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How do I know which gamma my film has?


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#1 Alexandra Wesche

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Posted 15 November 2010 - 01:25 PM

Hello all,

I have no traditional film lab education and I'm trying to get my head around all things light, grading, sensitometry etc.
I have read a lot and I've experimented a bit, but there are still many questions and noone to ask...

Let's say I have an old b/w positive print and I want to transfer the contrast values as close as possible. How do I know which gamma value my original has and how can I measure the aim gamma of the dupe negative?

I hope somebody can shed some light... I'm also grateful for any literature references (possibly not too mathematical).

Thanks,
AW
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#2 Brian Pritchard

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Posted 15 November 2010 - 02:35 PM

Hi Alexandra
Unfortunately there is no way of knowing what the gamma of an old B/W Positive. You have to assume that it has a normal gamma of around 2.4 to 2.6.

A dupe negative is normally processed to a gamma of 0.73 because it is designed to be printed from a duplicating positive which should have a gamma of 1.4. This gives a reproduction gamma of 1.0 which will give you a duplicate negative that is a close reproduction of the original negative.

If you are making a duplicate negative from a print with a gamma of 2.4 then the duplicate negative should be processed to a gamma of around 0.4. Most labs I have worked in actually made negatives from positives at a gamma of 0.45. Duplicate negatives are always denser than original negatives as they are printed on the straight line portion of the curve.

Brian
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#3 Alexandra Wesche

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Posted 15 November 2010 - 04:14 PM

Thanks a lot! Can I control the aim gamma of the dupe negative with grey patches attached to the positive? If so how many are reasonable and what densities should they have?
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#4 Charles MacDonald

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Posted 15 November 2010 - 07:56 PM

The gamma is normaly calculated in the first place by plotting the results from a test exposure of a series of levels. For example by running a lab test strip.

The lab that is making the dupe negative will undoubtedly run a test making a negative of a step wedge sample to get their exposure and processing in the right range. IF the job is critical that might even do some tests from the original and tweak the process to get the results that they are looking for. (example restoration of a faded colour original)
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#5 Alexandra Wesche

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Posted 16 November 2010 - 05:38 AM

Unfortunately there are only daily wedge tests on the b/w negative process independent from the exposure of the film that is to be processed. The lab tests are pre-exposed by Kodak.

To get the actual gamma of the b/w negative that I want to expose and process I would have to attach a wedge to the positive and expose it with the same light as the picture right?

Where would I get such a wedge or would I have to produce it myself with the estimated gamma of the positive?

Please correct me if I'm on an altogether wrong path.
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#6 Dominic Case

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Posted 16 November 2010 - 06:37 AM

I'm also grateful for any literature references (possibly not too mathematical).


Alexandra - check Brian Pritchard's website - linked on his reply to you above - there is a richness of information on it.

My own won't be ready for a while yet (probably not until everyone has stopped using film;-( but in the meantime look for my book Film Technology in Post Production which is in print, or for more detailed (but not too mathematical I hope) stuff on sensitometry, gamma etc, my earlier book Motion Picture Film Processing - now out of print but you can often pick up a second hand copy on line.
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#7 Alexandra Wesche

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Posted 16 November 2010 - 12:50 PM

Thanks, I'm halfway through the site and Brian's book and I read your book, before I started working any practical work on film. Might be a good idea to reread it now...

I'll look into the other one.


Just out of interest... do you still use your method to copy tinted/toned films as opposed to the Noel Desmet method?
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#8 Brian Pritchard

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Posted 17 November 2010 - 03:38 AM

Alexandra
You will need a step wedge to attach to your positive film. I presume you have access to a densitometer.

Depending how quickly you need a wedge, if you contact me off-list I can send you one next time I visit the lab I work at occasionally.
Brian
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#9 Brian Pritchard

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Posted 17 November 2010 - 09:56 AM

I meant to say in my last reply that the SMPTE book, 'Control Techniques in Film Processing' gives all the information you need about control techniques.
I have two SMPTE books, the one I mentioned which deals only with B/W and 'Principles of Color Sensitometry' for colour. I believe that the first book is out of print but you might find a second-hand copy.

Brian
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#10 Alexandra Wesche

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Posted 17 November 2010 - 10:25 AM

I meant to say in my last reply that the SMPTE book, 'Control Techniques in Film Processing' gives all the information you need about control techniques.
I have two SMPTE books, the one I mentioned which deals only with B/W and 'Principles of Color Sensitometry' for colour. I believe that the first book is out of print but you might find a second-hand copy.

Brian



Thank you!
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