Hi. This may be a stupid question, but I know very little about the specs of film scanners. From time to time I see people comment on a scanner's dmax rating. What is this and what are good values? Is a higher or lower number better and how will it affect picture quality? Thanks!

-- Brad Grant (bradandsteph@home.com), June 19, 2000

Answers

Dmax, in photographic terms, stands for maximum density. Scanner manufacturers tend to use it interchangeably with Dynamic range, although they're not strictly the same thing. Density is a measurement of the degree of blackness, or opacity of a film, and is expressed as a logarithmic value. The Dmax of an unexposed, developed slide film, would be typically about 3.8.

The Density range, which is what's really important, and more closely relates to Dynamic range, can be from about 0.1 (clear film) to approaching 3.8 in the deepest shadows, but this all depends on the type of film, the subject matter, how much optical flare there is in the camera and lens, and a few other factors in reality. Since the values are logarithms we can just subtract one value from the other and this gives us a dynamic range of about 3.7 (just over 5000 to 1) The closer a scanners dynamic range can get to this value will be a good indication of how much shadow detail it can get from a slide film. Many scanner manufacturers make claims for a Dynamic range of 3.6 (4095 to 1), which is still a bit short of what the most contrasty slide films can hold. However, most users find that this figure isn't borne out in practice. A more realistic figure would be 3.2 to 3.4. You may find that a more honest makers scanner with a claimed 3.2 Dmax outperforms one with a claimed Dmax of 3.6 since the dynamic range depends where the "white-point" is set. Setting the white-point at 0.3D, for instance, falsely increases the dynamic range by 0.2.

-- Pete Andrews (p.l.andrews@bham.ac.uk), June 20, 2000.

Thanks a lot, Pete, for that incredibly detailed answer! You hit all my questions and then some. Thanks again.

-- Brad Grant (bradandsteph@home.com), June 20, 2000.

After reading these answers on this forum, then I was wondering if anyone can tell me what scanner on the market that they would suggest that I buy. I'm looking in the 1,500 price range, and was considering the Coolscan or Polaroid. The Coolscan supposedly has a dynamic range of 3.6, and the Sprintscan has a 3.4 range. Any suggestions?

Ryan

-- Ryan Fox (rfox9@hotmail.com), June 21, 2000.