testing film speed

Lucita Jones · March 22, 2005

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Hi,

I want to shoot some tests for several short ends that I have stored for over a year and would appreciate any help in performing these tests.

One book (Ricardo Aronovich) suggests doing a SPEED TEST by adjusting the ISO setting of the light meter to a lower speed than the nominal speed of the film stock that will be tested and increasing it gradually. For example, if I`m testing a 100 iSO film stock, I should start from from ISO 50 and increase the setting to up to ISO 200. As the ISO is increased, the aperture on the lens must be closed in 1/3 increments. As far as I know, adjusting simultaneously ISO and aperture will render equal exposures because of a compensation of exposure. If I am right and all my exposures turned out to be correct and equal, then how would I be able to determine the differences in ISO? The only differences I can imagine seeing are those in the depth of field due to the closing of the aperture.

Another book (The Camera Assistant, Douglas C.Hart) explains how to perform a film latitude test and suggests under- and overexposing a scene. Now, would this latitude test also help me to determine the film speed? (If, for instance, the overexposed shot looks better than the "normal", I could establish that those 2 stops of overexposure could be equal to an increase in the film speed of two stops).

Which test is most reliable for determining film speed?

What should I look for in the gamma of the film to judge its speed?

I know its a broad question, but any help will be welcome.

Thanks,

Lucita Jones

Dominic Case · March 22, 2005

When you alter the ISO speed on your meter, you aren't actually changing the behaviour of the film itself. So if you meter at - say - ISO 50 and the meter tells you f/2.8, then you should expose at f/2.8. When you change the meter to ISO 64, then the meter will indicate f/3.2. SEt the lens to f/3.2 and shoot your next test. And so on.

Douglas Hart's book is just explaining the same test in different terms. I'm sure you could work out how to alter the aperture without using the meter too!

Cinematographers seem to be divided on how they report what they do. If they use a filmstock that the manufacturer rates at 500EI, but they expose it a stop over, some will meter it at 500 and the over-expose a stop, others will meter at 250EI and exposeas per the meter. THe first ones might say they always overexpose a stop, the others will say they always rate the film a stop slower. All comes to the same thing.

By the way, if your "2 stops over-exposed" shot looks better than the normal one, then you have DECREASED the speed of the film by two stops, not increased as in your post.

Lucita Jones · March 23, 2005

When you alter the ISO speed on your meter, you aren't actually changing the behaviour of the film itself. So if you meter at - say - ISO 50 and the meter tells you f/2.8, then you should expose at f/2.8. When you change the meter to ISO 64, then the meter will indicate f/3.2. SEt the lens to f/3.2 and shoot your next test. And so on.

Douglas Hart's book is just explaining the same test in different terms. I'm sure you could work out how to alter the aperture without using the meter too!

Cinematographers seem to be divided on how they report what they do. If they use a filmstock that the manufacturer rates at 500EI, but they expose it a stop over, some will meter it at 500 and the over-expose a stop, others will meter at 250EI and exposeas per the meter. THe first ones might say they always overexpose a stop, the others will say they always rate the film a stop slower. All comes to the same thing.

By the way, if your "2 stops over-exposed" shot looks better than the normal one, then you have DECREASED the speed of the film by two stops, not increased as in your post.

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Lucita Jones · March 23, 2005

Thank you! You are absolutely right about overexposing meaning the same as rating the film slower, or decreasing the EI. Your explanation made me see how confused I was about the whole thing. I understand now that both tests aim at the same result. In the end it is about bracketing the film and evaluating at which reading I will obtain the "best" exposure. So, if the nominal EI rating asks for a T4 and the "correct" exposure according to my test is at T2.8, then I would establish that for that particular stock I would have to either overexpose one stop or rate the film slower to achieve the desired results.

Thanks so much.

I am reviewing all information I have about gamma and density so as to better understand film stock. If someone could recommend a site with a good explanation on the subject I would appreciate it greatly.

All the best to everyone out there. :)