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Push Film Stop


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#1 Andy Yeomans

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Posted 29 December 2005 - 08:37 PM

I am fresh out of film school and the one thing I am unclear on that is really bothering me is the pushing of film stops. I tried using the search option but I came up with nothing, so sorry if this has been covered before. Okay, maybe two things...first, can someone explain quickly the stop ratio, ie. ND filter, correct exposure by 1/3 stop? If I am shooting at f5.6, what do I go to? I just don't get the stop ladder.

Second, If I am shooting on 500 speed film and I want to push it one stop in developing, do I meter and expose for the stop difference?

Thanks
Andy Yeomans
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#2 David Mullen ASC

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Posted 29 December 2005 - 08:50 PM

I'm not sure I understand all your questions...

Each f-stop is the equivalent of doubling or halving the amount of light coming through the lens. So if you open up from f/5.6 to f/4.0 -- i.e. by one stop -- you've doubled the amount of light reaching the film.

Now I'm not sure what you mean by "1/3 stop" but perhaps you are referring to the recommendation that you overexpose negative by 1/3 of a stop, i.e. rate it 1/3 of a stop slower. The reason why 1/3 as opposed to 1/2 or 1/4 is just that light meters input ASA ratings in one-third stop increments. For example, if you wanted to overexpose 500 ASA stock all the time by one full stop, you'd just rate it at 250 ASA on your meter and pretend it's a 250 ASA stock, thereby overexposing it by one stop. So between 250 and 500 ASA on a meter is 320 and then 400 ASA, one-third of a stop increments. So to rate a 500 ASA stock one-third of a stop slower, you'd set your meter to 400 ASA. It's simpler than trying to always remember to overexpose by 1/3 of a stop after you get your meter reading.

A push-processing, aka extended or forced development, causes an increase in density formed on the film as it is being processed. So a one-stop push increases the density as if you had overexposed by one stop. So if you rated the stock normally but asked for a one-stop push, the negative would come out one-stop denser-than-normal, as if you had overexposed by one stop.

However, most people push-process to compensate for underexposure, so that they can shoot in lower-light than the film is designed for, but still end up with a negative of normal density. So they will underexpose by one-stop; whereas if then they developed normally, they would have ended up with a negative that was thinner-than-normal, instead they push by one stop and end up back to a normal density on the negative (although with an increase in grain and contrast.)

Now pushing is not completely accurate or consistent, so it's safer to underexpose by slightly less than what you are pushing by to compensate, so at least you don't run the risk of getting a thinner (less dense) negative. For example, someone might rate a 500 ASA stock at 800 ASA rather than 1000 ASA when asking for a one-stop push. This gives them a little leeway in case the push falls slightly short of gaining you a full stop's worth of density.

So you'd set your meter to 800 ASA instead of 500 ASA (if that's the normal rating), thus underexposing everything by 2/3's of a stop, and then ask for a one-stop push, ending up with a negative that is maybe 1/3 of a stop overly-dense, or maybe closer to normal. Now maybe through testing, you discover that the lab is pretty accurate about getting you exactly one more stop of extra density when you push by one stop, so you could rate a 500 ASA by 1000 ASA and thus be underexposing by exactly one stop before you push it to compensate.

Edited by David Mullen, 29 December 2005 - 09:14 PM.

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#3 Andy Yeomans

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Posted 29 December 2005 - 09:10 PM

My questions were a little unclear, but your answer is perfect.

Thanks

Andy Yeomans
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#4 Greg Gross

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Posted 02 January 2006 - 08:47 AM

With my professional still (digital Canon) cameras,I can choose to bracket in 1/3 or 1/2 increments
for example. Of course I have the advantage of immediately reviewing the image and making the
choice of a finely tuned exposure if so desired.

Greg Gross

Edited by pd170user, 02 January 2006 - 08:56 AM.

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#5 Tony Brown

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Posted 03 January 2006 - 03:40 PM

So glad I never knew about film school........
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#6 Andy Yeomans

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Posted 05 January 2006 - 09:27 PM

Too bad I did...at least the one I went to.
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#7 John Pytlak RIP

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Posted 05 January 2006 - 10:54 PM

The normally accepted full stop increments are:

1.0
1.4
2.0
2.8
4.0
5.6
8.0
11
16
22
32

Do you see any pattern?

Each full stop value is equal to the square root of 2 times the previous value (rounded to two significant figures).

A stop is calculated by dividing the diameter of the effective lens opening (iris) by the effective focal length of the lens. So if you have a 50mm lens, that has an effective optical iris opening of 25mm, it is f/2.0. If you close the opening to only 12.5mm, it is f/4.0. An iris of 25mm has four times the area of an iris of 12.5mm, and so lets four times the light through the lens. One stop is twice the light, two stops is four times the light. The difference between f/2.0 and f/4.0 is two stops, or four times the light.
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#8 Chris Keth

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Posted 06 January 2006 - 12:03 AM

So glad I never knew about film school........


I wouldn't judge all film schools like that.

To the OP: I would seriously ask for your money back!


The normally accepted full stop increments are:

1.0
1.4
2.0
2.8
4.0
5.6
8.0
11
16
22
32

Do you see any pattern?

Each full stop value is equal to the square root of 2 times the previous value (rounded to two significant figures).

A stop is calculated by dividing the diameter of the effective lens opening (iris) by the effective focal length of the lens. So if you have a 50mm lens, that has an effective optical iris opening of 25mm, it is f/2.0. If you close the opening to only 12.5mm, it is f/4.0. An iris of 25mm has four times the area of an iris of 12.5mm, and so lets four times the light through the lens. One stop is twice the light, two stops is four times the light. The difference between f/2.0 and f/4.0 is two stops, or four times the light.



Read this a couple times and remember it. Hell, do the geometrical math with a couple focal lengths and figure out the area of the circle inside the iris at several stops. It's a good way to beat it into your head and remember it.
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#9 Mario C. Jackson

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Posted 06 January 2006 - 10:08 AM

You can also push film at the lab. Or if you were shooting a 320T stock you could set your meter to 400 ASA, 500 ASA and so forth.
Hope this helps
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#10 Scott Bullock

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Posted 07 January 2006 - 12:30 AM

"You can also push film at the lab. Or if you were shooting a 320T stock you could set your meter to 400 ASA, 500 ASA and so forth."

Mario, for clarity's sake, if you're going to push and/or pull your film you have to do it at the lab in conjunction with your meter settings. If you simply change your meter to a higher ISO and process the film normally all you are going to get is an underexposed negative. If you deliberately underexpose your film by one stop let's say, you must tell the lab to push the processing by one stop also for it to be a successful push. I'm not trying to be snobbish, just helpful. Please refer to David Mullen's post above; it is textbook.
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#11 David Mullen ASC

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Posted 07 January 2006 - 02:30 AM

With the existence of several color negative stocks of different speeds, a lab doesn't give a rat's a--- whether you shot 500T but rated it a 800 ASA, etc. It all goes into the same ECN-2 processing line no matter which color neg stock it is.

All they want to know is whether you want normal development, or push or pull development, and by how many stops. How you compensate for a requested development by exposing the negative in a certain way is up to you.

For example, you may expose a film normally -- but ask for a one-stop push so that you end up with a negative with one-stop extra density.

Push or pull processing affects DENSITY during development; exposure also affects potential density. So you have to think of these as two distinct steps and how much you want adjust the development and how much you want to adjust the exposure, to end up with a final density that you want, which may or may not be normal.
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#12 Scott Bullock

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Posted 07 January 2006 - 06:32 PM

David:

I can't tell if we are agreeing with one another or not. I completely understand what you are saying, but why in the world would someone expose normally and then have their film pushed by one stop when all they'd have to do instead is overexpose it by one stop? To me, the whole idea of having your negative pushed after underexposing it by, let's say one stop, is so that your negative will end up with the same relative density of a film stock that's one stop faster but exposed normally. I really don't see the advantage of exposing normally and then push processing. I mean, if one was going to do that, why not just overexpose by however much you want and be done with it? Then again, I don't have your experience under my belt either, so maybe there is a practical reason for doing this that I've simply never encountered.
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#13 David Mullen ASC

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Posted 07 January 2006 - 07:30 PM

I can't tell if we are agreeing with one another or not. I completely understand what you are saying, but why in the world would someone expose normally and then have their film pushed by one stop when all they'd have to do instead is overexpose it by one stop?


For an effect, like any other reason. There would be an increase in contrast, grain, and perhaps saturation. And maybe it's not practical for them to rate the stock a whole stop slower.

I remember that Dante Spinotti shot "Red Dragon" on 5279 (500T) rated at 800 ASA, I believe, but pushed by two stops, to end up with a negative that was 1 1/3 of a stop over-exposed. This created a very contrasty image with "hard" colors.
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#14 Scott Bullock

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Posted 07 January 2006 - 08:12 PM

Excellent example, David; thanks for sharing that. Now I understand it better than I ever have before. I've always been an 'expose it 1/3 of a stop slower and process it normally' kind of guy. I really need to experiment more often than I do.
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#15 Tony Brown

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Posted 08 January 2006 - 10:12 AM

But this is all numbers mumbo jumbo. What one cameraman rates his stock at compared to kodak or any other cameraman means nothing. A well know UK D.o.P. used to rate F500 @1000asa to the amazement of others. It merely meant his old Spectra was knackered and was a stop out.

Don't read too much into stories of people rating stocks at odd numbers, there usually a bit more (or less) to it than techincal craft.

Personally I always rate Kodak stocks at Kodak rating for the purposes of balancing and general readings, but on the whole I underexpose all my shots by at LEAST a stop, do discussing ratings is valid only as part of a much bigger picture.
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#16 Micah Fernandez

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Posted 09 January 2006 - 12:56 AM

I know that video's latitude and sensitivity is still far inferior to film's, but is pushing and pulling done on video, with more or less the same effect as it has on film?
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#17 David Mullen ASC

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Posted 09 January 2006 - 01:06 AM

I know that video's latitude and sensitivity is still far inferior to film's, but is pushing and pulling done on video, with more or less the same effect as it has on film?


You can boost the gain in video, and sometimes go to minus-gain, but it does not change the contrast as with film, just changes the noise level. Some people would say that is similar to how film gets grainier when pushed and less grainy when pulled, but pushing and pulling film gives you other effects on contrast, black level, color saturation, etc.
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