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5222 - one stop push, gamma value and calculation


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#1 tom meyer

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Posted 24 October 2010 - 12:37 AM

Hello all

I will be processing some 5222 at a somewhat sketchy lab (no way around that).

I'm planning to test their processing extensively. At least that will give me a
some insurance in terms of the final outcome. As part of these tests, I'll push
process the stock by one stop. I'll be drawing the HD curve myself and from that
I'll calculate the resultant gamma.

What I need at this point is some information on two key points:

1)
If nominal gamma is "0.65", what is the gamma for one stop push?
I've had some trouble getting an authoritative answer on this. Some sources
mention "0.75". Is that accurate, and if so, what would the gamma be for a
two-stop push?

2)
How should I go about calculating gamma? I.e., should I use the "straight-line",
or the "average gradient" method? Some sources state that the latter is to be
used for "modern emulsions" - and 5222 is definitely not modern. For that matter,
what method is Kodak referring to (in the context of 5222), when they state gamma
values?


Thanks for your help.

Tom
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#2 Brian Pritchard

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Posted 24 October 2010 - 04:01 AM

I believe that you should be calculating Overall Contrast for B/W negatives. Average Gradient is used for colour negatives.
Overall contrast is the slope of the straight line portion between density point 0.10 above minimium density and the point corresponding to Log E1.50 of the exposure scale. If you are using a step wedge with 0.15 increments then that is 10 steps up the curve.

If you are push processing you need to calculate the change in speed. It is not usual to measure push processing by change of contrast. The curve should move two steps to the left (if you are using a 0.15 step wedge). To be strictly accurate you should calculate the Exposure Rating : 100(1-Log E) where E is representing the exposure at a point of the density curve 0.20 above the D Min. It would be sufficient to see how far this point has moved to calculate the change in speed. The contrast you get will depend on the particular developer mix the lab is using.
Brian
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