W-Speed in action

[ic-racer]

A while back I used fresh name brand film to calibrate my sensitometer.

Today I had a chance to use it to test a partial roll of bulk 35mm film that I received. The film is about 25 feet of 35mm Delta 400 of unknown age.

Test revealed a base density of 0.66
The ISO speed based on W criteria was about 160.

Why do this? Saves me from shooting a series of zone I tests. I know from experience that with my equipment my exposure index is usually one-half the ISO. So by doing a single test strip (12" of film) I now know that if I rate this film between 80 and 100 I should get adequate exposure.

Why use W-crieteria? Because, unlike the ISO test, W-criteria is not based on a specified gamma. This saves me from having to do a family of curves just to find the ISO curve.

Why not use Delta-X criteria? Because in my hands and with my programming skill in Excel, it was easier to implement the W-criteria equation.

Isn't there a lot of math involved? Yes, but once the spreadsheet is programmed, I just plug in the density values and it spits out all the rest.

 

[nworth]

I'm not acquainted with the W specification. There is, of course, a reason for developing to a specific gamma: film speed varies a bit with development.

 

[Stephen Benskin]

I salute you for your achivement with w-speed. While w speed has just as good of a correlation with the fractional gradient method as the Delta-X method, it was never implemented. Whereas the Delta-X is part of the current ISO criteria and I therefore I suggest it is a more pertinent method.

 

[ic-racer]

I'm not acquainted with the W specification. There is, of course, a reason for developing to a specific gamma: film speed varies a bit with development.

The results of the ISO test vary with degree development, thats why ISO specify the required gamma. And that is why Jones, et al based the film speed on a fraction of the gamma. The ISO method, Delta-X and W are all approximations of the Jones' 0.3G method. The ISO method is the only one of those 3 that requires the film be processed to a specific gamma to make the measurement.

 

[ic-racer]

In terms of the computing involved, the W method involves obtaining only a single point that intersects the H&D curve (the W point).( I came up with a step-wise two-point interpolation to get the W point in my spreadsheet.) A computer-driven Delta-X method would involve finding two points on the H&D curve. Once to get the 0.1 intercept and a second time to get a point on your H&D curve that is 20 times greater than the result of the 0.1 intercept point.