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Why Does Reversal Have Less Latitude than Negative?


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#1 grant mcphee

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Posted 05 March 2008 - 12:15 PM

Hello,

I've always taken it for granted but I don't know the exact reason why reversal has less latitude than negative. I assume it's got something to do with one being an additive and one a subtractive.

I spent about half an hour having a look but could not really find anything in-depth.

If anyone know or has a link to a good site it would be appreciated.

Thanks.
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#2 Dirk DeJonghe

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Posted 05 March 2008 - 12:35 PM

The main reason is that negative has a much lower contrast (gamma) than reversal. Reversal is projection contrast and negative is low contrast, the final projection contrast comes from printing on positive stock or digital or telecine transfer.
So for a given change in exposure the negative will become less dense. During grading for print or digital, the correct zone of interest will be selected and this will be what is visible on the final output.

On reversal, you have to fit the zone of interest during the shooting with very little room for error. Even with a correctly exposed reversal original, it will be hard to keep bright highlights and deep shadows in balance.

In the old days there used to be a low contrast reversal ECO 7252 that was a beautiful low contrast reversal stock with very low speed (EI 25 at 3200K).
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#3 grant mcphee

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Posted 05 March 2008 - 12:50 PM

Thanks Dirk. That makes sense.
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#4 David Mullen ASC

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Posted 05 March 2008 - 03:08 PM

Thanks Dirk. That makes sense.


Yes, just remember that reversal's main purpose traditionally has been for direct projection of the original film after development, so the contrast is deliberately very high so that black reproduces as black when the image is projected with a bright light onto a screen.

Negative's traditional purpose, on the other hand, has been for duplication onto another piece of film, so its contrast is very low to minimize contrast build-up in duplication. The print stock adds the necessary contrast so that black reproduces dark enough on a screen.
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#5 Anthony Schilling

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Posted 05 March 2008 - 03:22 PM

Well, it depends on what you are comparing. Negative film is designed to be low contrast because the raw negative is useless for viewing, you have to print it or transfer to digital in which you have a build up of contrast. When you print or transfer a reversal film you still get the build up of contrast, which results in more contrast build up than if you started with a negative. Ideally, if you were to compare a properly exposed reversal camera original projected, the contrast would be in the same ball park as a positive printed negative projected. But one thing to consider is the technology of the most modern reversal is 10 years old now, and the newer negatives have made advances in latatude during that time.

Sorry, I wasn't trying to repeat David's answer, he got in there while I type slower.

Edited by Anthony Schilling, 05 March 2008 - 03:24 PM.

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#6 John Sprung

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Posted 05 March 2008 - 03:26 PM

It's the same for black and white, so additive and subtractive have nothing to do with it.

Negative has a lot of extra headroom. Underexpose it, and you're just making clear plastic. Overexpose it, and it keeps on getting denser and denser. In printing or telecine, you can push more photons through it and pull detail out of that density.

With reversal, you don't have that option. It's the processing, not the raw stock, that determines whether the image is positive or negative. Either stock can be processed either way. They make the raw stock for negative with a gamma of about 0.65, and for reversal with a gamma of about 1.4, so processing negative as reversal would make a very flat image, while processing reversal as negative would be high contrast.

In negative processing, you develop the exposed grains and fix them, and the unexposed grains are washed away.

In reversal processing, the exposed negative image is washed out of the film, then the remaining grains are exposed, developed, and fixed. Unlike printing from a negative, you don't have the option to choose how that second step that makes the positive image is done. It's a sort of built-in one-light.



-- J.S.
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#7 Matthew Buick

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Posted 05 March 2008 - 05:02 PM

I was so close to creating such a thread, seems I don't have to any more.

Thanks everyone, more knowledge to store for another time.
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#8 Charles MacDonald

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Posted 05 March 2008 - 09:03 PM

JUst to amplify, the negative can be printed up or down, once the reversal is processed you are stuck with what you have. This probaly gives the negative an extra stop of latitude, as the film will be tweeked in printing or transfer. Once reversal is processed, if you are too light or dark, there is not a subsequent step.

the much higher contrast is also acting like a fence arround the latitude as exposure that might lewve detail on the negative is swamped out in max black or max white in reversal
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#9 Dirk DeJonghe

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Posted 06 March 2008 - 02:06 AM

Just to continue on the same lines:

-regular video is like reversal,
-negative one-light log scanned digital is like negative,
-telecine scan from negative is halfway in that you can do grading adjustments before committing to video, the resulting video no longer has the latitude of the original negative.

I have a very convincing demo on my Baselight I show to visitors. It is a digital 2K Log scan from a Super 16 negative.

It starts as a very bright white image with an even brighter circle on it. Waw, a nice sun shot is the reaction.
I then start to pull down the brightness and it ends up as a dark moonshot with all detail visible on a pitch black night sky. A range of 80 printer points.
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#10 Dominic Case

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Posted 07 March 2008 - 01:35 AM

And one more contribution - the way I see it.

Negative has low gamma which means that for a given increase in exposure there is only a small increase in density. And that density can go on building up quite a long way before you run out of silver at d-max. So, lots of exposure range. The contrast is built up when you make the print. The print covers about the same exposure range as a reversal original would - but you have been able to select that range from the greater range that the negative has captured, by choosing the right printer light.

Reversal has a fixed starting point (the d-max). No exposure, maximum amount of silver (or dye). As you increase the exposure, you have less and less density in the processed result, until you run out of density entirely and you get d-min or clear film. That's a serious limit! But because the gamma is higher, to give a projectable image, you get from d-max to d-min quicker (i.e. less exposure range).
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#11 F Bulgarelli

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Posted 08 March 2008 - 03:07 PM

Yes, just remember that reversal's main purpose traditionally has been for direct projection of the original film after development, so the contrast is deliberately very high so that black reproduces as black when the image is projected with a bright light onto a screen.

Negative's traditional purpose, on the other hand, has been for duplication onto another piece of film, so its contrast is very low to minimize contrast build-up in duplication. The print stock adds the necessary contrast so that black reproduces dark enough on a screen.


David,

Why is there an increase in contrast during duplication.

Thanks
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#12 Anthony Schilling

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Posted 08 March 2008 - 05:11 PM

-regular video is like reversal,

Reversal does not blow out and burn out nearly as bad as video.
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#13 Dirk DeJonghe

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Posted 09 March 2008 - 02:20 AM

David,

Why is there an increase in contrast during duplication.

Thanks


The contrast of the print stock is built-in, it is designed to give a pleasing look when prints from a negative are made.
In the duplication process you multiply the gamma of each stock used to get the final gamma.

Negative stocks have a gamma of about 0.55
Positive stock for projection have a gamma of about 2.70.
Intermediate stocks have exactly gamma 1.00.

If you print a negative to direct positive you get a gamma of 0.55 x 2.70 = 1.5 (rounded)
If you print negative to interpositive to intermediate negative to positive you get a gamma of 0.55 x 1.00 x 1.00 x 2.70 = 1.5 (rounded)

Gamma 1.5 was found to give a pleasing image when projected taking into acount losses in projection lenses, flare etc.

Dominic and David, I am doing this from memory so some of the gamma values may be off by a few hundreds but is shows the point.
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