock-up of the extra glass inside box to check for internal reflections

Back wallInner box window

3 extra glass windows

High Power Targets GroupMike Fitton

Test setupWhite paint with black ink grid Shot blast and engraved aluminium

Resolution chart at forward most trough position

Back wall of box

Grid 10 - 0.28mm wide lines, 0.14mm gaps

High Power Targets GroupMike Fitton

LED lighting

Remaining reflections are a long way off the camera axisFinal test should be done with outer box once modifications complete

Position and angle of LED’s adjusted to reduce reflections. Final positions converted into engineering design of LED heatsink

High Power Targets GroupMike Fitton

Focal length range at 9.5m

840mm

210mm

Easy to frame the shot exactly as you want without moving the camera

High Power Targets GroupMike Fitton

Transmission comparisonBoth lenses at f8 and the exposure time set in the camera to 90 microseconds.Camera sensitivity was set to the lowest setting of ISO 200This would be good for 11,100 frames per second

Mirror lens Optical lens

Optical lens has slightly better transmission (brighter), but only justOptical lens has much better contrast and resolution (foreground and background)NOTE: Optical lens not zoomed enough so disadvantage in resolution comparison

Crop area for resolution comparison (next slide)

High Power Targets GroupMike Fitton

Resolution + DOF comparisonMirror lens f890s (11k fps)

Optical lens f890s (11k fps)

Optical lens ~f16+500s (2k fps)

Central frame - ISO 200Focused on foreground resolution chart

White gaps are 0.14mm wide

High Power Targets GroupMike Fitton

Resolution + DOF comparisonMirror lens f890s (11k fps)

Optical lens f890s (11k fps)

Optical lens ~f16+500s (2k fps)

Edge of frame – ISO 200Focused on foreground resolution chart

High Power Targets GroupMike Fitton

Summary• With the high speed cameras at RAL we have plenty of light, even at ISO 200

• Megaspeed HHC X7 ISO 200 – 3200• Phantom v7.1 ISO 4800• RedLake HG-100k ISO ???? This would be useful

information

• Mockup of experiment with 4 layers of glass showed no problematic reflections

• Optical lens has superior sharpness and contrast (centre to edge of frame)

• Optical lens has ability to stop down aperture to further increase sharpness and depth of field.

• Optical lens has ability to optimise image size regardless of optical path length

• Optical lens has slightly better transmission (brighter)

• We recommend the use of this optical lens