calculating development time

[chrisf]

I've done my film tests for BTZS, put the readings on a graph paper with the SBR numbers. But I can't find the section in the book with the method for calculating the development times. Can someone direct me to the formula?

thanks,
chris

 

[gainer]

You know I'm sure, that SBR is scene brightness range. If you can measure the SBR of a scene you want to photograph, you simply look up the time you developed your test strip to get that SBR number.

Using an ordinary exposure meter or one built into your camera, you aim the meter at the shadow you want to preserve, note the f-stop number, aim it at the highlight and note that number. The difference between the two f-stop numbers is one form of SBR. Let's say you measured f/5.6 in the shadow and f/11 in the highlight. That difference of 5.4 stops can be multiplied by 0.303 to get the logarithm of the SBR = 1.64. The actual brightness range is the antilog of 1.64 which you can find on most pocket calculators and any slide rule to be 43.3. It's been a long time since I looked at BTZS, but I know the principles. You must develop the film so that its translation of the brightness range you measured from the scene will fit the density range of the printing paper. Depending on what form you used to label the SBR axis of your plot, you can use the f-stop difference or the logarithm (1.64 in this case) or the actual brightness, 43.3. My guess is that it's one of the logarithmic numbers, f-stop difference or the logarithm thereof.

Hope I helped.

 

[chrisf]

Yes,

Sometimes the easiest things are right in front and you can't see them until you step back.

thanks,
chris

 

[nworth]

In the fourth edition of Davis' book, it's on pp 54-55 and is listed in the index. The method assumes that you have done full film tests with various development times in your chosen developer and that you have derived the SBR values for each curve. There is no formula, per se. You just replot the SBR vs the development time from your film test curves, and connect the points with a smooth curve.

 

[Paul Verizzo]

any slide rule

What's that?

 

[gainer]

What's that?

The technical name for it is "slip stick". My 10" Log-Log-duplex slip stick was stolen. I replaced it with an 8" slip disk, equivalent to a 25" slip stick.

 

[Stephen Benskin]

Chris,

Way back on Dec. 1, 2007 in this forum, I uploaded an article titled, "What is Normal?" It might be what you are looking for.

Steve

 

[Stephen Benskin]

To make things easier, I've taken the liberty to upload the article to this thread. The basic formula to calculate the aim CI for development is:

LER/SLR - Flare = CI

CI = Contrast Index
LER = Log Exposure Range of the paper (can also be considered the aim negative density range)
SLR = Subject Luminance Range

Whatever time gives you the required CI is your development time. Of course, finding the right variables isn't as easy as you might think. Flare is the monkey wrench in photography that makes it impossible to make any definitive statement about almost anything from exposure to processing. It is also covered in the attached article.

There needs to be a small correction to the paper. On the last page the sentence "In other words, it’s not only acceptable, but preferable to have a slightly contrastier negative for contrasty scenes and a slightly flatter negative for flatter scenes.", should read. "In other words, it’s not only acceptable, but preferable to have a slightly flatter negative for contrasty scenes and a slightly contrastier negative for flatter scenes."