Bokeh (out of focus areas, including highlights)! • Color: no color casts! •
Chromatic aberrations (sagittal & axial)! • Flare & Ghosting! • Light fall off (
vignetting, ...
Recap! • Depth of field!
The Art, Science and Algorithms of Photography!
– Parameters?!
Lenses ! Optics II! CSCI 4900/6900! Maria Hybinette!
1!
2!
SLRs and Other Cameras!
Single-Lens Reflect Camera (SLRs)! Typical SLRs:! • Moving mirror that enables you to see the scene through the lens ! Pre SLRs:! • 2 separate light paths:!
• See the scene through the lens how the sensor sees it! • Shorter shutter lag! • Customizations (how about CHDK?)! – Interchangeable parts (e.g., lenses)!
– Capturing: Through lens to film! – Viewing (‘image preview’ realistic for long distances)!
• Thought: Great LCDs cannot compete with seeing a ‘real’ image preview.!
• Above (twin-lens reflect TLR)! • to the side (rangefinder)!
– Is this changing?!
3!
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Camera Cross Section: Optics! /+'01)2*&%3)
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Pentaprism (1955)! • Correct light to the eye piece! • Interchangeable?!
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– Macro specific! !*"'063"$'0).+'%)
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7!
Price and Quality?! • EF 70-200 mm f/2.8L IS USM ($1.7K)! • EF 70-200mm f/4-5.6 IS USM ($550)! What matters?! • Optical quality (from the middle of lens to edges)! • Aperture (@ biggest, stopped down)! • Focusing motor! • Zoom! • Nearest focusing distance! • Built & Handling! 9!
Optical Quality? Evaluating Lenses!
• Typically price is correlated to optical quality but
sometimes price can be misleading.! • Sharpness (resolution, acutance, diffraction limits)! • Contrast! • Bokeh (out of focus areas, including highlights)! • Color: no color casts! • Chromatic aberrations (sagittal & axial)! • Flare & Ghosting! • Light fall off (vignetting, shading)! • Distortion! 10!
Sharpness (perceived)!
Acutance (contrast)!
• Clarity of detail in a photo!
• Acutance is about how sharply an edge
– Resolution, and ! – Acutance (contrast)!
transitions to the white area.! M&(>) N#$01'#+) O""-)L+'%)
• Resolution: Resolve details, texture •
(camera bottle neck), Large prints! Acutance (crispness): Edge transition (lens), Important for Online!
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• Sharpening! – In-camera! – post processing (Yay!)!
12!
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Resolution!
Acutance to Resolving Power!
• How many lines per mm
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can be distinguished? (lines thinner & thinner)!
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– smallest details a lens can resolve! – Microcontrast!
• Sharpening?!
– Cannot really be improved in post processing! 13!
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Quantitative Measure! •
Modulation Transfer Function (MTF)!
•
How much contrast is retained between the lines in the reproduced image, compared to the original pattern, a MTF of:!
Acutance (contrast) & Resolution! • Sharp images = High Acutance & High
– Spatial measure of acutance & resolution! – Measure lines per millimeter !
Resolution! • Camera’s with high resolution does not necessarily produce images that are perceived as sharp.! • Example images here:!
– [1.00, 100%] : no difference between original patter and image.! – [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)!
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Acutance (contrast) & Resolution!
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• • • •
Green 10 lp/mm (acutance)! Magenta 30 lp / mm (resolution)!
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18!
High resolution & high contrast! Low resolution & high contrast! High resolution & bad contrast! Conclusion: High contrast visually more important!
Example MTF Charts!
MTF & Bokeh!
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• If the dotted (M10) and solid lines (M30) are close together!
20!
Quantitative Measure of Sharpness!
– the out-of-focus areas of the lens will be smooth and pleasing (good ‘bokeh’), while if they are far apart the out-offocus areas tend to be distorted and less pleasing (bad ‘bokeh’). !
Example: Low Frequency to High Frequency Details (simplified)! P>&0+)
• Spatial Frequency Response (audio, hertz)!
– Lens manufacturer posts these!
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• It is the contrast at a given spatial frequency (cycles or lines per mm (lp/mm) ~ Hertz) relative to low frequencies. [cycles/pixel]! H;I)
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Modulation Transfer Function (MTF) " & Resolving Power!
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ReCap!
• 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!
23!
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• Modulation Transfer function (MTF) (now)!
– Cycles! – line pairs (white/black) !
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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 spread function, and the line spread function is calculated by differentiating the edge response function, which is what we measure on an image.!
• Problem: Difficult to produce smooth transitions! • Solution: Use bar patterns:! – 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:! → Modulation Transfer Function (MTF).!
•
– Wavelength of light used:!
• 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 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 is the thing to do to then get MTF vs ν, should also be obvious to you if you have looked at Fourier transforms.!
– Color temperature: ! • Tungsten lower than daylight (blue)! – Tungsten has longer wavelength components!
– Detector!
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• Sensors more sensitive to blue light than red light! 26!
Aperture: The Sweet spot!
– Will have a higher MTF than with a detector more sensitive to red light!
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? !
29!
30!
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Portraits!
Landscapes (General rule of thumb)!
• Wide-Angle ! • Aperture stopped down to get the most
• Subjects close to center (unless you're doing environmental portraits)! • Moderate telephoto lens. ! • Aperture is often fully open or almost fully open to blur the background. ! • Recommendation:!
depth-of-field (DOF) and sharpness. ! • Recommendation: ! – Focal length of 36mm or less that is evenly sharp throughout the frame when the aperture is stopped down (f/8 or smaller).!
31!
– (70mm-150mm) that is sharp in the center at its widest aperture (f/4 or larger).! 32!
Street & Candids!
Action / Sports!
• Zoom lens 35-100mm! • Sharp throughout the frame when it is
• Same as candids but longer! – 200-600mm!
• Sharp wide open? Or panning?!
stopped down to f/8! – Candids often include filling the frame with the subject(s) [both in terms of depth and frame !
33!
34!
Nature!
Distortions!
• Extremely long lenses 300mm or more! • Wide aperture f/4 or larger to isolate the subject! • Need wide aperture and is sharp wide open.! – Most expensive!!
35!
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• Zoom (barrel at wide end)! • Zoom (no distortion, in-between)! • Zoom (pincushion at the long end)! • Wave – ugly, but uncommon! >?2@AACCCBCCCB2>"0"W"'+B-+A.+'%60+*3&'"."(5) 36!
Wide Angle & Fish Eye (extreme barrel distortions)!
37!
Vignetting!
• Darker Illumination towards corners! • Most obvious wide open especially for wide
• Tilted upwards! • Tilted downwards! • Fisheye!
angle lenses!
– Minimize by stopping down! 38!
Color/’Tinted’ Lenses!
Flare!
• Contrast deterioration (haze)! • Ghosting !
• Cheap glass, tend to transmit on the warm site! • Portrait lenses may be warmer… warmer skin
– aperture shaped blobs due to inner reflections! – Multiple images of strong light sources!
tend to look healthier, no need for a tanning bed!
39!
– Warming filters (1A or 1B)!
40!
Blur!
Bokeh! • Spherical aberration not focused at the same point (later)! • Gaussian blur !
• Controlled by aperture!
41!
42!
Can be appealing:!
Bad Bokeh!
• Not the same as unfocused.! • Bokeh ! – Size (amount of blur)! – Shape! – Light distribution of disk (aberration)! 43!
44!
Good Bokeh!
Chromatic Aberration! • Light of different color doesn’t converge (focus) at the same point (more later)! – Lenses have different refractive index for different wavelengths of light! – The refractive index decreases with increasing wavelength.!
• Fringes of color along boundaries that
separate dark and bright part of the image! 46! >?2@AACCCB2*&3+6j$'01B'+0A2"'0AM"CF0"A>1FV+%R'(F.+'%+%A1FM"CF0"F0+%0F1F.+'%B>03.)
45!
Chromattic Aberration: Examples!
Spherical aberration!
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Shape! • Shape of aperture diaphragm! – Round, Hexagon!
49!
50!
Gaussian lens formula!
Resources! • • • • • • • • • •
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http://www.prime-junta.net/pont/How_to/ha_Testing_lenses/ a_How_to_test_a_lens.html! http://www.slrgear.com/reviews/index.php! – http://www.slrgear.com/reviews/zarticles/intros/howwetest.html!
http://www.bythom.com/rationallenses.htm! http://www.bobatkins.com/photography/technical/ testing_lenses.html! http://en.wikipedia.org/wiki/Critical_sharpness! http://en.wikipedia.org/wiki/Aperture! http://en.wikipedia.org/wiki/Acutance#Sharpness! http://luminous-landscape.com/tutorials/resolution.shtml! http://www.imatest.com/docs/blur_mtf.html! http://photo.net/learn/optics/mtf/!
