Wednesday, 18 February 2009

What is Correct Exposure Part 1

his is an oft repeated question by newbies to photography. An oft repeated reply by people who have “seen the light” is that the “correct” exposure for a scene is what you want it to be.

Huh?

We often point to the exposure meter in the camera (classic film SLRs) or histograms in the DSLRs. and we tend to say, you must try not to “burn” highlists (overexposure) by avoiding histograms peaks pushed flush against the right edge. And you must not push histogram peaks against the left edge otherwise you get inky dark shadows that have no detail (underexposure). Heck, even some photo competitions state one of the rules is that you must not submit a shot where the Photoshop eyedropper goes 0 or 255.

Well, that’s generally true. Except that often, you can’t avoid one or the other or both. That’s because, regardless of the medium – negative film > transparency film > 24x36mm sensor DSLR > APS-C DSLR > Four Thirds DSLR > small sensor compact, the natural outdoors or indoors harsh scene will have a Scene Brightness that exceeds the Dynamic Range handling ability of the camera. I was musing on that when I compared my photos, with technically correct histograms against someone else’s photos where there was so much “pop” in them. Having a technically correct histogram does not make the photo visually interesting.

That’s because the histogram is only a two dimensional report of scene and subject brightness (well, maybe two and a half because you can have separate R G B histograms as well). The histogram does not tell you:

  1. which part(s) of the scene is causing the spike(s)
  2. about what curves, shapes and visual geometry make the subject interesting
  3. whether the face or interesting part of the scene is “correctly” exposed – can you see the detail, texture of of the face and so on.

In a recent article, The Online Photographer describes and discusses Dynamic Range and even Local Contrast. It’s a good read.

So, what is Correct Exposure?

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