Recap ! • Depth of field ! The Art, Science and Algorithms – Parameters? ! of Photography ! Lenses ! Optics II ! CSCI 4900/6900 ! Maria Hybinette ! 1 ! 2 ! SLRs and Other Cameras ! Single-Lens Reflect Camera (SLRs) ! Typical SLRs: ! • See the scene through the lens how the • Moving mirror that enables you to see the scene sensor sees it ! through the lens ! • Shorter shutter lag ! Pre SLRs: ! • Customizations (how about CHDK?) ! • 2 separate light paths: ! – Interchangeable parts (e.g., lenses) ! – Capturing : Through lens to film ! • Thought: Great LCDs cannot compete with – Viewing (‘image preview’ realistic for long distances) ! seeing a ‘real’ image preview. ! • Above (twin-lens reflect TLR) ! – Is this changing? ! • to the side (rangefinder) ! 3 ! 4 ! Camera Cross Section: Optics ! Pentaprism (1955) ! /+'01)2*&%3) 45+)2&+#+) • Correct light to the eye piece ! ,"'-+'%+*).+'%) • Interchangeable? ! !"#$%&'()%#*++') – Macro specific ! 7+8+9)3&**"*)))) :;< ° =) • !"#1.)2.1'+) !*"'063"$'0).+'%) Mirror is a bottleneck !&.3)"*) %>$?+*) %+'%"*) (shutter lag) ! • Larger than other cameras (mirror/pentaprism) ! • Limit the ‘wideness of >?2@AA+'BC&D&2+-&1B"*(AC&D&AE&'(.+6.+'%F*+8+9F#13+*1) lens’ unless mirror lockup) ! 5 ! 6 !
Focusing Screen ! Lenses ! • Ground glass • Made of many different typically ! lenses ! • Split screen (not • Different prices ! auto focus – Primes (generally better cameras) ! than zoom at their • Plain (auto focus) ! ‘specialized’ length) ! – Zooms (more expensive) ! – architectural ! • Gridded ! >?2@AA>$G21(+%B#"3A>$GAHI6J"%06492+'%&K+6 ,13+*16L+'%+) • Markings ! >?2@AACCCB#1'"'B#"3A#13+*163$%+$3A0+#>A .F2.1'0A&'-+9B>03.) >?2@AA+'BC&D&2+-&1B"*(AC&D&A,13+*1F.+'%) 7 ! 8 ! Price and Quality? ! Optical Quality? Evaluating Lenses ! • Typically price is correlated to optical quality but • EF 70-200 mm f/2.8L IS USM ($1.7K) ! sometimes price can be misleading. ! • EF 70-200mm f/4-5.6 IS USM ($550) ! • Sharpness (resolution, acutance, diffraction limits) ! • Contrast ! What matters? ! • Bokeh (out of focus areas, including highlights) ! • Optical quality (from the middle of lens to edges) ! • Aperture (@ biggest, stopped down) ! • Color : no color casts ! • Focusing motor ! • Chromatic aberrations (sagittal & axial) ! • Zoom ! • Flare & Ghosting ! • Nearest focusing distance ! • Light fall off (vignetting, shading) ! • Built & Handling ! • Distortion ! 9 ! 10 ! Sharpness (perceived) ! Acutance (contrast) ! • Clarity of detail in a photo ! • Acutance is about how sharply an edge – Resolution, and ! transitions to the white area. ! – Acutance (contrast) ! M&(>) L+%%) • Resolution : Resolve details, texture N#$01'#+) N#$01'#+) O""-)L+'%) P"*%+)L+'%) (camera bottle neck), Large prints ! • Acutance (crispness): Edge transition • Sharpening ! (lens), Important for Online ! – In-camera ! – post processing (Yay!) ! >?2@AACCCB#13G*&-(+&'#"."$*B#"3A0$0"*&1.%A%>1*2'+%%B>03) 11 ! 12 !
Resolution ! Acutance to Resolving Power ! • How many lines per mm N#$01'#+):Q+C+*).&'+%)2+*)33=) 7+%".$R"'):3"*+).&'+%)2+*)33=) can be distinguished? (lines HI).2A33) SI6;I).2A33) thinner & thinner) ! – smallest details a lens can resolve ! – Microcontrast ! • Sharpening? ! – Cannot really be improved in post processing ! 13 ! 14 ! Quantitative Measure ! Acutance (contrast) & Resolution ! • Modulation Transfer Function (MTF) ! • Sharp images = High Acutance & High – Spatial measure of acutance & resolution ! – Measure lines per millimeter ! Resolution ! • How much contrast is retained between the lines in the reproduced • Camera’s with high resolution does not image, compared to the original pattern, a MTF of: ! – [1.00, 100%] : no difference between original patter and image. ! necessarily produce images that are – [0.85, or 85%] : 85% of the original lines are retained in the image ! – [0.00, 0%] : Fine lines are indistinguishable in the image (grey) ! perceived as sharp. ! • Example images here: ! >?2@AACCCB#13G*&-(+&'#"."$*B#"3A0$0"*&1.%A%>1*2'+%%B>03) 15 ! >?2@AACCCB*+K&%+3*&B#"3AT$+%R"'%A+T$&2FT1A3U) >?2@AACCCB'"*31'D"*+'B#"3AV$0"*&1.%AJV!<B>03.) 16 ! Acutance (contrast) & Resolution ! L+%%)*+%".$R"') /+*Q+#0)#"'0*1%0) O""-)#"'0*1%0)X) L+%%)#"'0*1%0)X) Y1-)#"'0*1%0)X) • High resolution & high contrast ! X)*+%".$R"') .+%%)*+%".$R"') G+?+*)*+%".$R"') G1-*+%".$R"') • Low resolution & high contrast ! • Green 10 lp/mm (acutance) ! • High resolution & bad contrast ! • Magenta 30 lp / mm (resolution) ! • Conclusion: High contrast visually more important ! 17 ! >?2@AACCCB2>"0"W"'+B-+A3U) 18 !
Example MTF Charts ! MTF & Bokeh ! O""-) *+%".$R"'Z) O""-) ,"'0*1%0) L+\)&%)0>+)]I6;II)^7)10);II33):_HB`a=B)b')0>+)*&(>0)&%)0>+);II33)QAcB])^7)):_HIa=)) ["0)%")(""-) *+%".$R"'Z) • If the dotted (M10) and solid lines (M30) are close ["0)%")(""-) together ! ,"'0*1%0) – the out-of-focus areas of the lens will be smooth and pleasing (good ‘bokeh’), while if they are far apart the out-of- focus areas tend to be distorted and less pleasing (bad ‘bokeh’). ! 19 ! 20 ! Example : Low Frequency to High b*&(&'1.)/1?+*')/*"g.+) Quantitative Measure of Sharpness ! V+%0)/1?+*') Frequency Details (simplified) ! f31(+)"Q)/1?+*') f31(+)/1?+*')/*"g.+) • Modulation Transfer function (MTF) (now) ! P>&0+) • Spatial Frequency Response (audio, hertz) ! – Lens manufacturer posts these ! Y.1#D) • It is the contrast at a given spatial frequency (cycles or lines per mm (lp/mm) ~ Hertz) relative to low frequencies. [cycles/pixel] ! – Cycles! – line pairs (white/black) ! H;I) H<) >?2@AACCCB.+'%*+'01.%B#"3A'+C%AcIIdBIeBHSA>1K+65"$6%++'63561#$01'#+) 21 ! 22 ! >?2@AACCCBG"G10D&'%B#"3A2>"0"(*12>5A0+#>'.A3UA3UHB>03.) Modulation Transfer Function (MTF) " ReCap ! & Resolving Power ! • Simplified MTF Definition: ! – MTF = (maximum intensity - minimum intensity)/ (maximum intensity + minimum intensity) ! • Imagine lines get finer and finer (as in figure) ! – 255-0/255+0 = 1.0 [first part of plot] ! – 140-15/140+15= 125/155 = 0.8 [second part] ! – 0.5 ! – 0.1 * this lens ! – . ! – MTF = 0 uniform grey patch - no variation [127] -> 128-126/254 => 2/254 = 0.007 ! • This is the resolving power of the lens ! >?2@AACCCB+-3$'-"2R#%B#"3A0+#>'.6 %$22"*0A&31(&'(A$'-+*%01'-&'(63U60+%R'(A) 23 ! 24 !
Finally: What is MTF Really? ! Test Charts & Caveats ! • Ideally: Test pattern is a “sine wave” pattern not bars: ! • The MTF is calculated by performing a Fourier transform of the line • Problem : Difficult to produce smooth transitions ! spread function, and the line spread function is calculated by differentiating the edge response function, which is what we measure on • Solution : Use bar patterns: ! an image. ! – Bar patterns unites are lines per mm ! – Sinusoidal patterns unites are cycles per mm ! • Caveats: MTF depends on (but the variation is small) ! Edge Response Function (ERF) (measured) ....differentiate: ! → Line Spread Function (LSF) ....Fourier transform: ! – Wavelength of light used: ! → Modulation Transfer Function (MTF). ! • Blue has a higher MTF than red’s MTF ! • Normally white light is used ! • The ERF is dead simple. It's just a plot of the image intensity across the – Color temperature: ! edge of an angled block in an image. The differentiation of this ERF shape is obvious (or should be, to a physicist!). Why a Fourier transform • Tungsten lower than daylight (blue) ! is the thing to do to then get MTF vs ν , should also be obvious to you – Tungsten has longer wavelength components ! if you have looked at Fourier transforms. ! – Detector ! h"'i0)'++-)0")D'"C)Q"*) • Sensors more sensitive to blue light than red light ! 0+%0) – Will have a higher MTF than with a detector more sensitive to red light ! >?2@AACCCB*+K&%+3*&B#"3AT$+%R"'%A+T$&2FT1A3+1%$*&'(F3U) 25 ! 26 ! Aperture: The Sweet spot ! Aperture & Quality ! • Typically 2 stops down ! • f/2.8 on an f/1.4 lens (f/1.4, f/2.0,f/2.8) ! • f/11 on an f/5.6 one. (f/5.6, f/8,f/11) ! • Better lenses peak earlier. ! • f/4-f/11 sweetspot ! 27 ! 28 ! Ideal Lens ! Considerations (in MTF terms) ! • Diffraction Limited ! • Example : Rule of thirds – off-centered, corner sharpness becomes more important. ! – Lesson: Combinations of aperture, focal length, and subject placement do you find yourself at most of the time? ! 30 ! >?2@AAQ"0"(+'+R#B-+1*&'(g.3B#"3A>"CF0"F#>""%+F1F.+'%B>03.) 29 !
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