Roy Pertchik
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 p.2 #3 · 20D Split/Micro Prism Focusing Screen available! | |
Nuyek, you are raising very good points. The fact that the prism is only in the center is a big limit on the usefuess. Still, for whatever it's worth, whenever it's useful, I think it would be nice to use it to at least confirm somewhat that the focus is what i think it is.
In our portrait example, if I had the focusing circle, one thing I would try is to AF on the eyes with the center point, and check that the prism looked good, then I could look at the belly and see if it was good too...the MF prism is now working like a DOF checker, I knew it would come in handy!
Anyway, as you hinted, at f/8 a 17mm on my 1.6 camera (28mm equivelent) focused at 10' has a DoF from 6' to 40' (according to Don Flemings DoFmaster for palm pilot). I don't think focus and recompose is a problem under these circumstances. I could focus on the eyes, and then sneeze, dance a jig, and then point the camera at the belly and still have DoF cover. I also don't think AF should be too tough here either, but sometimes the camera thinks it's tough! If I got a little hinky about the camera's focus for some reason, like low light, or if the subjects eyes are in profile against a plaid background, and maybe my AF sensor is not exactly ligned up with the little red box in the viewfinder (they're not necessarily dead on, you know) well, then I could just look at the prism area and be reassured the plaid didn't distract the AF.
Now, if it were my 85 at 1.8, the DoF would be from 9'-8" to 10'-2", a 4" spread. That would be tougher to focus and recompose. So I'd try it with an off center AF point on the eyes, and I'd look at the prism on the belly, or maybe the arm, and try to find something else that is in focus when the camera says the eyes are in focus, and then I would think if it's reasonable that these two points are in the same plane, and again I'd have a pretty good confirmation that things are ok.
For my really slow set up shots, right now I put C.Fn 4 on 1, which means focus is on the * button. Then I go about my business setting things up, taking exposures and test shots as often as I like, and I do a final focus at the last minute. Sometimes I have to put something into the scene at a point where I want to center the DoF and that happens to hit an AF point as well... the other day I put a ketchup bottle there, focused, and took it away. But I had no visual confirmation! I had no way to know for sure that the camera got it right until I broke the set, packed, took a train home, and down loaded to the PC. It would have been nice to look at that ketchup bottle with a focusing prism. 
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