1. Digital Camera Essential Elements Part 2

2. Digital Image Quality Image Quality - more comprehensive than resolution and more meaningful 4 Pillars of Quality –http://www.normankoren.com/Tutorials/MTF7.HTM L#Pillarshttp://www.normankoren.com/Tutorials/MTF7.HTM L#Pillars Resolution- a function of: lens+pixel number+file processing software Noise - analogous to grain in film photography Dynamic Range – Maximum signal / Minimum “usable” signal- usually expressed in powers of 2 (f-stops) Color Accuracy – White Balance, Saturation,

3. Digital Camera Resolution Megapixels – only one of many factors that limit resolution Other factors –Pixel size or pitch –Lens quality –Interpolation software –System noise (electronic and photon) –Image data file compression

4. Resolution http://www.dpreview.com/learn/?/Glo ssary/Digital_Imaging/Resolution_01. htmhttp://www.dpreview.com/learn/?/Glo ssary/Digital_Imaging/Resolution_01. htm Measured in line pairs / mm –Vertical (primary), Horizontal & –High contrast bar chart input –Low ISO (low noise) exposure –In camera processed, then printed

5. Digital Noise http://clarkvision.com/imagedetail/does.pi xel.size.matter/http://clarkvision.com/imagedetail/does.pi xel.size.matter/ CCD Noise – Photon “Shot” noise –Photons arrive at random intervals, so there is some uncertainty about the “true number” collected –Photon Noise: (# Photons collected) 1/2 Total Noise: ((Photon Noise) 2 +(Electronic Noise) 2 ) 1/2 Effective S/N: #Photons/Total Noise – Photon noise is dominant in a usable signal

6. (2) Digital Noise Cont’d Why do I care about noise? –Because we see it as luminance variation in uniform areas within the photo –ISO 12232.1998 standard S/N >~10: “acceptable” quality for 18% gray patch (R,B,G = 46) –S/N >~32: “noise” not visible –S?N >0.3: white patch just visible on black background

7. Digital Noise Example S/N = 10 @ 18% gray (R,B,G=46) (~219 photons)

8. Digital Noise Example S/N = 4 @ 18% gray (R,B,G=46) (~73 photons)

9. (3)Digital Noise Cont’d Noise standard sets a ”floor” on the number of photons that need to be collected –Signal:219, Shot Noise:14.8, Read noise:16, –Total noise: (475) 1/2 = 21.8 S/N = 219/21.8 =10 CCD is a linear amplifier, so (256/46)x219=1219 photons is minimum “full well” at level 256 for an “acceptable” image, at least from the stand point of noise.

10. (4)Digital Noise Cont’d Oops! –The inherent ISO of the CCD is ~50, but most cameras set the default ISO to 100 Therefore we need to double the full well capacity of the pixel to 2438 photons in order to maintain the minimum acceptable noise standard

11. (5)Digital Noise Cont’d High ISO – aka photon multiplier –Since fewer photons are counted to create a given luminance value, the S/N is also lower compared to a lower ISO setting

12. Digital Noise Conclusions Larger pixels enable more photons to be collected > higher S/N Shadows > lower S/N High ISO > lower S/N

13. Dynamic Range Definition: Maximum photon capacity / minimum (usable) photon count. Maximum photon capacity is limited by pixel size – small pixels collect fewer photons Minimum (usable) photon count is noise limited

14. (2)Dynamic Range Cont’d It is convenient to discuss dynamic range in terms of f-stops, i.e doubling the exposure >1 f-stop, or “zone”. Printed images are limited to ~ 5 1/2 zones A good monitor is limited to ~ 8 zones Real scenes may exceed 10 zones 8 bit/ color digital file can contain 8 zone exposure range (if the camera can deliver!)

15. (3)Dynamic Range Cont’d Pixel photon collection requirements –6.02n +1.8 @ pixel saturation n= digital bits /channel 8 bits required for full range jpg image, but a 10 bit A-D converter is used to “fill” the the shadows when the tonal correction is applied to linear data collected. Therefore n=10 in the equation above S/N min = 62 or 4100 photons @ saturation – vs. 2438@ saturation for acceptable noise

16. Minimum Pixel Size Current CCD (or CMOS) collect ~835 photons/ micrometer 2 –Minimum pixel area = 4.9 micrometer 2 –For square pixels: pitch = (4.9) 1/2 = 2.2 micrometer Cameras with pixels this small have barely enough dynamic range for a JPG photo and will require noise reduction for any ISO greater than 100

17. (4)Dynamic Range cont’d JPG is limited to 8 bits/channel but the actual dynamic range is less, depending on the A-D converter and the “noise floor” After A-D conversion, the signal from the linear CCD undergoes “tonal correction” to match our eye response :S out = K x (S in ) 1/Gamma Gamma ~ 2 K = 16 for 8bit S out

18. (5)Dynamic Range cont’d A-D converter & Tone conversion have a big effect on dynamic range –10 bit A-D after tone conversion allows only 5 stops, because the first non-zero level is 8. 256/8=32 = 2 5 –12 bit A-D allows 6 stops –14 bit A-D allows 7 stops

19. Tone Correction

20. Tone Correction from 8 Bit A-D Converter

21. Tone Correction from 10 Bit A-D Converter

22. Noise & Dynamic Range (before tone conversion) 3.4 micrometer pixel pitch

23. Noise & Dynamic Range (before tone conversion) 5.8 micrometer pixel pitch

24. Color Quality How accurately does the camera interpret the color in the image? –RBG filter cut-off –Bayer Interpolation accuracy –White Balance –“Over flow” artifacts –Tone Curve

25. Color Quality Cont’d RBG filter cut-off –Bayer filters well controlled –Foveon filter - technical challenge since the filter/sensors are “stacked” Bayer interpolation – How many surrounding pixels are “read” to determine RBG channel luminance? –Circle of confusion &/or diffraction means the image information spans several pixels. –Interpolation is computation intensive > camera computer is limited vs. PC

26. Color Quality Cont’d http://www.dpreview.com/learn/?/Glossary /Digital_Imaging/White_Balance_01.htmhttp://www.dpreview.com/learn/?/Glossary /Digital_Imaging/White_Balance_01.htm White Balance – How well does the camera “gauge” the luminance distribution –Camera software is remarkably ingenious, but not perfect and there may be significant differences between cameras manufacturers and across the spectrum of luminance distribution.

27. Color Quality Cont’d Overflow Artifacts –When a pixel is over-exposed, excess electrons may “overflow” to adjacent pixels causing color fringing, especially in high contrast areas –Inherent to CCD construction

28. Color Quality Cont’d Chromatic Aberrations –A function of lens design: red, blue and green light may not focus at same point in the image. –After interpolation, the effect is very similar to color fringing and most obvious in high contrast areas farthest from the lens axis

29. Color Quality Cont’d http://www.normankoren.com/digital_tonal ity.html#Raw_conversionhttp://www.normankoren.com/digital_tonal ity.html#Raw_conversion CCD is a linear device, but we “see” as a log function –CCD: luminance proportional to exposure –Visual: Log 10 luminance proportional to Log 10 exposure Tone Curve “converts” from CCD to visual

30. Power Source Batteries –Alkaline - not rechargeable, readily available –Ni-MH – Rechargeable & available to replace alkaline –NI-MH – custom form factor unique to camera –Lithium Ion – custom form factor unique to camera

31. Illuminator Electronic Flash is universal –Fixed location (entry level) –Elevated operating position –improved red eye –Hot shoe – to accept more powerful, more flexible flash ( articulated, power modulated)

32. Focus Auto focus is standard on all but the most basic (fixed focus) cameras Most cameras have auxiliary lamp to aid focus operation in low light ambient More expensive cameras offer multiple focus programs (macro, portrait, landscape) + manual focus.

33. Exposure Control Digital Camera exposure latitude is far less than print film (and about equal to slide film). Auto exposure adjust is universal –More expensive cameras offer aperture dominant + shutter speed dominant exposure modes –More expensive cameras offer multiple “scene averaging” programs

34. Aiming Devices LCD (fixed or articulated) Auxiliary optics (point & shoot) Parallax error ~80% true field of view Electronic view finder (LCD + lens) –Accurate field of view >98% –Relatively low resolution, both color and detail –Supplementary data (histograms, shooting data)

35. Aiming Devices Cont’d Reflex (SLR) –High end cameras with multiple lens options –Accurate field of view –Excellent color and detail –LCD view mode not available until “after the shot”

36. Data Storage Fixed in-camera – Entry level Removable flash memory –Type 1, Type 2 compact flash (CF) cards –SD, XD & Memory stick (all smaller than CF cards)

37. Summary Camera Properties –Ergonomics- Does it fit your hand/eye? –Controls- Intuitive vs. Learning Required Adjustments for special needs Features adequate for intended use –Image Quality vs. compact size trade-off Pocket size >> photo captured –Output Format: +JPG High quality? +TIF? +RAW?

38. Summary Cont’d Big Sensor + Big lens + computer power > Highest image quality –Big camera, Big $ Small Sensor + Small lens + limited computer power > Pocket size convenience, Adequate image quality, –Less $, –Less control over camera output

39. Summary Continued Do research camera choices: –Phil Askey: extensive image quality review –Steve’s Digicam: less extensive image quality, but concise summary with comparison to other camera options. Many more cameras reviewed –Google: “Camera model” + “review” –Specific camera forums – users own them, reveal the warts along with the good