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Can someone explain rating film stocks to me?


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#1 Scott Fritzshall

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Posted 26 November 2005 - 04:17 PM

I've read quite a few posts on here about rating film at different ASA, and I'm kind of unclear about it. I understand that you basically use the film as though it was a different ASA, for instance 500T as if it were 320T or 800T, but I don't exactly understand if you process it any different, or how exactly it affects your image.

Could someone give me a rundown on exactly how this all works? I've got a project for school I'm shooting on Monday; shooting on Vision 500T. I'd like really deep shadows, and I was considering rating it differently based on some discussions I've seen around here, but I'd like to have a better understanding of it before I decide.
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#2 Laurent Andrieux

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Posted 26 November 2005 - 04:53 PM

Why don't you just rate normally, as a first step, then ? ;)

You see, my students and I are shooting 5218 next week, we just rate it normally. What is important is what is in the frame, not the sophisticated lab process and stuff... A beginner should begin by the beginning... I think...

Deep shadows will be there if your shadows are deep... (use a spotmeter).
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#3 Scott Fritzshall

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Posted 26 November 2005 - 05:14 PM

Why don't you just rate normally, as a first step, then ? ;)

You see, my students and I are shooting 5218 next week, we just rate it normally. What is important is what is in the frame, not the sophisticated lab process and stuff... A beginner should begin by the beginning... I think...

Deep shadows will be there if your shadows are deep... (use a spotmeter).

In all likelyhood, that's what I'll end up doing for this project; I'd just like to learn about this subject for future reference.

Also, I don't have a spotmeter, and can't get one from the school for this project, sadly. :(

Edited by Scott Fritzshall, 26 November 2005 - 05:15 PM.

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#4 Joseph White

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Posted 26 November 2005 - 05:28 PM

in essence, the lab does nothing special or different unless you instruct them to. if you want to overexpose a bit by rating 500 asa film at say 400 asa, you're simply diong just that - overexposing - if the lab corrected for it, it wouldn't be overexposing. the same rule applies to underexposing, when you rate a film at a higher than intended iso - say rating 500 asa film at 800 asa.

now if you want to push process or pull process, this changes things. pushing or pulling doesn't "change" the asa of the film stock - 500 asa film is 500 asa film - but you rate it differently to compensate for the different time used in processing the film. typically, the normal compensation for pushing a film one stop is a one stop difference in how you rate the film. so if you're shooting 500t film and you push it a stop (for more grain, contrast, and saturation) if you were rating normally you would then rate your film at 1000 asa. a lot of people like to underexpose slightly when doing this, hence why you hear people say they rate 500 asa film pushed one stop at something like 800 asa (to print down later to get richer blacks). the same principle applies to pull processing (to reduce contrast, grain, and saturation) where you would rate 500 asa film pulled one stop normally at 250 asa.

the iso rating on the can is a guideline, it's what the manufacturer reccomends you rate your film at in order to shoot "normally"; rating differently is simply a matter of taste - there's no right or wrong to it - it's just what you enjoy and what's appropriate for the piece.

hope this helps - happy shooting!
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#5 Laurent Andrieux

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Posted 26 November 2005 - 08:05 PM

In all likelyhood, that's what I'll end up doing for this project; I'd just like to learn about this subject for future reference.


Good ! Don't put pressure on yourself. As you're talking of a very soon shoot, that's a clever choice.

And I'm pretty sure you'll have enough worries of many kind, don't give yourself more worries again.

Now, the question you ask - that one can study out of rush - is, of course of interest. Make a research on these forums and I'm pretty sure you will already find a lot of valuable matter !

As to get deep shadows without a spotmeter, then meter them with the incident light meter, consider 3 stop under key light should be on the toe of the curve. After that, it depends on the reflectance/albedo of the subject.
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#6 David Mullen ASC

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Posted 26 November 2005 - 08:28 PM

The recommended ASA by the manufacturer means that you will end up with a negative, if developed normally, of average density with the widest range from bright to dark information that the negative can hold.

As you overexpose a negative, you expose more of the smaller (thus less sensitive) grains as well as the larger ones that got exposed originally. The more you underexpose, only the larger (thus more sensitive) grains have time to get exposed.

So some slight overexposure leads to some tightening up of the grain structure, which makes the image look less grainy (though the large grains are always there.) It also exposes more shadow information at the expense of some bright highlight information (no free lunch.) When this slightly overexposed negative is developed normally, then printed down, the blacks tend to look richer in the print, and thus the contrast looks snappier, which in turn makes the image look a little sharper, plus the colors look richer because of the better blacks, etc. Now if you're not printing, there is less benefit to overexposing.

By the way, rating a stock 1/3 of a stop slower is so conservative that it's within a margin of error, i.e. you could easily accidently underexpose a shot by 1/3 of a stop and counteract the slower rating. So if you want to rate a 500 ASA stock at 400 ASA, you could do so without too much worry.
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#7 Charles MacDonald

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Posted 26 November 2005 - 08:57 PM

I'd just like to learn about this subject for future reference.


One sugestion, if you can "get away with it" is to shoot some experimental tests after you get what you need to make your main shoot. Just try a short shot at the end of a roll that does not have enough for another take. (you may have to buy the rest of yoru crew a couple more beers) of course you will have to be sure the test shot will get transfered/printed so you can see what it looks like, but I suspect that would notbe a big hastle on a "student" film, part of the reason you are out with a camera is to learn how the media reacts to the controls that are available. Part is to build the confidence to go out and spend the day working hard and trusting you will have something to show for it when you get the film back.

Charles, who often figures every shot is a test of something...
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#8 Scott Fritzshall

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Posted 26 November 2005 - 09:27 PM

So some slight overexposure leads to some tightening up of the grain structure, which makes the image look less grainy (though the large grains are always there.) It also exposes more shadow information at the expense of some bright highlight information (no free lunch.) When this slightly overexposed negative is developed normally, then printed down, the blacks tend to look richer in the print, and thus the contrast looks snappier, which in turn makes the image look a little sharper, plus the colors look richer because of the better blacks, etc. Now if you're not printing, there is less benefit to overexposing.

When you say "developed normally," do you mean developed at its intended ASA or at the ASA I'm rating it at?

For this project I am getting a workprint made; are there special instructions I would need to give the lab to have it printed down properly? Also, in the event that I ever had this transferred, it's unlikely I would be able to supervise it; would my transfer be really bright-looking or something? I know this probably doesn't represent a typical scenario; I'm just wondering. :)
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#9 Laurent Andrieux

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Posted 26 November 2005 - 09:50 PM

Right, I actually think that you should be allowed to make these tests as an entire exercise by itself, at your school. I do Keylight tests with my students just as much/often as I can.

It consists in shooting a face (usually a lady) in beetween mid shot and close-up, with a 6 grey chart, at different values, say from - 5 to + 5 around key light, by steps of 1 stop. you can do 1/3 steps in the -1 to +1 range. Shoot 10 sec for each shot.

Have this print at a "one light " print, so there is no correction at the lab. Then look at the positive on a screen.

You could even do it with Tungsten reversal still 35 mm slides as a first step, as to give you an idea of what you will have to eyeball, for cheap.

It will allow you to determine at what rating you like the stock the better.

You can determine how many stops over you like for a exterior day or how many stops under you like for a night effect.

Also it will help you out determining what latitude you have under and over keylight for your highlights and shadows.

The 18 % value will help you out determining how many stops over and under KL the film can assume from black to white, so that you can determine where things will be on the curve, from a spotmetering.

A good thing to do at the same time is contrast tests :

Keep one side of the lady's face at the KL, and under/overexpose the other side only as described below.

Then you can determine what contrast you like for what situation : contrast/soft day exterior ; contrast/soft interior etc.

After you know how to set your meter and lights, you can begin to interest yourself in the lab techniques such as push and pull process etc. But I think you should first learn how to control and master the stock itself.


When you say "developed normally," do you mean developed at its intended ASA or at the ASA I'm rating it at?


He means "at its intended ASA"

For this project I am getting a workprint made; are there special instructions I would need to give the lab to have it printed down properly? Also, in the event that I ever had this transferred, it's unlikely I would be able to supervise it; would my transfer be really bright-looking or something? I know this probably doesn't represent a typical scenario; I'm just wondering. :)


It depends on what your school has asked the lab.

2 possibilities :

1) They have asked a "one light" print. It's cheaper. There are no corrections made at the printing. So an overexposed shot will be overexposed and vice-versa. It's also good when you are a student to see your mistakes, better than having them corrected. But as David Mullen said, a 1/3 overexposition won't be very noticable on the screen

2) They've asked for a color timed/graded print. That's what is usually done on normal /major professional productions, so that little offsets are corrected (and sometimes big ones BTW...) That's when an overexposed shot would be "print down".

But there necesarly no need to esp. tell the lab in that case, usually, they see it and correct it. If one wants to be sure, he can always write on the printing sheets whatever he likes for the person who will pass the cans

Edited by laurent.a, 26 November 2005 - 09:59 PM.

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#10 Scott Fritzshall

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Posted 26 November 2005 - 10:22 PM

Right, I actually think that you should be allowed to make these tests as an entire exercise by itself, at your school. I do Keylight tests with my students just as much/often as I can.

I think there is a class where we eventually do something of the sort, but I'm not sure. You're right, though, I kind of wish they had showed us this earlier on.

He means "at its intended ASA"

Ah, ok. What would happen if I had it processed at the ASA I rated it at?



It depends on what your school has asked the lab.

2 possibilities :

1) They have asked a "one light" print. It's cheaper. There are no corrections made at the printing. So an overexposed shot will be overexposed and vice-versa. It's also good when you are a student to see your mistakes, better than having them corrected. But as David Mullen said, a 1/3 overexposition won't be very noticable on the screen

2) They've asked for a color timed/graded print. That's what is usually done on normal /major professional productions, so that little offsets are corrected (and sometimes big ones BTW...) That's when an overexposed shot would be "print down".

But there necesarly no need to esp. tell the lab in that case, usually, they see it and correct it. If one wants to be sure, he can always write on the printing sheets whatever he likes for the person who will pass the cans

Ah, ok. I think it's #2, because they will time it to gray cards if we ask them to, and we get a printer light report. So basically this would just happen automatically without me needing to tell them.
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#11 David Mullen ASC

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Posted 26 November 2005 - 11:56 PM

All motion picture color negative gets sent through the same ECN-2 process -- the lab doesn't care whether it was 100T or 500T or 250D emulsion and they don't care what ASA you rated the stocks at. The only question is whether you want normal processing time & temperature (the same for all stocks) or you want push or pull processing (i.e. extended or reduced development) and by how many stops.

So in most cases, if you rate a 500 ASA stock at 320 ASA, you'd ask the lab for normal development, which would then produce a negative with 2/3's of a stop of extra density since it was overexposed. Now with this denser-than-normal negative, you would probably darken the image down to normal brightness, not leave it overexposed-looking, by printing it "down" using higher printer lights. Or if this was just for telecine transfer, you'd just color-correct it to look normal in brightness.

Let's round things up to one-stop over or underexposure. Let's say you rate a 500 ASA stock at 250 ASA.

You could ask the lab for NORMAL processing. So you shoot a gray scale at 250 ASA so that the timer / colorist knows that the bright image is meant to be corrected to look normal. This is just in case they see an overexposed image on the negative and assume it's supposed to look that bright.

Instead, you could ask the lab for ONE-STOP PULL-PROCESSING. Now they will reduce the processing time so that you get one-stop's worth of less density, which in your case, will be compensated for the fact that you overexposed by one stop. So the final negative that was pull-processed ends up with normal density, not extra density. It does not look overexposed. You still have the benefits of reduced graininess to a minor degree, but you also have less contrast and your colors are more pastel.

So to repeat, the lab does not care whether you rated a 500 ASA stock at 320 ASA, or even what the speed of the color negative was to begin with, because that has no bearing on the ECN-2 processing, which is the same for all color negative stocks. All you can ask for is normal processing, or push or pull-processing by "x" number of stops. As to how you expose your negative with these processing options in mind, that's your business.
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#12 Scott Fritzshall

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Posted 27 November 2005 - 03:21 PM

Ok, I understand now. I'm sure I'll learn all about this next semester; I'll be taking Photo Theory and Lab Practice, but it never hurts to get a head start! Thank you, that was very helpful :)
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#13 John Pytlak RIP

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Posted 27 November 2005 - 11:21 PM

Exposure Index:

http://www.kodak.com...t/faq/#preprod8

Why is the speed rating of motion picture camera films given in Exposure Indices rather than ASA or DIN values?
There is no ANSI standard to determine the speed of these films.
The speed of motion picture camera films and the suggested filtrations are determined on the basis of practical picture tests. Suitable safety factors have been included to allow for differences in cameras, variation in lighting, etc. The exposure index values should not be regarded as numbers which express the absolute speed or sensitivity of the film, neither should they be regarded as fixed values which can not be changed if the results of repeated tests indicate the need for such changes.


http://www.kodak.com...structure.shtml

Sensitometric and Image-Structure Data

* Understanding Sensitometric Information
* Characteristic Curves
* Spectral Sensitivity
* Spectral-Dye-Density Curves
* Printing Conditions
* Sound-Track Printing

"Sensitometry" is the science of measuring the response of photographic emulsions to light. "Image structure" refers to the properties that determine how well the film can faithfully record detail. The appearance and utility of a photographic record are closely associated with the sensitometric and image-structure characteristics of the film used to make that record. The ways in which a film is exposed, processed, and viewed affect the degree to which the film's sensitometric and image-structure potential is realized. ...


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#14 Laurent Andrieux

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Posted 28 November 2005 - 12:29 AM

he exposure index values should not be regarded as numbers which express the absolute speed or sensitivity of the film, neither should they be regarded as fixed values which can not be changed if the results of repeated tests indicate the need for such changes.


I remember I read statistics about the usage of stock by operators. Far from them all use the stock at its recommended value.

I also remember that when the 5248 issued, Kodak at first rated it at EI 125, but as operators mainly used it at EI 100, likewise the older 47, Kodak changed the 48's rating after a while and rated it at 100.
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