Use of Step Wedges

[ragnar58]

Here is what I’ve done.
Using a 31-step wedge with my normal paper at the Grade 2 filtration, I see the first distinct non-black as Step 23 (2.27D) and the first non-white as Step 12.5 (12 was not enough but 13 was too much; average at 1.19D). This results in a density/exposure range of 1.08. This is in line with the published 0.95 to 1.15 range for Grade 2 that I’ve seen.

Then I contact printed the step wedge on strips of film. Things easily seen: shift in speed point (0.10d above fb+F) with development changes, changes with base fog with longer development, and small shifts in the upper zones with minor changes in the agitation technique.

I’ve plotted the results on MS Excel and have used the Kodak method to determine the Contrast Index values. Although, my CI values are quite different from the Kodak CI/development times values (mine are longer to achieve the same CI).

Now for my questions.

If each step is approximately 0.10 density and 7 stops is considered a “normal” exposure range and a change of 0.30 is 1 stop; then the normal exposure range should be 21 steps. But this method does not account for the effects of flare in the camera and enlarger (lower contrast), would 6-1/3 stops (19 steps on the wedge) be a better target range? Should the target highlight value be determined by the density 19 (or 21) steps from the 0.10d above fb+F step? Oddly, this approach appears to be more in line with the published times.

It appears the objective is to find a development time that gets a Grade 2 density range with 6-1/3 stops on the film test strip. This would seem to be the definition of normal exposure and development.

If one wishes to determine the development time for a 1-stop push, would the best technique be based on a Grade 2 range in 5-1/3 stops or a Grade 1 range in 6-1/3 stops? Both techniques require a longer development time.

I’ve run casual tests before but I’m trying to be more “scientific” this time for my own peace of mind. I purposely omitted the actual film and developer to keep the topic focused on the test practice and not the merits of a particular film or developer.

 

[ic-racer]

But this method does not account for the effects of flare in the camera and enlarger .

Long thread on that subject: (there was a url link here which no longer exists)

My opinion is that one can define 'speed' in a manner that does or does not show the effects of flare.

Having said that I read the word "FILTRATION" in your post. Are you using a multigrade system? If so, then your development for 'grade 2' should satisfy most all conditions. That is, no need for N+ or N- development and no need for an exact, specific contrast index for all your films. That is the advantage of MG printing.

In practical terms, once you have your exposure index (which should be independent of development) and a contrast index that matches grade 2, you are all set.

 

[Stephen Benskin]

You're close with your numbers. A log 2.2 luminance range (7 1/3 stops) is considered statistically normal. And a flare factor of 1 to 1 1/3 stops is normal for flare. This is what, at least Kodak, uses to define their normal processing aims. By assuming a 7 stop luminance range with a 3/4 stop flare factor is giving you the same results as the 7 1/3 stop luminance range and a one stop flare factor. In any case, the log exposure range at the film plane should be around 1.80 for normal.

An additional factor to consider is that flare tends to vary depending on the luminance range. It is higher for larger luminance ranges and lower for lower luminance ranges. On the average the difference is 1/3 stop per stop change in luminance range.

 

[Stephen Benskin]

Forgot to give the equation dealing with all the variables.

Aim CI = Desired Negative Density range / Log Subject Luminance Range - Flare

example:

1.05 / 2.2 - 0.34 = 0.56