How to correleate densitometer reading with f-stops?

[losheng]

Hi, guys,

I have been working with E6 processing for a while now, and I've been trying to calibrate push processing timings. I'm hoping to base the calibration on E6 control strip readings. But I have one question - how does the densitometer reading relate to f-stop? For example, when the speed step is, say -0.1 units off the aim point - is it roughly 1/3 stop, or some x number of stops, slow? In another words, if I were to push process the control strip by +1 stop, how much deviation am I expected to see in the speed step - would it be -0.3 units from the aim point?

I would most appreciate some help or pointers in interpreting densitometer units.

Thanks!
ls

 

[Chan Tran]

Hi, guys,

I have been working with E6 processing for a while now, and I've been trying to calibrate push processing timings. I'm hoping to base the calibration on E6 control strip readings. But I have one question - how does the densitometer reading relate to f-stop? For example, when the speed step is, say -0.1 units off the aim point - is it roughly 1/3 stop, or some x number of stops, slow? In another words, if I were to push process the control strip by +1 stop, how much deviation am I expected to see in the speed step - would it be -0.3 units from the aim point?

I would most appreciate some help or pointers in interpreting densitometer units.

Thanks!
ls

There are a lot of experts here but it seems that they didn't want to answer your question. If you increase exposure by 1 stop the density would be lower by a little bit more than 0.3 density. I don't know if you push 1 stop it would be the same or not as pushing and pulling affect the contrast.

 

[polyglot]

Densitometer readings are in log10. What that means is D = - log10 ( fraction of light passing through film ). For example if the transmission is 10%, D=1. For 1% transmission, D=2, etc.

Stops are log2 therefore you can convert D to stops by multiplying by log(10)/log(2) = 3.3219. So yes, an error of 0.1 is basically 1/3 of a stop... in DENSITY. Not in exposure, not in processing time. They're related but a 1/3 stop change in one will not cause that same change in the other two, it's all a bit non-linear, depends on the film type you're using, the contrast you're developing to and the starting density you're adjusting from.

Chromes are high contrast (>1) therefore achieving a 0.1D change in density generally requires less than 1/3 stop change in exposure. If it's at the toe (speed point), it differs though because of the toe shape. Negatives are usually low contrast (0.5 to 0.7), which means that a 0.1D change in density usually requires about a 2/3 stop exposure change.

So that doesn't get you anywhere. Since you're trying to correct a chemical process, you're adding a certain quantity of chemicals, e.g. a pH change to CD in order to cause a colour shift or a pH change (or processing time change) to FD in order to change contrast and/or speed. How much you need to add and how much you need to change your processing times is specified in the process documentation, which you absolutely must read if you're doing process control. Make sure you get the documentation from the appropriate manufacturer, i.e. Fuji or Kodak or Tetenal or whoever, as the suggested corrections differ slightly.

 

[losheng]

Hi, guys,

Thank you so much for the information. While I understand the dynamic curve changes with the processing time, I was trying to find out what's the objective measure for push processed slide films? Although Fujihunt chemicals come with recommended processing parameters, I've found that they are merely starting point, and they needed to be tuned for each processor. For example, the recommended First Developer time for normal processing is 6:00, but I've found that using the Phototherm SSK-8r, it is necessary to run it for 6:30 - otherwise film speeds will tend to be on the low side. Likewise, going with the official recommendation for push processing, it will result in films that look under developed.

I figure that there must be an official standard for E6 film speed, something that I can establish with the control strips. If anyone knows of such documentation, I would really appreciate the pointers.

ls

 

[polyglot]

I suspect you have to keep Newton's Method in mind. Try a bunch of variations on processing time (from normal development to twice the extension recommended for a 1-stop push), plot them, figure out the slope (of contrast and speed vs FD time) and then either interpolate or extrapolate to get the right answer. Then test again with a few points clustered around where you expect the right answer to be.

And then deal with the fact that 5+ sequential processes (assuming rotary or inversion) would oxidise the developer somewhat so maybe consider single-shot development for this testing to avoid that problem, but keep all the once-used developer to go back into your main stock for use on real films, because the single-shot won't have harmed it much.

 

[Bill Burk]

I figure that there must be an official standard for E6 film speed, something that I can establish with the control strips. If anyone knows of such documentation, I would really appreciate the pointers.

ls

Hi losheng,

I usually develop black and white negative, so I don't have the standards for E6 handy. But sure, control strips are useful and they would come with documentation of the proper readings. If you were going to "do-it-yourself" I don't know. Maybe shoot some test target shots, like Zone System users often do, and send one roll to a lab - process one roll yourself and check the differences...