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Overexposure


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#1 Patrick Ryan

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Posted 11 July 2007 - 07:50 PM

Sorry for posting this in two different forums, but this is more of a newbie question.

I understand that underexposing allows for a denser negative and it can also act as a backup so you don't go crazy with underexposure. I'd like to shoot some test to see if I'd like the result of a desner neg.

Being that I'm pretty horrible in math, I don't know the caculation of underexposing a stock by 2/3? I see that 2/3's of a stop of ASA 500 is 320, how do you arrive at that?
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#2 Michael Nash

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Posted 11 July 2007 - 08:08 PM

Underexposing only gives you more density when you're shooting reversal film. When you're shooting negative, OVERexposure gives you more density on the film. Over- or under-exposure needs to be combined with higher or lower than normal printer lights to give you a normal looking brightness on the print. If you just do a "straight" print your image will just look too bright (or too dark, depending on what you did).

The ASA number is doubled as the film gets more sensitive by one stop. 200 ASA is one stop more sensitive than 100 ASA; 500 ASA is one stop more sensitive than 250 ASA. ASA's are typically handled in thirds of a stop, so the numbers are straight forward multiplication/division. You'll see these common numbers:

50, 64, 80, 125, 160, 200, 250, 320, 400, 500, 640, 800...

Notice how the first two digits eventually repeat? That makes it even easier to commit to memory.

You might find some more useful information in some of the books here.
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