How autofocus and exposure metering work on DSLR?

[Panama Hat & Camera]

On DSLR, the light reaches the viewfinder via a 45º mirror and a pentaprism (or a pentamirror). The mirror flips out of the way to allow the light to reach the sensor direct during exposure.
During the optical viewfinder operation, how the light reaches autofocus sensor(s) and exposure sensor(s)?
Cheers,
Antonio.

[inkista]

The mirror isn't 100% effective. While it doesn't let as much light through as a pellical mirror in an SLT, it does let some through, and some is redirected down to the "floor" of the camera, where the autofocus sensor array is, and some of the light headed for the viewfinder is directed via the pentaprism/pentamirror to the metering sensor. From imaging-resource.com's article on the 30D, http://www.imaging-resource.com/PRODS/D30/D30A3.HTM. here's the cross-section of a 30D:

[Manfred M]

Interesting illustration Kathy; the light metering looks very similar to my rather ancient Leica R3; a semi-silvered main mirror that has a hinged secondary mirror that lets the light hit the meter cell in th base of the camera. I seem to remember it was used in spot-metering mode.

It also had additional meter cells up at the pentaprism for average metering functionality (I suspect the bottom mirror was used as well).

[NorthernFocus]

Mirrorless certainly removes a lot of complexity. It's hard to figure out why Canikon don't get on with it and produce APS-C and FF mirrorless bodies that will accept their lenses. Investment in lenses is really the only thing keeping many DSLR people from jumping ship. If they produced their own mirrorless bodies to fit their lenses, I have to think it would retain a lot of existing customers.

[Panama Hat & Camera]

The mirror isn't 100% effective. While it doesn't let as much light through as a pellical mirror in an SLT, it does let some through, and some is redirected down to the "floor" of the camera, where the autofocus sensor array is, and some of the light headed for the viewfinder is directed via the pentaprism/pentamirror to the metering sensor. From imaging-resource.com's article on the 30D, http://www.imaging-resource.com/PRODS/D30/D30A3.HTM. here's the cross-section of a 30D:

Kathy,
Thank you very much for your answer.
After your explanation, now I understand the Dan's advice about the use of alternative focusing screens in the thread Manual focus on a DSLR.

Second, I have read that alternative focusing screens can degrade the performance of the AF system.

Cheers,
Antonio.

[Otavio]

The mirror isn't 100% effective. While it doesn't let as much light through as a pellical mirror in an SLT, it does let some through, and some is redirected down to the "floor" of the camera, where the autofocus sensor array is, and some of the light headed for the viewfinder is directed via the pentaprism/pentamirror to the metering sensor. From imaging-resource.com's article on the 30D, http://www.imaging-resource.com/PRODS/D30/D30A3.HTM. here's the cross-section of a 30D:

Very nice, indeed, Kathy. Helpful information. Regards,

[blakemcguire]

I wonder why split image focus screens would interfere with autofocus if the AF sensor is at the base of the camera

[inkista]

I use one, and I never found or heard that they did. The problem with using a split-circle focus screen in a dSLR is, as I said, a mirror that isn't 100% efficient, you've got a much darker view through the viewfinder than you did in a manual focus camera. Make it a crop body with a pentamirror, and it's not only darker, it's also smaller. A lot less light. At lower light levels, split screens and prism collars will black out. This is why you don't use them with f/5.6 lenses.

My Katzeye is great until I wanna go birding with my EF 400mm f/5.6L USM. That's the point at which I'll swap back to the default screen. Having swappable focus screens was one of the main reasons I got a 50D, because I used to play a lot with adapted manual focus lenses. When I got my 5DMkII, I actually found that the precision matte screen from Canon helped me manually focus my Olympus OM-mount 50/1.2 wide open just as well as the Katzeye on my 50D for less than half the cost, and I pretty much gave up on the split-circle thing.

[ajohnw]

I had a Topcon Unirex once with split image. I thought it was great until available light turned it's area black. The micro prism arrangements only worked as they should when there was plenty of light as well. I would say split image won on that score but a black disc in the centre of the view is a little disconcerting at times.

The usual arrangement was a small circular main focusing aid in the centre with a ring of one grade of ground glass round that and another on the rest of the focusing screen. The idea being that something would always work whatever lens was on in any realistic light level.

A wonderful camera

http://www.thecamerasite.net/01_SLR_...ges/topcon.htm

John
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