I dont have a densitometer (too expensive). I do have a 31 step wedge from which I can take readings on prints by means of a Umax 1200s flatbed scanner and the density readings provided by Photoshop. Is there a method of correlating these readings with conventional densitometric readings? I was led to consider using the computer after reading John P Schaeffer's Basic Techniques of Photography where he advocates a complicated technique for establishing densitometric values from prints using rollfilm exposed at different exposure values, a reference print and matching other prints at different exposures by means of the darkroom timer. He provides a formula for converting the variations in time to exposure values. The drawback to his systme is it requires exact eye matching of prints and use of an entire film each time. My method (if feasible) would work exactly like a densitometer reading and would require only one frame exposure through a density wedge stuck to the first frame of the film. Anybody got any ideas for translating the 255 step scale of a computer density reading into sensitometric form. Or is the computer reading too crude?

Thanks in advance

-- Brendan Kenny (Brendan@woodpath.freeserve.co.uk), February 18, 2001

Answers

Brendan

Where do you live. Most cities have professional photo labs that have densitometers and most of those labs charge a very reasonable fee for having your negs read on the densitometer. I use to use a lab to read my negs all of the time before I bought a densitometer for myself. If you absolutely cannot find a solution write me back and I will read your negs for you. Kevin

-- Kevin Kolosky (kjkolosky@kjkolosky.com), February 18, 2001.

Each make and model of scanner will be different, and they'll probably vary from sample to sample as well. Most scanner software applies a gamma correction which will make the density scale non-linear. In addition, if you're very unlucky, the software will also apply an automatic correction for the brightness range of the scan, rendering the results useless.
If you calibrate your individual scanner carefully to a known density wedge, and ensure that the software doesn't make adjustments that you can't control, then it's perfectly possible to get approximate reflective density readings, but there won't be much resolution in the shadows areas. Typically, a 0.3 density change in the dark region only causes a 3 to 6 level change in pixel value.

-- Pete Andrews (p.l.andrews@bham.ac.uk), February 19, 2001.