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ASA and F stops


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#1 Michael Ryan

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Posted 16 February 2009 - 05:17 PM

Hello All,

I can't get my mind around this, and maybe this isn't the way I should be thinking of it, but here is my question:

I'm going to use Kodak's Ektachrome 100D reversal film in a movie camera that has an automatic shutter. The deal is you have to set the film speed on a wheel that is on the camera. This ring doesn't have 100 ASA. It can be set at 125 or 80. If I use 100 ASA film but set the ASA for 125 does that work out to a full F stop? Or if I set it to 80 is that one F stop the other way?

I wanted to get an image that didn't have as much grain, so I have been told to overexpose by one F stop with this film.

Thanks for your help.

Mike
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#2 Shubham Kasera

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Posted 16 February 2009 - 05:30 PM

well
i didnt really understand what u said
but all i know is that if u need to overexpose yr 100D
u need to rate it 50ASA
which im sure you would know
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#3 David Mullen ASC

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Posted 16 February 2009 - 05:32 PM

You don't want to overexpose reversal film, just negative film.

The 125 ASA setting would only underexpose a 100 ASA stock by 1/3 of a stop... the 80 ASA setting would only overexpose it by 1/3 of a stop. So either is pretty safe, but I'd shoot a test roll to see what worked better. Maybe you could switch between them scene by scene or shot by shot... depending on if you wanted to expose more shadow detail (by using the 125 ASA setting) or the highlight detail (by using the 80 ASA setting.)

Doubling or halving the ASA rating is the equivalent of one-stop more or less sensitivity.
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#4 Michael Ryan

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Posted 16 February 2009 - 05:53 PM

You don't want to overexpose reversal film, just negative film.

The 125 ASA setting would only underexpose a 100 ASA stock by 1/3 of a stop... the 80 ASA setting would only overexpose it by 1/3 of a stop. So either is pretty safe, but I'd shoot a test roll to see what worked better. Maybe you could switch between them scene by scene or shot by shot... depending on if you wanted to expose more shadow detail (by using the 125 ASA setting) or the highlight detail (by using the 80 ASA setting.)

Doubling or halving the ASA rating is the equivalent of one-stop more or less sensitivity.



Thank you, David.

It makes more sense to me now.

The camera that I'm testing is a very rare Elmo C300 Double Super 8. It takes a 100ft roll of of DS8 film in the magazine (which looks like mickey mouse ears) and the film stops right before the first hundred feet goes through the film gate. So, instead of having to re-thread the film for the second hundred feet you just flip the film magazine over and shoot the second hundred feet. This is really sweet! The C300 has no film weave or jitter because it has a proper 16mm film gate in there.

I'm going to test a roll of the 100D, but I have been told with this film if you expose this film at the proper ASA/F stop it comes out a little too grainy. If you overexpose it by one stop you get way less grain. Does this sound right?

BTW this camera was completely overhauled by Bernie at Super 16.


Mike
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#5 David Mullen ASC

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Posted 16 February 2009 - 06:55 PM

No, overexposing reversal film doesn't necessarily reduce the grain by much and since it has a limited dynamic range, overexposing will cause highlights to burn out unrealistically.

You need to expose reversal film close to the recommended rating. 80 ASA may be OK for 100D, but I wouldn't overexpose a reversal film by one stop.

BTW, overexposing negative film doesn't really reduce graininess, it just improves the appearance of graininess by filling in the large grains with more of the smaller and thus slower grains in between, so the overall effect is a "tighter" grain structure.
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#6 Ryan Thomas

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Posted 17 February 2009 - 12:22 AM

No, overexposing reversal film doesn't necessarily reduce the grain by much and since it has a limited dynamic range, overexposing will cause highlights to burn out unrealistically.

You need to expose reversal film close to the recommended rating. 80 ASA may be OK for 100D, but I wouldn't overexpose a reversal film by one stop.

BTW, overexposing negative film doesn't really reduce graininess, it just improves the appearance of graininess by filling in the large grains with more of the smaller and thus slower grains in between, so the overall effect is a "tighter" grain structure.


Does overexposing negative film a stop, then printing up to compensate tighten the grain structure? Or is it only when you overexpose and then pull the film in processing?
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#7 Chris Keth

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Posted 17 February 2009 - 01:00 AM

Does overexposing negative film a stop, then printing up to compensate tighten the grain structure?


Yes it does. The overexposure itself is what tightens up the grain. Smaller grains of film are slower than larger ones so a bit of overexposure exposes the smaller grains in the spaces between the big grains.
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#8 David Mullen ASC

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Posted 17 February 2009 - 01:30 AM

Does overexposing negative film a stop, then printing up to compensate tighten the grain structure? Or is it only when you overexpose and then pull the film in processing?


First of all, you'd print "down", not "up", to compensate for an overexposed (dense) negative -- because you're bringing the bright image down to normal brightness. But to do that in printing, you use higher printer light numbers.

It's the overexposure that primarily causes the negative to seem less grainy -- again, I say "seem" because the size of the grains is determined by the speed of the stock. You can't make the large grains any smaller in size, all you can do is expose so that the smaller grains in between the larger ones also get developed and fill-in the gaps. To truly get smaller grains only, you need to use a slower-speed stock.

Whether you print down or under-develop (pull-process) to compensate for the extra exposure, well, that mainly has an effect on contrast and density more than grain, though I suppose there may be some slight grain reduction from pull-processing as well. But it's really the extra exposure that helps. But pulling will lower contrast, and since your final negative ends up with normal density, you don't get the denser blacks that comes from printing down.

But since reversal has a limited exposure range, it's not a good idea to over or under-expose much, you don't have the latitude.
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#9 Ryan Thomas

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Posted 17 February 2009 - 01:39 AM

Ah, makes sense. Thanks.
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#10 K Borowski

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Posted 17 February 2009 - 10:28 AM

You would want to slightly underexpose reversal film. So given a choice between 80 and 125, expose at 125. That would be what I would shoot at anyway even if I could shoot at box speed instead.

125 will give you even more saturation because it makes for a denser image. Don't know how this would effect scanners though. I'm sure all of the scanners out there, designed for negative film, have enough trouble with reversal so as it is, so the extra third of a stop might cause shadows to block up in scans.
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#11 David Auner aac

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Posted 17 February 2009 - 12:23 PM

125 will give you even more saturation because it makes for a denser image. Don't know how this would effect scanners though. I'm sure all of the scanners out there, designed for negative film, have enough trouble with reversal so as it is, so the extra third of a stop might cause shadows to block up in scans.


I was about to post that too. In stills, my grandpa, who was a pro photographer, would always underexpose reversal by 1/3rd of a stop to get more deeply saturated colors. Especially useful for plants and landscapes if you want that look. Reversal, again from a stills perspective, is way easier to scan, yields finer grain in scanning so I'd assume that you could "de-feature" a negative scanner by turning all bells 'n' whistles off and scan nice reversal, no? I have no idea how that works for cine film though.

Cheers, Dave
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