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I posted another question which sparked a bit of a discussion in the comments. Basically, in a high contrast (high dynamic range) picture, if the difference in distance from the camera to the different parts of the subject is negligible, then does a fill flash actually help bring out details in the darker areas of the subject?

The way I see it, if the difference in distance is negligible, then using a fill flash will add a uniform amount of light across the entire subject. If different parts of the subject are considerably but not extremely different in ambient illumination (say, something like 4-5 EV difference; as an example, in shade versus sunlit), this should help bring out details in the darker areas, since those are illuminated more by the flash relative to the ambient light level than are the highlights. But it's been argued in the comments that that's not the case, since the absolute difference in illumination remains the same with or without a fill flash (which would mean it's all about the exposure settings).

So, which is it? And why?

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An example to illustrate specifically what I am thinking about is the third picture from the top here. In that picture, I would have wanted to illuminate the dog's head without adding unnecessarily to the lighter portions, while the background is largely irrelevant (and could even have benefited from being even darker relative to the main subject, along the lines of the bottommost photo at that page). –  Michael Kjörling Jul 29 '11 at 6:39

2 Answers 2

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Michael, you are mixing two different things here. In the previous question we were discussing the effectiveness of using fill light to distinguish areas on the subject. Traditionally, a fill flash is used to bring a subject's brightness level to be less contrasty with the background, such that metering for the subject will not blow out the background, and metering for the surroundings will not underexpose the subject.

So, the answer to the question is yes, fill flash certainly helps bringing out details of a subject over a bright background.

EDIT: reading the comments below, I think I need to clarify the terms I use in this discussion. By referring to dark and bright areas on the subject I mean dark colors (i.e, black) or bright colors (i.e, white). Think Dalmatian. The argument in the previous question was that shedding light on the subject will not reduce the dynamic range of the light reflected from the different areas. Through the comments there one can see that a conclusion is that given multiple lights, then increasing one source's intensity can indeed reduce the DR.

In this question, the OP asks about the effectiveness of fill flash, and here, the problem is not the DR of the subject, but rather the whole scene.

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Flash is also commonly used to bring up the shadows of a contrasty object regardless of the background. This technique was especially prevalent in the film days when it was harder to pull up the shadows after a shot had been taken (nowadays there's a fill light slider in Lightroom that makes it very easy, back then you would have to try and dodge each shadow in the darkroom). –  Matt Grum Jul 28 '11 at 16:55
    
@Matt - see my comment in the other question. How does flash helps in bringing up dark areas of a subject (that is, w/o pulling the bright areas as well) if reflectance is assumed linear? –  ysap Jul 28 '11 at 17:07
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If the flash is on a different axis to the main (as is almost always the case), then the shadows will receive a different amount of light to the highlights, the shadows are shadows because they're pointing away from the main light, vice versa with the highlights. –  Matt Grum Jul 28 '11 at 18:40
    
@ysap Imagine a cube and its lit harshly from the top. The top is light and all the sides are dark. Now you take a flash and flash it from the side. The shadows on the side were reduced and the top was nearly untouched. Because the angle of light is different. (This is basically Matt Grum's example about the midday sun in his answer.) –  rfusca Jul 28 '11 at 18:50
    
@rfusca, Matt - OK, I see where the possible misunderstanding come from. When I say "dark areas" I mean "dark colored" (say, black or deep gray) as opposed to dark due to lack of lighting. I fully agree that lighting from the side helps bring out the parts that are dark due to lack of lighting. However, if the dark shaded areas have both dark and light colored surfaces, then the additional lighting will not brighten the dark spots w/o possibly overexposing the bright spots. –  ysap Jul 28 '11 at 20:10

Yes a fill flash can add detail to dark areas, provided the flash lighting angle is different to the main illumination angle.

If the main light and flash are co-incident, then the flash will have a multiplicitive effect on the object brightness and do nothing to tame the DR. However if your flash comes in from a different angle then it's possible for it to light up dark areas relatively more than it adds light to bright areas, reducing the DR of the scene, i.e your intuition is correct.

it's been argued in the comments that that's not the case, since the absolute difference in illumination remains the same with or without a fill flash

Dynamic range is defined as the relative difference between the darkest and lightest parts, not the absolute difference.

A good example of this in practice is the use of fill flash in midday shooting when there is no cloud cover. Here there is a large difference (up to 90 degrees) between the sun and flashgun, which enables the flash to fill in shadows without adding that much light to areas directly illuminated by the sun.

Flash is not the magic bullet for high dynamic range scenes, as even if your subject is within range of the flash, as flash will change the character of the lighting, especially if the main light is soft.

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