DMax and Dynamic Range.

A few weeks back I was asked some questions about particular scanners and their capabilities. One feature mentioned was Dynamic Range (DR) and the DMax specification. I'd promised I'd get back in short order with a post discussing DMax, DMin and the whole dynamic Range issue. This is a tough nut to crack mind you, therefore, I've decided, I would try to keep it simple.

DMax relates to the Maximum Readable Density or the maximum "opacity" achievable in a given medium.
DMin equates to the Minimum Density or a films maximum "transparency".

It has been suggested that the DMin value should reflect the density reading where the detail is still retained in the light areas of an image. Whereas DMax has been described as the darkest area of an image that a scanner can still discern detail from.

Back when I was pulling film for typesetting and printing, we would try to ensure our films were all checked by densitometer and that the darkest areas were kept around a DMax of 4.0.

The ideal range for scanners would be to be a DMin of 0 and a DMax of 4.0 giving a DR of 4. One thing to bear in mind, the capture of dynamic range varies greatly. Factors like sensor type, colour channel, sensor noise and bit conversion depth all play a part. View all manufacturer specifications, with a prove it to me mind set. Probably the best bit of advice I have to offer is, that you should look to third party reviews for information on how well a scanner performs.

One of the best descriptions of Dynamic Range, that I've come across, can be found at photo.net.