cgardner
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 Re: Strobist | |
UK: So say my image is \"underexposed\". What is your definition of optimally exposed? Gray card image in print matching actual gray card when laid side-by-side?
In technical terms on a digital sensor optimally exposure occurs then the full reflective range of solid objects hit by the illuminant record digital values between 1 and 254 with 0 reproducing voids and 255 specular reflections.
On a digital camera 18% grey card with a digital value of X will fall somewhere within the range of 0-255 when exposed about 2.5 - 3 f/stops below clipping. That is to as if one where to overexpose the 18% card to the point of clipping and bracketed to make it darker, took those files and made prints of them, then laid out the prints and compared with the actual card, one of those prints of the card would reflect the same 18% of light when measured with a densitometer. The card and print would have densitometer reading of log(1/.18) = .745
On a Canon camera when a white object is exposed accurately, with detail, the large spike created by an 18% gray card will not fall exactly in the middle of the histogram representing the sensor range. Thus if you define \"correct\" exposure as one that centers an 18% card on the histogram that is an incorrect assumption.
The 18% value dates back at into the 1920\'s or 30\'s. Apparently even Kodak doesn\'t know when it first started selling them. Years later when there was a better understanding of human perception it was determined with testing that 12% was a more accurate \"perceptual\" mid point in the overall tonal range. That is to say of you put a white and black card in front of a subject and asked them to pick a the gray tone that was in the exact middle most would pick 12-13% not 18% which perceptually appears a bit too bright. The ANSI standard used for meters and cameras adopted 12% as the \"middle\".
Photographers were not aware of this until digital cameras put histograms on the playback. Thinking that a centered 18% card spike was \"correct\" exposure they found doing that would blow the highlights. Why? The camera sensor response is engineer to put 12% in the middle of the range. That is why boys and girls my white towel exposed just below clipping IN FLAT LIGHTING will result in the spike from the card winds up to the right of center. 18% is lighter in tone that the 12% the sensor will render in the middle.
This is a typical exposure confirmation shot — not rendering a gray card 18% on my print — but as a check to see I\'m getting the highlights and shadow end point exposed correctly...
The card spike winds up right of center on the histogram because the center of the histogram is based on 12% and my 18% card reflects more light than that. Had I used a WhiBal gray card or other more recent products designed specifically for digital exposure the spike would have fallen in the middle of the histogram in that same shot. Why? Those cards are manufactured to reflect 12% not 18%.
This has confused photographers, even pros, for years.
The reason the Kodak card has remained at 18% over the years is, according to legend, because Ansel Adams badgered Kodak to keep it that way. Kodak which participated in the research leading to the new 12% perceptual based midpoint value was at some point considering changing the value on the card from 18% to 12%. Adams by then had created the cult of Zone V = 18%. Adams according to anecdotal accounts when to Rochester and lobbied Kodak and convince it that changing the 18% card would cause confusion when those reading his Zone System books based on 18% cards tried to use it with new 12% cards. Ironically the Kodak card staying at 18% but meters and camera sensor switching the 12% mid point of the response curve / histogram is causing today\'s generation of digital camera users exactly the confusion Adams feared because they assume \"correct\" exposure is when the card is center in the histogram. It is, if you use a 12% card like WhiBal but not if you still use an 18% card as shown in the shot above. If I center the 18% card on my camera histogram the resulting exposure blows the highlights.
Why don\'t I buy an 12% card? Because I don\'t set exposure per the card. It\'s just used for setting Custom WB and then in the next step of my workflow as a convenient way to use the towels I do use to set expose so scene overall matches sensor. After achieving that I have the subject hold the card for a post processing baseline shot so I can verify that that exposure is in fact correct on both ends on the towels and rest of the foreground. The color chart is for evaluating camera styles and guiding the tweeking of color selectively without changing WB or tint globally. If you don\'t understand the difference or why you\'d want to do the things I do you really don\'t understand the process well enough to challenge it. The best way to do that and convince me to do things differently is SHOW ME A BETTER WAY by posting photographs you\'ve taken illustrating your methods.
Adams used 18% as his perceptual mid-point for reasons he explains in his book \"The Negative\". The zone system as originally conceived consisted of ten zones 0 - 9, not including the base white of the print because in Adams words, \"A gray scale of 10 steps seems to be most convenient.\" In later editions Adams assigned Zone 10 to specular highlights making it an 11-step scale from max black on print/black void in scene to pure white paper base/specular highlight of scene.
In the next paragraph he wrote: \"The term \'middle-gray\' as I use it is more of an emotional value than a quantitative physical value.\" Further in the same paragraph he explains, \"A surface of surface of average tonality — such as the middle step of tone of a full range glossy print — would have an average reflectance of about 18% and relates to Zone V of the exposure scale. Of course the actual brightness of this gray tone would depend on the intensity of the light falling upon it. But we are concerned here with the proportionate value of tone. \'Middle Gray\' represents the geometrical mean of the extremes of useful values.\"
Source: Ansel Adams, The Negative, 1968 Edition — Fifth Printing 1971, page 16.
Adams was gilding the lily there. In practical terms he used18% because the 18% Kodak cards where the only reliable and widely available process control process tool available for exposure at the time. That is to say you could take your Weston meter, point it at the card in the same light as the scene, and the middle tone values in the resulting print would wind up similar to those seen by eye. So the card compared to the scene was a perceptual baseline, and the exposure was set to the card so the mid-tones on the print matched what was seen by eye in the scene. All of then wound up on what called \"Zone V\" in flat lighting. But as Adams say the the rendering of the card would depend on whether it was in sun or shade.
Also to put Adams use of 18% and Zone V as a benchmark it is necessary to understand how in his system the overall tonal range — shadow and highlight values on the print — where controlled and matched to the scene by development of the negative to fit the scene-to-print. There was no \"middle Levels\" slider with the zone system or any film based capture. The best we could do AT CAPTURE is try to record detail in shadows and highlights at the same time. As Adams said what happens in the middle is this .... \"But we are concerned here with the proportionate value of tone. \'Middle Gray\' represents the geometrical mean of the extremes of useful values.\"
Again Adams is gilding the lilly. A more straight forward way to describe the Zone System is this:
Expose so the negative has density in Zone 1 shadows above voids (Zone 1). Develop your film so specular highlights in the scene (Zone 10) wind up reproduced by just the paper base and a solid white object like the hood of a car you see the specular highlight on (Zone 9) is the lightest gray tone of silver on the print you can discern the specular highlights against. Where an 18% scene value (Gray Card) will wind up in the print will depend entirely on how it is lit. If hit by sun it will, on a B&W print, wind up looking to a card seen in sunlight. If the card is held half in sun and half in shadow the sunny part of the card will be rendered similarly per comparison of actual 18% card on but the shaded parts of the card will not. They will be rendered darker with the tone in the shadow from the sun is controlled by the amount of fill they receive, outdoors from the indirect skylight or indoors from fill hitting from directions different from the \"key\" source creating the shadow.
If you did the zone system correctly and made a proof sheet on #2 print paper exposed so the clear film was maximum black on the print you would get results that looked like this:
The reason I would make proof sheets very systematically on #2 paper exposed to render film base = max. black was because that was the baseline in the Zone System for judging if the results were \"correct\" or not. When a proof sheet is exposed in that manner any frames which were underexposed would be seen to have no detail in the shadows. On inspection you would find no density in the shadows in those areas — the film didn\'t get enough light there to record and image above the \"fog\" left behind after the undeveloped silver crystals were dissolved by the fixer. If the shadow detail was fine but the highlights on the print were blown out it meant the film had been developed too long for that scene contrast, which in that artificially lit shot was controlled with the OVERLAP of key on top of fill coming from behind the camera, per the advice of the Kodak \"How to Shoot Portraits\" book I was using as my guide in that my first every artificially lit portrait session. If on inspection the highlights of the proof sheet were gray, not white, it was an indication that the film had not been developed long enough to build up density in the highlight areas on the negative. By the time the shadows were completely black on the print too much light would pass in the highlights over exposing them on the print which made the highlights gray.
The significant thing to note above is that the appearance of the midtones wasn\'t a criteria for a correctly executed workflow with B&W negative film. What matters was: 1) enough exposure for shadow detail, controlled with aperture/shutter, and 2) the negative range, controlled with development with B&W, matched the paper.
How the midtones are rendered is a function of the lighting ratio used. From that \"fit range to print\" starting baseline seen on the proof sheet a B&W practitioner would look at it and if they decided the clothing should be darker or shadows a bit lighter on the final print they would dodge and burn with paddles and cards with holes in them to give areas of the print less or more net exposure than the proof sheet. The manipulation of the midtones was done in post processing.
As today with digital some would argue the not dodging and burning was a \"purer\" form of the art of photography and what the camera captured should be the \"benchmark\" of correctness in technique. Adams didn\'t subscribe to that philosophy. He would dodge and burn very systematically taking notes. Once he had made a print to his satisfaction he would take that print and mark it with a tissue overlay with instructions tell his assistants where and how much to dodge and burn. That was the more holistic goal behind Adams \"System\" approach, to allow him to predictably and easily get to the baseline proof stage on a proof print, and from there create a blueprint of how to alter the image from that \"fit scene to sensor baseline\" to the final print he envisioned with dodging and burning.
I found Adams method matched my goals of spending as little time in the darkroom so I could spend more time capturing images. The proof sheet above is evidence it worked for my even 40 years ago because any of the frames on that proof sheet would make a very nice print with minimal effort because range fits #2 paper and the lighting ratio rendered the girl next door in a flattering way with correctly exposed highlights and flattering light shadow tone on the face with detail in her dark hair. The lighter clothing blends into the background and the dark hair acts as a frame for the face. Apart from refining the facial angles there\'s not much I\'d change if shooting it today.
Did I understand completely what I was doing back then? No. But I got that far, that early by reading books by Kodak, Adams, and many others and following their advice. Along the learning curve I\'ve come to understand the things suggested, like the 3:1 ratio used for those portraits worked. 3:1 happened to be what fit the overall scene range to the #2 print. I was shooting in my parents livingroom (I had just graduated high school) and there was a good bit of extra \"spill fill\" bouncing off the 8\' ceiling contributing to the fill. Everything in the middle fell into place by itself and would up looking \"perceptually\" correct. Would an 18% card if used in that shot reflect 18% in the printed image. Don\'t know and don\'t care any more than I do with digital.
In that shot I set ratio by distance because I was using two 150W bulbs in aluminum clip-on shop lights attached to step ladders as stands as my lighting with the \"key\" light closer than the centered fill, at distances suggested by Kodak. One of the reasons Kodak suggested that approach was that in those days all a hobbyist could afford was tungsten lighting. That was still the case back in 2002 when I started giving lighting advice and suggesting speedlights. It wasn\'t to convince people that my speedlight approach was better than Hobby\'s or anyone else but to try to convince them not to waste money on a set of 500W shop lights on stands from Home Depot and to buy a pair of the Vivitar flashes I was using at the time instead.
40+ years ago I learned to fit scene overall to print and adjust the appearance in the middle with the lighting ratio. The reason I advocate using flash outdoors is because without it you can\'t control the lighting ratio. Sky fill is three stops darker on a clear day, an 8:1 incident ratio. The more overcast the sky the lower the ratio becomes until on an overcast day the contrast is even less than a 2:1. There is direction that models shape but not very strongly.
The lighting on the face in highlight and shadow wound up as it did in that first B&W session of mine because I exposed and developed for a full range of tone and used the 3:1 lighting ratio. Had I instead opted to use 1:4 lighting ratio by making the key light brighter it would not change the exposure in the the fill only shadows. If I used the same development time as before the highlights wouldn\'t fit the print because I had pegged exposure to the shadows, and the range of the negative, developed for a 3:1 scene no longer fit the range of the #2 paper. This would be very obvious to me from my standard baseline #2 proof sheet because the shadows would be fine but the highlights would be blown out with just the paper base showing. If I had bracketed exposure there would be frames that looks fine in the highlights, but would have a loss of detail in the shadows because the negative was clear.
The equivalent of \"fixing it in Photoshop\" would be to use a different grade paper. By making a proof sheet the same way for max. black on clear border on #1, #3, or #4 paper or by changing paper contrast with colored filters I could flnd the paper which would render the highlights correctly.
Some like RDKirk who I\'ve known on the forums for years understand my approach because they understand its zone system roots of fitting range to sensor then manipulating what that produces in the middle tones with a combination lighting and post processing.
Indoors it\'s as simple as finding the ratio of key / fill that will put detail in white and black towels at the same time...
then tweeking the middle slider if that \"sensor full range baseline ratio\" isn\'t producing the desired look on the face..
Using rim light changes perception of the lighting and shifts the Zone 9 / 245 from the front of the towel and face to the back hit by the sun. There is also the option to leave the front on Zone 9 / 245 and just blow the highlights on the back to Zone 10 specular / 255. Or even blow the highlights on the front to 255, but I don\'t choose to do that.
By using examples I already have on line, such as the HSS shots, or putting targets I don\'t use for what I am measuring when I took the shot is apparently confusing UK and others. But the big disconnect is different definitions of \"correct\" exposure. Like Adams my definition for correct exposure has always been that it starts with getting the end points correct, not reproducing a gray card accurately. If you can get past that notion and understand that\'s not my goal you should understand my approach.
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