Wednesday, October 31, 2012

Image Quality? Image Depth.

In the previous post I pointed you to Mike's remarks over on ToP, which, if you have not, you should still go read.

This may or may not be an accurate paraphrase, but it's what I think he said, and what I want to think about now:

Image Quality isn't a real thing, as an objective measure of goodness in any meaningful way. Photographs simply have properties, which are neither good nor bad, they simply are. There is no use to be gained by assigning subjective qualities to these properties, such as sharpness, dynamic range, contrast, saturation, and so on and so forth. These are certainly measurable things, but we should not use them to measure the quality of the image in any way.

This is a point I have harped on at some length, although not quite in these words. The image exists as it is, and these technical properties either serve and support the image, or they do not. If they serve and support the image, we could describe them as good, to be sure. However, the exactly opposite quality might serve and support a different image. Simply measuring something like sharpness yields no insight into the goodness of the image. Sharpness is neither good nor bad by itself, it is good or bad only in the context of a specific image.

So far, so good. This has all been said before. In fact, this next part has been said before as well, but perhaps not quite in this context.

Photographic properties come in three flavors, for my purpose here:

  • Properties which can be altered in post, but only "uni-directionally" for lack of a better word. I can remove sharpness, I can remove color, but it is difficult to add sharpness or color when it it not present.
  • Properties which can be altered "bi-directionally" in post. I can alter apparent contrast or saturation in either direction.
  • Properties which cannot be meaningfully altered in post. Things like the relationship of the various objects in the frame, the expression on the subject's face, the position of the key light.

Of course what I am saying here is not strictly true. With enough effort you can do pretty much anything to a photograph in post. Further, as I have noted, the line between what can and cannot be done in post is constantly moving (for example the light field camera allows us to focus in post). In broad strokes, though, there are these three families of properties.

What does make sense is to lump the first family of properties together into some sort of named bucket. It is a fact that a camera which produces higher resolution images is a more flexible tool than one which produces lower resolution images. Is the phrase "image quality" a good one to use here? I agree with Mike, essentially, that it is not. The phrase sounds too much like "good photograph" which is a completely different thing almost entirely concerned with the third family of photographic properties.

Perhaps we can measure things like sharpness, dynamic range, color gamut, and so on (in general, the things we can alter only in one direction in post, given the tools of today, whenever that is) into something called "Image Depth" or "Imaging Potential" or similar.

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