canonloader wrote in post #2495696
What blackout range would that be? Are you saying the Depth of Field preview button won't work with a 2X TC on the camera?
It has nothing to do with the depth of field preview. 1/2 of the split screen itself will black out when the maximum aperture (or effective maximum aperture when using TCs) is smaller than f/5.6 in most cases. I know of at least one screen that will work down to f/8.0, but I've never seen a split screen that won't black out by f/11. 1/2 of the split screen section literally goes black. You can generally see the same effect with any slowish lens by moving your eye off center from the viewfinder. You will see 1/2 of the split screen circle go black. This is the blackout I'm referring to. At smaller maximum apertures, you cannot find an eye position that will relieve the blackout. Split screens are useless in these cases.
Remember, modern lenses focus and meter at the maximum aperture to give you the brightest view. That's why the depth of field preview button exists - so you can see what the depth of field will be when the lens stops down to your f-stop setting as it does when you take the picture. The limiting factor with the split screen focusing screens is the maximum aperture of the lens. For example, my 400mm f/5.6L lens is pretty slow with a maximum aperture of f/5.6. However with a 1.4x extender, that aperture goes to an effective f/8 (loss of 1 stop). With a 2x extender, it's f/11. There is no split screen focusing screen that I am aware of that will not black out with a maximum aperture of f/11. I don't fully understand the optics involved, but it's a fact of these screens.
Don't get me wrong, I love split screens and especially split screens with micro-prism collars. I miss them from my old Pentax cameras. I have a Katz-eye Split screen for my 20D, but I ended up not using it due to the metering issues, especially when using partial metering. When I get a 2nd body, I may install it on my 20D, but until then, I'll live without it.
= Ed =
