>>3464287>Does setting the shutter speed also affect the light meter calculations?Yes.
Basically, your light meter reads the light in the scene and figures out the exposure value (EV) that it thinks you need.
So say it's shortly after sunset. For ISO 800, your meter will figure out that you're at EV13 (exposure value 13. You don't actually need to remember that number or anything about EVs in general; this is just to explain how your meter works).
Then it looks at your shutter and aperture and figures out how close they are to combining to EV13. They each play an equal part in the so-called exposure triangle (ISO-Aperture-Shutter), so the light meter looks at all three and the actual light in the scene to figure out the exposure you've got dialed in vs. the exposure you want.
So, say the camera reads the light at EV13 and you've got your aperture set to f/4 and your shutter speed set to 1/2000th. Your camera will read underexposure.
You change your shutter to 1/1000th. Still underexposure.
1/500th. Reads exposed correctly.
Now, if you want to use a faster shutter speed (e.g., if you're photographing someone dancing really fast and you need the faster shutter to freeze the action), you can change your aperture to f/2.8 (letting in one stop more light than f/4) and then move your shutter back to 1/1000th (letting in one stop less light than 1/500th) and get the same exposure.
Or if you want more depth of field (i.e., the amount of your shot that's in focus in front and back of the plane you're actually focused on, which is the other thing aperture controls), you can stop your lens down from f/4 to f/5.6 and slow down your shutter speed from 1/500th to 1/250th.
To stay at one exposure, any increase in aperture needs to be balanced by a decrease in shutter speed and vice versa. The light meter lights up "O" when the combination you've selected match what it thinks is a good exposure for the light in the scene.
