BTZS Expodev question - am I being thick?

[Andrew_4548]

I've been using the expodev software for a while on and off and get some great results but keep coming back to a puzzle.

I understand for the filter factors, you enter, for example, "8" for a 3-stop filter, i.e. the number of times the light is reduced. That's OK for colour temp. filters etc but what about when you're using a red filter to bump up the contrast of a sky? Do you need to alter your zone placements in line with the filter used or just stick to the placements as seen and use the basic factor?

I haven't used B&W contrast filters much as most of my pictures have been inside so this is slightly new territory with landscapes...

Thanks

[Shen45]

Enter the factor as 8 as you have indicated. ExpoDev will take care of the adjustment needed. You have already entered the high and low exposure readings [incident or zone] prior to the compensation section of the software [filters,bellows, reciprocity etc].

Steve

[Andrew_4548]

But does putting a red filter on alter the exposure gradient rather than just the exposure time?

[Andrew_4548]

But does putting a red filter on alter the exposure gradient rather than just the exposure time?

[Doremus Scudder]

"Does putting a red filter on alter the exposure gradient rather than just the exposure time?"

I believe it does. My experience indicates that both sensitivity and contrast can be affected with strong filters. This probably has to do with the relative sensitivity/activity of the sensitizing dyes in the emulsion, etc. For example, I find that with Tri-X, a #25 red filter changes effective E.I. by 2/3 stop more than my through-the-filter reading, and increases contrast. Other films and colors respond differently.

Note that I do not use filter factors to compensate, but take a meter reading through the filter, adding the meter's spectral response to the mix.

However, I believe the sensitivity/contrast issue to be film-specific. The best way to find out for your combination is to do a few tests. I would do a development/exposure ring-around with your strongest filters to see what the differences were from the expected results and develop a set of "fudge factors" for the most affected combinations (e.g. my "fudge factor" for Tri-X/#25 red is "+2/3-stop and N-1. I don't know if this is programmable into the BTZS software, maybe it is...)

Hope this helps.

Doremus Scudder

[clay harmon]

If you are using the modified BTZS approach and using a spot meter (in other words, classic zone placement) you can meter through the filter to see what the relative luminance values are between different parts of your scene. If you have one of the Zone VI modified spot meters, they are supposedly calibrated and filtered internally in a way that makes this even more accurate. But even with any other spot meter, this exercise will give you an indication to the degree with which the filters mess around with your scene luminance values.

If you are using the stronger filters such red or dark orange, realize that they will really affect the shadow luminance, as in smack it in the head. Shadow areas are illuminated by a giant blue soft box called the sky, and the light is very cold and will be disproportionately affected by strong red or orange filtration. In short, it is easy to get underexposed shadow areas when using a dark red filter. Those dark Anselesque skies come at a cost.

[Andrew_4548]

Thanks for the info on taking readings through the filter - I'll give it a try as we're supposed to be going out and so far there's a blue sky and white clouds...