View Full Version : Defining one's personal ASA for a film

Robert Ruderman
14-May-1998, 21:29
I've been reading Ansel Adams' book "The Negative" in which he explains how to d etermine one's personal film speed given one's equpiment and developing techniqu e. His described process calls for a densitometer. I have to admit that I am lac king access to a densitometer, so I am wondering what other ways are there to de termine one's personal film speed for Zone system work/applications.

Any suggestions on other resources I should look at to determine film speed?

Thanks, Robert

Sergio Ortega
15-May-1998, 12:56
Robert, Your last question first.

In addition to Adams' wonderful book, The Negative, look at Fred Picker's book, The Zone VI Workshop. Picker's discussion of the Zone System, and his instruc tions for determining one's personal film speed, are excellent and very well ill ustrated. Steve Simmons' View Camera book also has good, sensible information on determining one's film speed, and the subsequent use of the Zone System. (B TW, why does everyone hate Fred Picker so much?) If you're only interested in determining your personal film speed, ONCE, to expo se and produce a Zone I density (approximately between .08 and .12 above film b ase fog), you might call your local photo lab, professional photography studio, or any nearby college photo schools/departments. Most reasonably well-equipped labs have a transmission densitometer available, and can quickly read your test negatives to determine densities. Just be sure you have them take a reading fr om the clear portion of the film area as well, to determine film base fog densit y.

Now for the long-winded stuff...forgive me, for I do tend to run on:

Kodak sells a very useful device: Product # 1523380, 1-A 11-step uncalibrated st ep tablet. Calumet sells it for $54.95. I've used it for years, to zero-in my densitometer dial needle to a predetermined density reading, whenever I turn on the unit. These densitometer units tend to "drift" over time, so it's best to e stablish a known reference point every time you use one.

This Kodak step tablet is simply an 11-step, sequential series of carefully expo sed (in a sensitometer) and developed negatives, on Kodak film stock. The 11 se quential negative densities range from 0.05 density (step 1) to 3.05 density (st ep 11), in perfectly spaced 0.30 density increments. (Kodak also sells a 21-ste p tablet, with more tightly-spaced density steps. It's more expensive.) The fir st five steps of the 11-step step tablet, densities 0.05, 0.35, 0.65, 0.95 and 1 .25, would be most useful for your B&W film tests, since they correspond to seve ral important Zone densities.

I have found it quite easy, and reasonably accurate, to place the step tablet on a light table and match it to test negatives by eye. It's surprising how sensi tive your eye is to slight differences in densities. Also, if you have a spotme ter, you can use it to make comparison readings between the known step tablet de nsity and the negative to you wish to measure. While this does not replace a de nsitometer reading, it can be a reasonably accurate substitute.

If you want to continue your Zone System testing, this same step tablet can also be used to measure and compare other zones, in a similar fashion, to determine your development temperature/agitation/developer dilution/times for the higher Z one densities. For example, if the density range of a typical negative would be from about 0.80 density to 1.25 density, with a middle Zone V density of aproxi mately 0.65, the step tablet has accurate density examples of 0.65 and 1.25 for comparison.

I hope this helps. I also hope this discussion does not elicit the usual Zone S ystem controversies. If you have any specific questions, please e-mail me direc tly.

Good luck, Sergio.

Sergio Ortega
15-May-1998, 13:05
Correction: sentence in 2nd to last paragraph should read: "the density range o f a typical negative will be about 0.80 to 1.25 , depending on the type of enlar ger used to print it". Sorry!

Alan Gibson
22-May-1998, 20:10
Or make your own densitometer, with a spot meter and a close-up lens. Put the ne gative on a ligh-box, and directly meter the parts of the negative. Each stop on the meter is a density shift of 0.3.

John Lehman
22-May-1998, 22:00
A practical rather than scientific way is:

Print half of a clear negative (and half no hegative) at various exposures to de termine the minimu exposure with that film base + fog density to produce maximu m black (where the part thru the negative matches the part with no negative).

Do trial exposures at 1/3 stop intervals for zone I (4 stops less than medium gr ay). For example, with a film which is nominally ISO100 try zone I exposures ranging from an EI of 25 to 100 at 1/3 stop intervals.

Contact print each negative next to a clear (unexposed but developed) negative. The EI at which you just notice a difference between the pure black from the clear negative and the test negative is the correct EI to pr oduce correct shadows