Is there a way to use a step wedge for testing the tonal gradation of film? If so, how is this done? Do I put a (transparent) step wedge on a light table and photograph it?

Thank you, /Peter

-- Peter Olsson (peter.olsson@lulebo.se), January 03, 2001

Answers

I contact print the step wedge on 4x5 film under the enlarger. They also make step wedges that will fit 120 and 35mm. You can photograph a step wedge on a light table but if your plan is to then read in on a densitometer make sure that the image is large enough on the film. I have my test results on my web site from testing using the contact method.

-- Jeff White (jeff@jeffsphotos.com), January 03, 2001.

Thank you Jeff. Are the results from enlarger light the same as those from flash/daylight? I mean, does it matter what kind of light source that is used for the contact printing?

-- Peter Olsson (peter.olsson@lulebo.se), January 03, 2001.

Yes the results are the same. I use a 4/10's of a second exposure to avoid any reciposity. I have used the system with a cold light and now use it with a dichroic head with the same results.

-- Jeff White (jeff@jeffsphotos.com), January 03, 2001.

The light source should make absolutely no difference to the result.
However, if you photograph the step wedge, then the flare factor of the camera and lens will affect the results quite noticeably, reducing the contrast range of the step wedge from its specification.
For instance, if you use a light box, then the amount of white light surrounding the step tablet will alter the contrast of the image on film. You'll get a truer representation by masking off all but the area of the step wedge itself.

If the object is to find the transfer function of the camera/lens/film combination, then photograph the step tablet. If you want to only test the film, then you should contact print, using a diffuse light source for exposure.
(Photographic density is always given as diffuse density, unless stated otherwise. Using a collimated source, such as an enlarger, for the exposure may give a misleading result.)

-- Pete Andrews (p.l.andrews@bham.ac.uk), January 03, 2001.

I remember Kachel mentioning (maybe in Darkroom Techniques) that color of light does influence an emulsions response. A traditional emulsion gains contrast when exposed with strong red light, (as when exposed through a 25 red filter), which is what I found in my tests and, if memory is serving me right, new technology films lose contrast. Might be worth keeping in mind if you're exposing under a color head. Good luck. DJ

-- N Dhananjay (ndhanu@umich.edu), January 03, 2001.