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Film Rating?


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#1 James Steven Beverly

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Posted 04 March 2010 - 08:54 PM

I have a kinda basic question that I should already know but I'm still uneasy with what I think I know. I'm still not sure I completely understand film rating. When one rates say a vision 3 stock with an asa of 500 at 800 or 320 you essentially pretend the asa number is 800 or 320 or whatever, ok here's where I get confused, why would you do that? If the film is rated at 500, the latitude is fixed within those perameters, the shoulder and toe stay the same and just saying they're different won't change that so how does ratig film work and when is it approprate to do so. Also what factors like stop compinsation are involved, how does it effect DOF and are filter stop compentation effected? I'm sorry if this is a weenie question but for some reason, I just can't wrap my head around this particular concept. :D

Edited by James Steven Beverly, 04 March 2010 - 08:55 PM.

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#2 Paul Bruening

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Posted 04 March 2010 - 10:15 PM

Steven, brotha'man, I'll throw-in what I know. The bottom line is you're thickening or thinning the density of the negative. You can restore some amount of that alteration in lab or let it go depending on your needs. Either way you're goofing with the standard rating and behavior of the the stock to get some benefit out of it. You will definitely shift the latitude one way or the other. You are, also, trading off the DR in one direction for the other. I'm still mulling over whether DI can buy you some expansion of the DR on that through some of the simple voodoo that comes with it.
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#3 Adrian Sierkowski

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Posted 04 March 2010 - 10:28 PM

You're essentially under or over exposing the film, with all the effects that go with that. If you're saying it's a 320 film, you treat it like a 320 film, so if you loose 2 stops from the filter, let's say, you just incorporate that (making it an 80 asa). When you're going 320, you're thickening the neg up and you're also tightening up the grain a little bit when you correct it back in the TC. When you're @ 800 you're thinning it and increasing grain (and also killing some saturation if you correct it in the TC). It's really something you do to get "your best results," if that makes sense. It's a trial and error thing till you get to a point with a particular emulsion that you know what it'll do for you and where you feel it's best to place exposure as well as rate it. Hope that helps a bit.
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#4 David Mullen ASC

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Posted 04 March 2010 - 10:30 PM

First of all, keep in mind that rating a stock 1/3 of a stop slower, like rating 500T at 400 ASA, is negligible, it's in the range of a 2-point printer light adjustment. It's within a margin of error in exposing, i.e. you may rate the stock 1/3 stop slower but you could easily underexpose a shot by 1/3 of a stop and cancel the effect.

So don't think that there's some big change in dynamic range when you are only talking about rating 500T at 400 ASA. It's more just extra insurance against underexposing rather than consistently overexposing.

You'd have to rate the stock 2/3's of a stop slower to be more consistently overexposing it and getting a denser negative, i.e. rating 500T at 320 ASA.

You are getting the same dynamic range but you are biasing information more towards the shadows when overexposing, and considering film's tremendous overexposure latitude, it's not a bad idea to worry more about the dark end of the scale and give it more density.

A denser-than-normal (overexposed) negative, printed at higher printing light numbers to compensate, i.e to print "down", gives you deeper blacks in the print, which improves the appearance of color saturation and contrast, plus tightens up the grain structure since you've exposed more of the tiny, slower grains in the film.

A simple test would be to take an unexposed roll of film, develop it normally so basically you have a totally black image (clear on the negative), and then print it at different printer light numbers. You'll see that at higher printer light numbers, the blacks in the print are deeper.

So ideally you'd want to expose your negative so that it prints in the mid to high 30's on average (except the blue record) rather than the mid to high 20's (the scale goes from 0 to 50).
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#5 Dominic Case

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Posted 05 March 2010 - 01:28 AM

If the film is rated at 500, the latitude is fixed within those perameters,

It's not a hard-edged science. It's not like cutting a piece of glass to fit a window frame. More like cutting a panel of wood to block a hole in the wall. The panel of wood will be bigger than the hole, and you can position it a little to the left or right, so long as the middle is over the hole.

Film has a very wide useful exposure range, which means that with a low to average contrast scene you can overrate or underrate the film and get away with it. Like a big panel of wood and a small hole.

And if you are shooting a very contrasty scene, with abrightness range outside the film's abilities, don't forget that the film characteristic curve has gradual slopes on the toe and shoulder, and you can nudge the image up and down the curve to control which end of the range gets better treatment.

In essence, exposing according to the recommended EI will get your mid greys exposed in the middle of the range. If you have no really extreme highlights, but you want to see into the shadows as much as possible, then you'd rate your 500 EI film at 250 or even lower. Or, you'd rate it at 500 and over-expose by 1 or 2 stops. It is exactly the same thing.

If you are doing a TV commercial in a white-tiled bathroom, no shadows, you might rate at 800 or 1000. Doesn't matter what midgreys look like, you want as much range in the whites as possible.

Other non-sensitometric reasons for varying the EI rating would be to do with minimising grain or trying to get away with fewer lights.





So
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#6 James Steven Beverly

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Posted 08 March 2010 - 07:35 PM

That clears it up immensely. AS ALWAYS, thank you, gentlemen. The information is much appreciated. This seems to really just a matter of balancing lighting and your ASA shift to something that serves the look. I guess it's also a matter of experience, and of course testing, in getting the balance right. B)
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