Recap ! Depth of field ! The Art, Science and Algorithms - - PDF document

The Art, Science and Algorithms

• f Photography!

Lenses ! Optics II! CSCI 4900/6900! Maria Hybinette!

1!

Recap!

• Depth of field!

– Parameters?!

2!

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:!

– Capturing: Through lens to film! – Viewing (‘image preview’ realistic for long distances)!

• Above (twin-lens reflect TLR)!
• to the side (rangefinder)!

3!

SLRs and Other Cameras!

• See the scene through the lens how the

sensor sees it!

• Shorter shutter lag!
• Customizations (how about CHDK?)!

– Interchangeable parts (e.g., lenses)!

• Thought: Great LCDs cannot compete with

seeing a ‘real’ image preview.!

– Is this changing?!

4!

Camera Cross Section: Optics!

• Mirror is a bottleneck

(shutter lag)!

• Larger than other cameras

(mirror/pentaprism)!

• Limit the ‘wideness of

lens’ unless mirror lockup)!

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Pentaprism (1955)!

• Correct light to

the eye piece!

• Interchangeable?!

– Macro specific!

6!

Focusing Screen!

• Ground glass

typically!

• Split screen (not

auto focus cameras)!

• Plain (auto focus)!

– architectural!

• Gridded!
• Markings!

7!

Lenses!

• Made of many different

lenses!

• Different prices!

– Primes (generally better than zoom at their ‘specialized’ length)! – Zooms (more expensive)!

8!

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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)!

• Clarity of detail in a photo!

– Resolution, and ! – Acutance (contrast)!

• Resolution: Resolve details, texture

(camera bottle neck), Large prints!

• Acutance (crispness): Edge transition

(lens), Important for Online!

11!

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Acutance (contrast)!

• Acutance is about how sharply an edge

transitions to the white area.!

• Sharpening!

– In-camera! – post processing (Yay!)!

12!

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Resolution!

• How many lines per mm

can be distinguished? (lines thinner & thinner)!

– smallest details a lens can resolve! – Microcontrast!

• Sharpening?!

– Cannot really be improved in post processing!

13!

Acutance to Resolving Power!

14!

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Quantitative Measure!

• Modulation Transfer Function (MTF)!

– Spatial measure of acutance & resolution! – Measure lines per millimeter !

• How much contrast is retained between the lines in the reproduced

image, compared to the original pattern, a MTF of:!

– [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)!

15!

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Acutance (contrast) & Resolution!

• Sharp images = High Acutance & High

Resolution!

• Camera’s with high resolution does not

necessarily produce images that are perceived as sharp.!

• Example images here:!

16!

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Acutance (contrast) & Resolution!

• Green 10 lp/mm (acutance)!
• Magenta 30 lp / mm (resolution)!

17!

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• High resolution & high contrast!
• Low resolution & high contrast!
• High resolution & bad contrast!
• Conclusion: High contrast visually more important!

18!

Example MTF Charts!

19!

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MTF & Bokeh!

• If the dotted (M10) and solid lines (M30) are close

together!

– 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’). !

20!

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Quantitative Measure of Sharpness!

• Modulation Transfer function (MTF) (now)!
• Spatial Frequency Response (audio, hertz)!

– Lens manufacturer posts these!

• 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) !

21!

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Example: Low Frequency to High Frequency Details (simplified)!

22!

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Modulation Transfer Function (MTF) " & 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!

23!

ReCap!

24!

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Finally: What is MTF Really?!

• 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.!

Edge Response Function (ERF) (measured) ....differentiate:! → Line Spread Function (LSF) ....Fourier transform:! → Modulation Transfer Function (MTF).!

• 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.!

25!

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Test Charts & Caveats!

• Ideally: Test pattern is a “sine wave” pattern not bars:!
• 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)!

– Wavelength of light used:!

• Blue has a higher MTF than red’s MTF !
• Normally white light is used!

– Color temperature: !

• Tungsten lower than daylight (blue)!

– Tungsten has longer wavelength components!

– Detector!

• Sensors more sensitive to blue light than red light!

– Will have a higher MTF than with a detector more sensitive to red light!

26!

Aperture: The Sweet spot!

• 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. !

27!

Aperture & Quality!

• f/4-f/11 sweetspot!

28!

Ideal Lens!

• Diffraction Limited!

29!

Considerations (in MTF terms)!

• 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? !

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Landscapes (General rule of thumb)!

• Wide-Angle !
• Aperture stopped down to get the most

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!

Portraits!

• Subjects close to center (unless you're doing

environmental portraits)!

• Moderate telephoto lens. !
• Aperture is often fully open or almost fully
• pen to blur the background. !
• Recommendation:!

– (70mm-150mm) that is sharp in the center at its widest aperture (f/4 or larger).!

32!

Street & Candids!

• Zoom lens 35-100mm!
• Sharp throughout the frame when it is

stopped down to f/8!

– Candids often include filling the frame with the subject(s) [both in terms of depth and frame !

33!

Action / Sports!

• Same as candids but longer!

– 200-600mm!

• Sharp wide open? Or panning?!

34!

Nature!

• 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!

Distortions!

• Zoom (barrel at wide end)!
• Zoom (no distortion, in-between)!
• Zoom (pincushion at the long end)!
• Wave – ugly, but uncommon!

36!

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Wide Angle & Fish Eye (extreme barrel distortions)!

• Tilted upwards!
• Tilted downwards!
• Fisheye!

37!

Vignetting!

• Darker Illumination towards corners!
• Most obvious wide open especially for wide

angle lenses!

– Minimize by stopping down!

38!

Color/’Tinted’ Lenses!

• Cheap glass, tend to transmit on the warm site!
• Portrait lenses may be warmer… warmer skin

tend to look healthier, no need for a tanning bed!

– Warming filters (1A or 1B)!

39!

Flare!

• Contrast deterioration (haze)!
• Ghosting !

– aperture shaped blobs due to inner reflections! – Multiple images of strong light sources!

40!

Blur!

• Controlled by aperture!

41!

Bokeh!

• Spherical aberration

not focused at the same point (later)!

• Gaussian blur !

42!

Can be appealing:!

• Not the same as

unfocused.!

• Bokeh !

– Size (amount of blur)! – Shape! – Light distribution of disk (aberration)!

43!

Bad Bokeh!

44!

Good Bokeh!

45!

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!

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Chromattic Aberration: Examples!

47!

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Spherical aberration!

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Shape!

• Shape of aperture diaphragm!

– Round, Hexagon!

49! 50!

Gaussian lens formula!

51!

Resources!

• 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/!

52!