Digital cameras are linear devices, so their internal electronics must be set up apply a gamma compensation curve in order to obtain an acceptable impage on-screen. Does the industry have a standard e.g. 2.2 as mid-way between PC and MAC? How about the white point?

I have a Dimage EX1500 and would dearly like to know the answer so that I can apply the proper correction in Photoshop to match my working colour space (BruceRGB). I've tried Minolta technical support, but haven't been able to get through to anyone who userstands the issue.

Ian Lancaster

-- Ian Lancaster (ilancas@ibm.net), July 30, 1999

Answers

I can't answer your question, sorry, but you should be aware in your investigations that unfortunately 'gamma' has two different meanings.

In a photographic sense, it refers to the slope of the characteristic curve (ie the input-output function), assuming it is a straight line.

In an image processing sense, it refers to the shape of that curve. Straight lines have this gamma=0. The desired value for this gamma will depend on your output device (MAC, PC, whatever).

When I download from my flat-bed scanner, I can twiddle both these settings. I would hope that you can do the same from your Dimage, but I don't know.

-- Alan Gibson (Alan.Gibson@technologist.com), July 30, 1999.

Gamma is only one parameter that, taken together with others, defines the complete characteristic curve. (S-shaped curves of this sort are commonly defined by at least three parameters.) Thus, characteristic curves of the same gamma are not necessarily identical. Similarly, digicams with the same gamma would not necessarily produce the same tonal range for a given image.

With digicams such as the EX1500, where there is no control over aperture and speed, the camera's meter will want to expose so that the overall scene is medium tone. This means that a correction in Photoshop will be required, the direction and magnitude of which will be a function of the tonalities in the original scene. For example, I recently photographed a Seconic lightmeter against a dark red felt background. The EX1500 images resulted in medium red backgrounds that had to be corrected in Photoshop. Thus, regardless of the EX1500's gamma, I had to adjust the gamma afterwards. (Incidentally, to give the EX1500 credit, I turned off the camera's on-board flash and used a Lowel halogen light. The camera's meter recorded a very accurate medium red tone, and the automatic white balance worked just fine.)

Finally, on another place on this site I have criticized Minolta Technical Support (an oxymoron) as being less than satisfactory.

-- Albert J. Klee (aklee@fuse.net), July 30, 1999.

FWIW, I've heard that the digicams are all gravitating to the "sRGB" color space popularized by Microsoft and HP. I *think* this uses a gamma of 2.5, which sounds very high: I haven't noticed colors and tone being as far out of whack on my Mac monitor as a gamma value of 2.5 would lead you to expect...

-- Dave Etchells (detchells@imaging-resource.com), August 05, 1999.