Friday, March 17, 2006

The Right Scale

In my previous posting I complained about the fact that Photoshop treats an image as a bucket of numbers.

Some people object that the only thing you can process with a computer is indeed numbers and that even if we use a graphical representation for the numbers, they are numbers all the same.

Indeed in the Zone System zones are numbers too, but my point is that there are numbers that are meaningful and numbers that are not, the Zone System numbers provide a piece of information that is relevant to the photographer, while the "Photoshop numbers" instead don't.

This depends on the scale that is used.

Photoshop uses a gamma adjusted representation for the pixel values. This is a very convenient representation when storing images on a 8-bits per channel format, since it provides a "perceptually adjusted" lossy compression scheme, but it is very bad for image processing instead.

It is bad for a number of reasons, first of all gamma adjusted images have a big handicap in the image processing side, images are captured by linear devices, like cameras and scanners, and processing them in a non linear color space produces several unwanted artifacts. This will be the subject of a future posting.

The badness reason I'd like to talk about right now is the way the photographer perceives the scale.

Historically people have been organizing quantities on scales. Big things seem to be distinguished from small and things need to be sorted out in convenient buckets.

Americans insist on using inches, yards, miles, ounces, quarts, gallons, etc. These units are not even directly commensurable with each other and to an european accustomed to the metric system it looks positively weird.

There is a reason for these units though: they set a scale and immediately make it possible for people to tell things apart. Roads are not measured in inches, that's it.

A "right scale" helps the thinking process and provides a concrete support for quick decision making.

The same is with images. Our eyes see things on a logarithmic scale, that is shades of gray seem to vary uniformly (perceptually) when they actually double in lightness intensity. This is similar to how hearing works too, we distinguish sounds in pitch or intensity on a similar logarithmic scale.

In Photography pretty much all quantities are organized in a logarithmic scale: f-stops, exposure times, exposure values, zones. The value of these scales is that they are well adapted to the way we perceive reality and they allow us to develop an intuition about them. With an intuition we can make decisions swiftly and precisely based on "gut feelings". This is how great pictures are always made.

The conclusion I'd like to make is that to adjust images conveniently any numeric value should be presented on a logarithmic scale, such as the Zone Mapper and the Histogram in LightZone.

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