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What does it mean to "rate" a camera or film


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#1 Nick Centera

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Posted 13 July 2009 - 02:28 AM

Hey, I have heard many different people from multiple sources saying they rate their camera at ____ or rate there film at ____. I am wondering if someone can help shed light on the subject for me? Any input is greatly appreciated. Thanks
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#2 Adrian Sierkowski

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Posted 13 July 2009 - 08:11 AM

Well for a camera, video, it's a rough approximation of it's effective speed. This varies a lot based on camera/settings/scene/contract.This will help me light a room without having to have the camera up off of my meter to get a rough estimation of stop. Also useful for location scouting when I won't have the camera with me and want to get an idea of the place's natural look.

As for film, it has to do with wanting to over/under expose film for certain effects. For an example, 500T film is pretty grainy overall. Now, film is made up of little and big particles which react to light. The bigger the particle the less light it needs to expose it, but the smaller ones need more. So, let's say I want to reduce the grain of my 500T stock. To do that I need to expose the big particles as well as the small ones but since the smaller ones are less reactive to light, I re-rate my 500T slower, at around 320, which means I'm effectively over-exposing the film (on purpose) to get these smaller grains to expose a bit and "fill in" the gaps between the big grains (which was see as film grain, it's the gaps between these particles). Later on, of course, I'll correct this in printing/color correction to make it back to "normal."

Re-rating is basically controlling over/under exposure, and all the attendant side-effects thereof. That's just 2 of my quick examples and what I rate cameras and re-rate film for the most.
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#3 K Borowski

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Posted 13 July 2009 - 09:14 AM

To further expound upon Adrian's explanation, with film, you get an ISO (ASA/DIN) speed rating. The ASA number is used pretty much exclusively now.

What an ISO # is is the minimum exposure index to produce in the ECN-2 process the proper shadow speed.

Even with ISO standards, film speeds are still somewhat overrated, especially with faster films, though they're getting better.

The EI, or exposure index is what you rate the film at, be it for a modified process or the standard process. This is what you set your light meter to.

The EI can be different from the ISO speed of the film, or the same. It all depends on the look you are going for and any deviations you take from standard processing, such as a push or a pull.
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