Hello everyone I'm new to the world of Cinematography. What I'd like to know is, what's the best ISO to shoot with during the day and or at night? I have the sony F55 camera. Should I be set at 800? 500? 1250?
Well, it's whatever looks best to you, and is dependent on your camera.
I don't shoot with the F55 so I couldn't say. But, just as an example, I shoot with the Canon C300PL where 850 is considered the "optimal" ISO... but honestly I often shoot much higher and much lower than that. Especially lower, that's just way too fast for daytime exteriors.
There is probably some documentation in your camera manual that spells it out? There certainly is in the Canon manual.
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Actually, it's best to shoot at the camera's "native" or "nominal" ISO, which in the case of the F55 is 1250. By changing to a higher speed you are lowering the middle gray and begin to loose dynamic range in the shadows. The opposite is true by lowering the ISO you are raising the middle gray and loosing dynamic range in the highlights.
Think of it this wayl when you use the nominal ISO there is an equal dynamic range above and below middle gray. This is how the manufacturer determines that number in the first place.
Use the built in ND's and others on the lens in a matte box to control bright day exteriors so that you can stay at the native ISO.
When it's really dark, of course you'll have to "gain up" as necessary but the sacrifice is both smaller dynamic range in the shadows and noise.
In RAW formats (of which the F55 is capable) when you change the ISO you are not actually changing the exposure on the recording, but only on the monitoring, but it is an approximation of what t will look like after you've made a similar adjustment in post color correction.