With the chip you have the option of relying on the camera's AF system to confirm focus (similar to switching off the autofocus on a Canon lens). Since the viewfinders of most current Canon DSLRs are not really designed for manual focusing this can be more accurate than relying on your eyes.
I use one with a Planar 85/1.4 to help get the really narrow DOF in the right place when when it's wide open, but I don't use one for my wider Contax Carl Zeiss lenses. I generally get good results with the original focus screen on the 5D, but find the Haoda split screen to be really helpful on the 20D.
The focus confirmation chip makes the body think that it has an AF-compatible lens attached, and so it blinks the little red AF indicator box in the viewfinder when the body thinks the image is in focus. As Cal says, this lets you use the camera's AF system as a visual aid to confirm when the image is in focus. It works fairly well when you select the central AF point only, and you focus with a lens at f/2.8 or faster. I find you get better results if you "home in" on the focus point, by focussing back and forth over the subject, to find the middle of the zone where the AF indicator light comes on - this is the normal MF procedure with an added visual cue from the blinky thing.
My Zeiss Planar 1.4/50 has a pretty narrow DOF, too, at least when wide open. So, will all of the AF points work in the viewfinder? FWIW, I've got a 40D I'll be using most often. Thanks for all the advice so far.
dcains wrote:
My Zeiss Planar 1.4/50 has a pretty narrow DOF, too, at least when wide open. So, will all of the AF points work in the viewfinder? FWIW, I've got a 40D I'll be using most often. Thanks for all the advice so far.
It'll work with any focus point, but not to the degree of accuracy that the centre point will. This is a function of the AF system, not the chip. Turn AF off on one of your Canon lenses and half press the shutter release while focusing. That is exactly what you'll get with a MF lens and AF confirm chip.
Another thing to consider is the Ef-S focusing screen, which is the 40D version of the 5D Ee-S screen. It is a high precision matt screen that really makes things "snap" into focus. I have not used the 40D version, but on the 5D it negates any need for other focusing aids (for me, anyway). I can focus my Minolta Rokkor 58/1.2, wide open, with at least 90% keeper rate.
