Light meter/sensor?

Not stupid at all.... you can't see it, it's hidden away up in the pentaprism - the bumpy bit on the top of the camera. The meter actually reads the light coming through the lens into the camera deflected up into the pentaprism by the mirror.

There's another sensor actually which just deals with metering flash, but you can't see that one either :)

canon_20d_diagram.jpg
 
Actually just to clarify that - when you look through the viewfinder and focus, you're actually looking at the focusing screen and the meter measures the exposure at the focusing screen. You can see the focusing screen if you remove the lens and look upwards into the camera lens opening. On the half -press of the shutter button, the meter reads the exposure. When you press the shutter all the way, the mirror flips up, the shutter opens, and the light through the lens being no longer diverted by the mirror, projects the image onto the sensor.
 
What happens if the focusing screen gets dirt on it?
 
My concern is would dirt on the focus screen affect the AF?
 
Dirt and dust on the focusing screen is irritating but it has no effect on the images. You're bound to get dust and hairs on the screen eventually and unless you really can't live with it, it's best ignored. It is possible to clean the screen yourself - even remove and replace it on some cameras, but if you've the slightest tendency to be a bit clumsy I wouldn't bother - you can do more harm than good. Get a pro to clean it if you're unsure. ;)
 
admirable said:
My concern is would dirt on the focus screen affect the AF?
Click to expand...

LOL. Good question - but highly unlikely I would think, You can after all get screens with lines on to aid composition, and they don't affect the AF system - I can't see that dust on the screen would be any different?

AF is a function of the lens with most cameras anyway, so the lens has already focused the image before it reaches the screen?
 
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