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The two biggest variables in exposure and contrast are scene lighting and content. Here's an example from a high speed flash test I did.
As a baseline I started with a test target facing the sun, ambient only:
The criteria for exposure was keeping the white towel (i.e. textured highlights) below clipping. Here's the same target with the same light hitting from the other direction, exposed exactly the same way: to retain the highlght detail.
Both are exposed the same way and "technically correct" in the sense of preserving highlight detail, but look radically different "perceptually" due to the way the content is presented in the photo. To a great extent what we perceive is driven not by what the eyes record, but rather by what the brain expects. Your brain know the towel should be white and if you've ever used a gray card you have a reasonable expectation of what it should look like. The first image where those "reference tones" are prominent and well exposed create the impression the entire scene is well exposed when in fact the shadows cast on the card in the first shot are similar to those on the shaded side of the card in the second.
The basic cause and effect of photography is using contrast patterns to attract the eye and create the illusion of 3D. The angle of the dominant "key" light creates the highlights and shadows which the brain uses to intuit shape, and the amount of detail in the shadows created by the fill source (whatever it is) provide additional clues about shape and texture. Over the history of photography its practitioners learned how to cope with scene contrast and use that contrast to their advantage.
The earliest photographers used orthochromatic films that could be developed under red safe lights. That allowed them to develop the negatives by eye to fit any scene to the range of the print paper. Adams, who started using ortho film, quantified that "seat of the pants / eyeball" method into the Zone System which allowed doing the same the same thing --adjust development of the negative to fit the paper -- in the total darkness needed for developing panchromatic films. The other method used for B&W was to change paper grades to fit the range of the negative would would vary due to scene range and development time.
Either way photographers of the B&W era didn't need to be concern much with film vs scene range because the underlying technology took care of matching both. Get the two end points - max black from clear areas of the neg, and pure white by matching neg. range to the paper range -- and all the tones in between fell normally both technically and perceptually because most of the photographic response curve is linear. In simpler terms if you shot a gray scale on B&W film it could be reproduced to look exactly the same.
Color negative changed the photographic paradigm two ways. The range of color print paper is shorter than that of B&W (5 f/stops vs 10-12) and color negatives have little latitude for processing variation because all the color recording layers must stay in sync to record neutral tones accurately (i.e. gray balance).
The movie industry had long been faced with the problem of keeping contrast consistent on the negatives over a wide range of scene contrast and pioneered the use of artificial lighting to alter scene range. The move to color in movies, with its much shorter dynamic range, is one of the reasons movie making moved in-doors: it was easier and cheaper to build an outdoor set indoors and control the lighting contrast with artificial lighting than it was trying to battle the contrast of natural light outdoors with "fill" light.
Still photographers found the direct fresnel sources they had used for portraits with B&W film where too harsh on the short range, more contrasty color film. Middle gray (18% reflectance) is 4 stops from the highlights on B&W film but only 2 stops from the highlights in color. That meant rethinking lighting ratios, which are based on f/stop differences between the highlight and shadow sides of a face [ i.e. 2:1 means 2x more light (1 f/stop) more light reflects from the highlights]. The change in contrast of the film lead to the use of more diffuse light sources such as dishes and umbrellas.
Changes in printing reproduction methods, namely the use of drum scanners for color separation, created another paradigm shift to transparencies. From a strictly photographic perspective color negative is better in many ways because it has greater range and latitude for exposure variation. But scanning from a transparency resulted in much better resolution because it eliminated the variable of optical resolution. I know this first-hand because back in the mid-1970s I worked at National Geographic making halftones and color separations manually using reflective / camera techniques and later worked at and managed operations which used first generation analog and digital scanners. Advertising is the economic engine that drives publishing and prior to the Internet color magazines were primary form of advertising using still photography. So the shift to transparencies wasn't made because it was a better way to capture photographic images but because it was a better way to reproduce them.
Back in the days of transparencies a collection of color correcting filters was to photography what a chef's knives are to cuisine: the mark of a professional photographer was his filter case and knowledge in using them to get the image right in the camera. Metering of both exposure and color temperature needed to be spot on because all the stuff now done in Photoshop was done manually. If there were 300 photos in a catalog they all needed to be as identical technically out of the camera. Just as the shift to B&W negative lead to changes in studio lighting the shift to transparencies with an even shorter dynamic range spawn a new generation of larger, more diffuse light modifiers.
Color catalog pages with multiple images were created on a light table by duplicating the original transparency in an enlarger to the size needed on the page on a special type of duplicate transparency film which allowed the image layers to be peeled up from the base. A layout technician working on a light table would strip each image from the dupe and re-paste it in position on a page-sized sheet of mylar. That saved time and money at the scanning stage and make it easier to get all the images to match.
Meanwhile outdoors still photographers were faced with the same light with ranges of 8 - 12 or more stops of contrast. To cope with that contrast new strategies were developed. Photographers realizing that they couldn't capture detail everywhere started exposing "perceptually" to make whatever was most important, usually the faces, correctly exposed. "Damage Control", preventing the fact the highlights and shadows were being blow, was a matter of composing the photo so those areas were not important to the message of the photo. That's what the illustration I started with illustrates. The same lighting contrast will be perceived differently depending on what the brain expects to see. In the second wide shot the brain expects the background to look normal and had it been exposed that way the fact the towel was totally blown out wouldn't be noticed or thought to be odd.
The other strategy photographers learned to use outdoors is "fill" flash. The flash falls off with distance and can only correctly exposed highlights at one distance. So in most situations it can't change the contrast range of the entire scene. But perceptually the brain focuses more on the foreground and tunes out the background when doing so in real life, and the same thing will work in a photo outdoors:
Starting by keeping the white highlights below clipping with shutter speed (- 2 EC in Av mode) I then reached up, turned on my 580ex flash set to FEC = 0 and let the camera's evaluative flash metering sort things out.
The shot at FEC =0 actually looks a bit overfilled perceptually, but mostly because of the contrast with the dark background which is larger. The brain still wants that background to look normal also. The shot above where I measured only the card area histogram shows it reproduces the range of the card almost perfectly in the technical sense. If I were to take that wide shot and crop in tight on the card it would look "normal" as I expanded the crop the darker background would show and actually work in a positive way to frame and pull attention to the card. But at some point where the card occupies less area in the photo than the background, or is moved out of the center of the frame the perceptual focal point will become the background and the card will start to look overexposed RELATIVE TO THE BACKGROUND and the the background underexposed.
A perceptual quirk of fill flash is that when flash exactly matches the ambient light it looks fake. Why? Because in real life we perceive a difference between the sunny back side and the sky filled shady side. The difference between what the eyes see and the camera without flash records is due to the face the eyes adapt to whatever the brain tells them to focus on. So if looking at that scene in person when you focus on the shaded card or face in shadow the lighting doesn't change, the brain tells the pupils to dilate more so the detail can be seen. The background in person would be nearly as overexposed as in a photo taken for a correctly exposed face but the in person the brain just tunes it out so its not noticed.
So photographers making the transition from B&W to color neg to transparency learned from experience that flash works best outdoors in photos with prominent foreground detail.
They also learned flash can only reduce the foreground contrast when the camera shoots into the shadows. Hence the convention of putting the back of a subject to the sun in direct sunlight which allows the flash to lift the shadow side without overlapping and blowing any significant sunlit areas in the foreground.
When a single flash is added to the shaded foreground it really isn't fill. Fill by definition raises the shadows. What a single flash does outdoors is create a highlight pattern over the fill light from the sky that is already there:
Blur the shot and the highlight pattern created by the flash is easier to see:
The shadows are still illuminated only by the sky and in that shot by the light reflecting up off the light shirt.
Perceptually the single flash works well in a full face pose IF it is raised above the camera to create a natural highlight pattern on the face. Natural light comes from above and so should the "key" light, which is the role the flash is playing. The shadows are actually quite dark, but OK perceptually because the face is mostly in highlight and the shadows work well to frame and slim the face.
But in an oblique pose in the same lighting conditions more of the side of the face would show and trigger a perception the lighting is too harsh. Dark shadows = perception of hardness. Light shadows = perception of softness. So outdoors with oblique poses I employ the same dual flash short lighting strategy I do indoors using fill over the camera to lift the facial shadows to the tonal level I desire, and overlapping the key light to control the contrast on the face:
You'll notice how distracting the control target is in the shot above? Again that shows the role of perception. Black out the card and the perception of the face changes....
Because the face is now the brightest, most strongly contrasting area in the photo. In both examples the face is lit in a way that creates a highlight "mask" on the front of the face the brain will immediately recognize as a face..
In the case of the shot with the white shirt I picked the background of the sunlit river so the shirt wouldn't do what the card did on the dark background - distract. I understand the role of contrast and perception and use contrasting tone and focus it to lead the eye of the viewer where I want them to go in the photo, and more importantly keep their attention focused there...
The message here is that perception of the final image in the mind of the viewer, not capturing every scintilla of shadow detail is what should drive the creative process. Ansel Adams system was based on pre-visualization of the outcome and getting past the technical limitations of the recording process. I learned the zone system shortly after starting photography in the late 1960s so I carried the same "ethic" into shooting on color negative, transparencies, and digital. The technical processes have changed, but its still pre-visualization of what is possible holistically which changes scenes seen by eye and recorded "accurately" like this:
Into finished images like this:
Chuck
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