Auto ISO

You say "How much light do I have in my scene?"

Answer, "Lots": Low as you can go on the ISO baby.

Answer, "Hrm, it's a little overcast, pretty dreary": 400 is probably better.

Answer, "Oh my, it's pretty dark in here. I need my flash too!": 800+ (well, in my experience, but one can whip a powerful flash into filling-in enough light with 400).

Answer, "Where the heck did my arm go?!?": 3200+

Okay, that's rough and very joking, but hopefully you get the picture. ;)

More seriously, I go about it by setting the lowest ISO I think I can get away with, meter, and try to expose correctly balancing shutter speed and aperture. If I just can't get a good exposure, I bump ISO up one or two stops; experience dictates how much of an increase is required (or just mathing-it-up and figuring-out how many more stops are required).
 
How would one really go about figuring out the ISO for the right setting? I have a 50D and I shoot in RAW but always set white balance but leave the ISO up the cameras metering system
i was thinking about the same thing as well and i've been told that even ISO 100 is good enough at night/dark areas as long as you use a tripod.
 
How would one really go about figuring out the ISO for the right setting? I have a 50D and I shoot in RAW but always set white balance but leave the ISO up the cameras metering system

What music said covers it, but here's a more direct way to approach it...

Set ISO as low as you can go (ideally no lower than native ISO*), and then check your exposure. If you can expose the shot you want with the aperature** and focal length*** you want, then shoot. If you can't because it would be underexposed, turn the ISO up.****

Now here are the notes:

* Some cameras have "boosted" high and low ISO. These are "effective" ISOs, but may introduce noise or other artifacts, so they should be used with caution.

** Aperature not only affects how much light comes into the camera, but also depth of field and sharpness of image. You need to understand both your composition and your lenses strong and weak points to make the decision on what aperature to use.

*** You need to keep in mind that you have to pick a shutter speed that is at LEAST 1/xxx where xxx is your focal length, or you will get camera shake that will blur your picture. This will affect this entire equation.

**** 2x the ISO means you can 1/2 your aperature. Take small steps in the ISO until you get the desired result.


The reason this is complicated is because it's like a big puzzle where any one piece moved affects all the others.
 
I used to use auto ISO...Not anymore. ISO, is super important in relation to light, just like aperture and the others. By adjusting your ISO you could get a great handheld shot that may have not been capable otherwise due to the available light. There's a reason film has ISO settings!

have a good one
3Eo
 

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