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Originally posted by robmanu7

Hi,

I recently got the Cannon 550d and wanted to clarify the exposure settings.

For an average, well lit scene how do you set up the exposure. I heard it was good to leave the shutter speed at 1/50 as it matches the motion of film cameras (1/48). Then also heard that when the ISO value gets higher, more noise appears in the image. So should I set the ISO as low as possible then adjust the aperture to get a decent exposure?

Hope someone can clarify.

Rob - UK

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Basic photography explains the idea of equivalent exposures.

For instance, in a given situation, you could get the "proper" exposure in various ways.

Situation A) shutter 1/48, T-stop 11, 24fps, ASA/ISO 800
Situation B) shutter 1/48, T-stop 2.8, 24fps, ASA/ISO ????

Okay, so looking at the two hypotheticals, you can see that the only thing that changed was the aperture. There is now a four stop difference which means that there is 16 times MORE light hitting your film (or sensor) than in situation A. This could be the result of more light in A than B or perhaps you put Neutral Density on the lens to knock down the amount of light entering the camera.

In any case, to compensate to get a "correct" exposure, you need to adjust something else. Your options are to adjust the shutter or the frame rate or the ASA/ISO. Being that you're shooting "movies," it's best to leave the frame rate and the shutter alone which leaves you only with the ASA/ISO. With film, you would simply use a different filmstock. In video, you sometimes have the option (depending on the camera) to change the effective rating of the camera.

So, what's the answer above? What does the new ASA/ISO have to be in Situation B to achieve an equivalent exposure? If I've done my math right 😉 , the answer should be an ASA/ISO of 50.

Also note, that you don't get something for nothing. Changing any of those elements affects something about the resulting pictures. If you change the aperture from more closed (T-11) to more open (T-2.8), your depth of field shrinks. If you close the aperture, you get more depth-of-field. If you change frame rate, that obviously creates either s slow-motion effect (if you shoot at higher than 24fps) or a jumpy effect (if you shoot at a lower fps than 24).

And yes, a higher ASA/ISO tends to create more "noise" in your picture. Some cameras are better than others about how much noise is introduced. This is the same concept as when film is used. The reason higher ASA rated filmstock was grainier is simply because more sensitive filmstocks have larger grains of silver to collect light. "Slower" filmstocks (ie, ASA 50, ASA 100) have smaller grains of silver crystals so there is less apparent grain. Electronic cameras collect light in a different way but the same basic affect occurs, but instead of grain you see "noise," typically showing up in the blacks first.

So, the key to being a good Cameraman and shooting the movie in the specific way YOU want it to look means understanding why the camera works the way that it does and understanding what every adjustment will do to your picture. This is partially why cameras with automatic functions are inherently bad for anyone wishing to create a narrative motion-picture. The only way to make the movie that YOU want is to have complete control over every aspect of the photography.

Brian Dzyak
Cameraman/Author
IATSE Local 600, SOC
http://www.whatireallywanttodo.com
http://www.realfilmcareer.com