Confusion between AF and Exposure

Hello everyone.

I am a little bit confused between AF and Metering. I know what each are - but when you press the shutter halfway down, it AF's AND it meters that point, does it not? So how are they different besides AF desiding what is in focus and Metering deciding .. what?

I'm confused, sorry :S

I am sure most here know a TON more then me, but as I understand it metering decides essentially how much light there is and how fast to open and shut the shutter or to deal with the "exposure".

They are totally different things happening at about the same time....

Metering decides what an appropriate shutter speed/aperture/ISO combination will work well for a good exposure. Of course, if you specify one or more of those parameters, the camera will pick the other(s); If you're on full manual it will at least suggest whether your choices will result in enough light hitting the sensor. Using a particular scene mode will shift the metering logic toward (for example) larger or smaller apertures to control depth of focus. It would be somewhat helpful if you provided a model number of the camera you're asking about; small cameras will probably use different assumptions about metering compared to a DSLR... but I think in general they do similar things when you half-press the shutter button.

So does this make sense:

Usually I use Av mode, so, whatever I meter on - it will decide a shutter speed for that amount of light (the thing i metered on) based on my chosen aperture and ISO? Whereas the AF will decide what is in focus based on my aperture?

Also - what is this FEL, i understand it locks the exposure - so would this be what you'd use it for: meter on something else other than your subject (some more neutral in color - say your subject is wearing all white), press FEL or * and then go to the subject, focus on them then take the picture?

Does this make sense or am I just super confused?

Thanks guys

I think you've pretty much got it... although I'm not sure I would say

The camera will try to focus based on the focus point(s) sensing edges or contrast in your subject matter. Focus is indeed influenced by aperture -- small apertures giving greater depth of focus, and therefore perhaps "better" focus -- but the camera will do it's best to focus on what's in its sights regardless of aperture. Different cameras will use different schemes to determine what the subject (and focus point) should be... you can specify a focus point in some cameras, or it will just take a guess based on some pre-programmed instructions about what/how users typically shoot.

OK I think I got it. I don't understand the difference between the 3 metering systems.

There is one with a box and a circle in it, one with a small box, one with a big box.

What is the difference?

Metering modes: Spot metering uses just a very tiny area in the center of the frame (or an area you can pick instead of the center) as the souce for metering. Handy for small objects against a big bright (or dark) background.

Center-weighted average takes a bigger area, finds an "average" amount of light, with some extra consideration given to what's in the middle of the frame -- where you'd usually put your subject.

Evaluative metering looks more at the whole image, makes some guesses as to what it may be -- landscape, or night scene, or person at the beach -- and calculates an exposure based on that guess... I think.

I don't have my camera or manual in front of me now, but I believe I've got the terms and their very informal descriptions correct for most Canon cameras.

in these enlightened times, are we still allowed to say 'RTFM'

chris clements wrote:
in these enlightened times, are we still allowed to say 'RTFM'

You took the words right out of my mouth.

Also, try the beginner threads.

Sorry guys - I did read the manual and I know that a lot of it is explained I just want to make sure that I am getting the right idea.

Also try reading Bryan Petersen's excellent book Understanding Exposure. I think it must get recommended 100 times a day in this forum.

Mark

Thanks for the help guys - one last question.

In manual mode - the metering mode doesn't do anything does it?

Yes it does: it shows you what exposure the camera would choose. And the mode of meterimg (partial, etc.) remains as set .

You then set your manual exposure with reference to the camera's suggestion. For instance, you expose more than the meter suggests if you're shooting a polar bear in a snowstorm, or less if you're shooting a black cat at midnight.

bcap, I want to thank you for asking these questions which have confused me also! I'm about ready to read "Understanding Exposure" for the second time, then maybe it will all click!

So how does this work?
If you push the button half the way to lock the focus, move the camera to get the composition you want, does that change the exposure or is it locked in place like the focus?