Canon ee-s focus screen helps focusing?

[kel_madics]

<p>of course i know it should help focusing because that's what it suppose to do but thing is since i changed to this focusing screen my 5d's AF itself seems to focus a lot better specially in back-lighted subjects. maybe i am just imagining things. So is there a relation between the camera's AF and the focusing screen? please confirm if true.</p>

[isaac sibson]

<p>No, there is not.</p>

[steve_dunn2]

<p>To expand on that, the focusing screen has absolutely nothing to do with AF. The focusing screen is located at the top of the mirror box and receives light which has been reflected upward by the main mirror. The AF system is located in the bottom of the mirror box and receives light which has passed through the main mirror (which is actually a half-mirror) and been reflected down to it by the sub-mirror which is mounted on the back of the main mirror. You could completely remove the focusing screen, or replace it with a piece of black paper, and the AF system would neither know nor care, though it would affect metering (which is done by a sensor aimed at the focusing screen).</p>

 

<p>In the following diagram, the main mirror is item 2 (or 3 when it's flipped up for exposure), the focusing screen is 4, the sub-mirror is 10, and the AF sensor is 11.</p>

 

<img src="http://www.sensorcleaning.com/pics/digital_slr_diagram2.jpg" alt="Simplified diagram of the layout of the mirror box">

[Charles_Webster]

<p>Nice explanation Steve!</p>

<p><Chas><br /></p>

[kel_madics]

<p>wow, i concur. i am imagining things. hehe. probably just excited using my new add-on.</p>

[diego_di_michele]

<p>It certainly does affect exposure readings, so adapted lenses (nikon, zeiss, leicaR, etc...) stopped down have precise meter readings, that with the standard sceen they do not. That's a fact.</p>

[kel_madics]

<p>haven't experimented the metering readings yet but compare it to the default focus, how much difference is it?</p>

[diego_di_michele]

<p>With fast lenses like 24mm 1.4, 35mm 1.4, 50mm 1.4 there is a lot of difference. You actually see the real DOF.<br>

<br /> With f/2.8 there is still improvement. Everything else is pretty much the same. Not recommended for lenses with f/4 or more because the viewfinder gets darker than the standard screen.</p>