Outline: Metering ! What makes metering hard? ! the meter doesnt - PDF document

Outline: Metering ! • What makes metering hard? ! – the meter doesn’t know what you’re looking at ! The Art, Science and Algorithms • histograms ! – Rule of thumb ! of Photography ! • metering technologies ! • metering modes (center, evaluative,...) shooting modes (Av, Tv, P , M) ! Exposure & Metering ! • exposure compensation ! • background topics ! CSCI 4900/6900 ! – Ansel Adams’ zone system ! Maria Hybinette ! – Zone system and digital photography ! • Quiz ! 1 ! 2 ! Quiz: Metering ! What Makes Metering Hard? ! 1. On Auto: you metered black on black (BB), white on white • light meters are not magical they don’t know (WW) and grey on grey (GG): How did the images differ? ! 2. For black subject on white background (WSWB), Which what you are looking at & what you want. ! metering from (1) worked best? ! • so they assume the scene is “middle” gray 3. For a white subject on a black background (WSBB) Which ( 18% reflective) [12%-14%] ! metering from (1) worked best? ! 4. Assume you had reading available from (BB), (WW) and (GG) • the world is full of hard metering problems... ! but now your meter is broken: ! a) For a black subject on a white background (BSWB) how would you expose your image? [you can still adjust shutter and aperture) ! b) How about a white subject black background? ! 5. Suppose you did not have a metered reading for (GG) but only for BB and WW, how would you now expose BSWB and WSBB? ! 3 ! 4 ! A!"#$%%B)+),)//-63+@';.C: !"#$%%&&&'(!)*+,)-*(.(',)/%+012.+)#%234!5647879'!+/: 5 ! 6 ! !"#$%%&&&'.((.6502;343+02,0/.*0',)/%<979%9=%4.>648.?#)(-*.8@)-8&06+8#0*+833'!+/22:

Another Problem: Contrast ! Real World Dynamic Range ! • The range of illumination levels that we • Foreground too dark ! encounter is 10 to 12 orders of magnitudes ! • Sky is too bright ! – Eye can adapt from ~10 -6 to 10 6 cd/m 2 ! – Often 100,000 levels in a scene ! • Negatives/sensors record 2-3 orders of magnitude ! • Contrast: ! • How do we center this window? Exposure problem? (today we will focus on this) ! – measure of the difference in brightness between light and dark areas in a scene. ! • Here the contrast is too great between the brightest bright and darkest dark: ! – High Dynamic Range. ! !"#$%%12)4'*D'6.+%<99E%9F%#*);-,3648!;*83/04.(%: 7 ! 8 ! Metering Modes ! Shooting Modes ! • Aperture priority (Av, A) ! • Centered average ! • Spot ! – Photographer sets aperture (so depth of field) an ! – Camera sets shutter ! • Smart metering ! • Shutter priority (Tv, S) ! – Learn from database of images (Nikon 30,000 images) , – Photographer sets shutter (so motion blur) ! decision process may depend on: ! – Camera sets aperture ! • How much brightness from different “areas” of grey? ! • Program (P) ! • Local contrast, where is the subject, foreground, background ! – Camera sets both, some trade-off options ! – Nikon 3D matrix (multiple segments) ! • Manual (M) ! – Canon evaluative ! – Photographer sets both (must meter) ! • Incident (hand held light meter) ! – Histogram may help ! – Measure incoming light ! !"#$%%&&&',0/1*3;4.36,)2)-*',)/%+-+)*302(%!3(+)4*0/(7'!+/: !"#$%%&&&'2-/36)-(8206;(,0#.',)/%+-+)*302(%-6;.*(+06;3648(.*3.(%-6;.*(+06;3648!3(+)4*0/('(!+/2: 9 ! 10 ! Histograms (simplified for now) ! Idea ! <G9: 11 ! 12 !

White on White: High Key ! What the Camera Did ! • Underexposed originally left (made it grey) ! • Corrected on the right ! 13 ! 14 ! High Key ! Low Key ! • Rule of thumb: ! – End up with underexposed images ! • No blown (or clipped) channels or highlights ! • More forgiving ! – Higher signal to noise ! 15 ! 16 ! Low Key ! • Prone to overexposure ! • Details cannot be recovered. ! • Overexposed: clipped/blown channels ! 17 ! 18 !

Contrast ! Quiz 2 ! • Narrow range low contrast e.g., (fog no texture on the left) ! • Broad range, high contras e.g., (texture water with a range of tones). ! !"#$%%&&&';0/3.6(@/)6;(',)/'0-%+-+H!3(+)4*0/'!+/2: 19 ! 20 ! Quiz 2: Answer ! Impossibilities ! E:I+)#(: !"#$%%&&&';0/3.6(@/)6;(',)/'0-%+-+H!3(+)4*0/'!+/2: 21 ! 22 ! What to do? ! Bad Histograms? ! • Fill flash ! • Graduated Neutral Density Filter ! • Take multiple exposures (and blend) ! • But a better camera (film) or a Nikon D3X ! • Go home ! 23 ! 24 !

How much latitude in Camera? ! Film Exposures ! • Make sure you are exposing the important areas within the film latitude ! • Nine stops (each stops is double in brightness from its neighbor) contrast range, plus all black and all white ! – Underexposing darkest will fall off ! – (12 stops Hasselblad, 10 stops Nikon D3X) ! – Overexposing you get flat areas without contrast • Seven stop contrast range, plus all black and all (no texture) ! white ! • Five stop contrast range, plus all black and all white ! !"#$%%&&&'B)*#!)+)4*0#!@',)/%!)&8+)(%J)6.%J)6.7'!+/2: 25 ! 26 ! Film ! Film Exposure ! • Best to expose for the shadows, to the left ! • Exposure controls shadows ! • Development time controls the highlights ! – Also: the higher the contrast, and the greater the difference between black and white. ! • Rule of thumb, ideal exposure is to place all the tones of the object except all black, just within the films threshold to the left. ! – Allows for smallest aperture ! – Not too dark ! – Fine grain, ! 27 ! 28 ! Digital Exposure ! Zone System ! • roughly 1 f/stop per zone ! • Expose to the left: Danger Noise! ! • X = “maximum white of the paper base” ! • IX = “slight tonality, but no texture: flat • Digital cameras have disproportional fewer darker snow in sunlight” ! • VIII = “textured snow, lightest wood at tones than we see with our eyes (lots of bright tones) ! right” ..... [digital exposure] ! • Full tonal range of scene: ! • VII = light/pale skin tones ! • V = 18% gray card, sky, light foliage ! • III textured shadows [film exposure] ! • II first hint of texture ! • I stuck with these cant contract or expand after exposure. ! • 0 = “maximum black that photographic paper can produce” ! • lesson for the digital age ! !"#$%%&&&',0/1*3;4.36,)2)-*',)/%+-+)*302(%;343+028.?#)(-*.8+.,!63K-.('!+/: 29 ! 30 !

Left to talk about ! Exposure/Metering ! • Gamma ! – S/N ! • Bits / RAW and dynamic range ! • HDR (beginning) ! Next up: ! • Lenses, focal lengths, distortion and DOF ! • Camera hacking ! 31 ! 32 ! Quiz: Metering ! 1. On Auto: you metered black on black (BB), white on white (WW) and grey on grey (GG): How did the images differ? ! 2. For black subject on white background (WSWB), Which metering from (1) worked best? ! 3. For a white subject on a black background (WSBB) Which metering from (1) worked best? ! 4. Assume you had reading available from (BB), (WW) and (GG) but now your meter is broke ! a) For a black subject on a white background (BSWB) how would you expose your image? [you can still adjust shutter and aperture) ! b) How about a white subject black background? ! 5. Suppose you did not have a metered reading for (GG) but only for BB and WW, how would you now expose BSWB and WSBB? ! 33 ! 34 ! Slide Credits/Resources ! • Prof. Fredo Durand ! • Prof. Marc Levoy ! • London, Stone, Upton Textbook ! • Wikipedia ! • http://www.luminous-landscape.com/tutorials/expose-right.shtml ! • http://www.cambridgeincolour.com/tutorials ! • http://www.bythom.com/graycards.htm ! – ANSI standard is 12% gray (1/2 stop less than 18% gray on Kodak Gray Cards). ! – http://david.spielman.com/Gray_Card/ ANSI_PH3_49_1971.PDF ! 35 !