----- Original Message ----- From: Rob Brigham Subject: RE: AF speed (was: Silly Photo.net thread)
> But DOF is not an issue for focussing - the camera focusses wide open. > It doesnt stop down until you take the shot. When you talk about the > subject leaving the plane of focus before the shutter fires this is > shutter lag. Lens performance aside, the camer always gets the subject > in focus at f2.8 if it is an f2.8 lens. DOF is an issue for focussing. You are treating the subject as if it is two separate issues, and you are wrong. Focus with an AF (or any) camera is dependant upon the subject being within the plane of acceptable focus at the moment of exposure. This is an interpendant issue of camera and lens, as they work as a single unit. With an AF camera, the sensor must be perfectly aligned to the focal plane to achieve perfect focus. This means absolutely no allowable margin of error. Remember, we are talking about wavelengths of light here, so any error, even a few millionths of an inch will make focus dependant on DOF. Then the focus electronics must be fast enough to make focus decisions, and be able to actuate the AF motor, which in turn must transfer that torque to the lens's focussing elements. Now focus is also dependant upon motor torque and AF gear ratios. Focus is also dependant on how fast the subject moves after the camera has confimed focus and begins the exposure cycle, as after the mirror starts to move, AF measurment is impossible. Predictive AF is the camera guessing about where the subject will be, based on where the subject has been. It may or may not be accurate. Most likely, not. Oh yes, since it is a mechanism, there must be an element of slop involved to allow the moving parts to move. Not much, but we are talking about wavelengths of light as being the limiting factor for allowable error. There is a reason for why Nikon F5's cost a lot of money. It has very little do do with the nameplate, and very much to do with how the body handles the tasks presented to it. I have been in situations with my MZ-5 where the camera has been unable to secure focus on a dog running towards me in a daylit outdoor area. I have found Pentax AF performance rather dissapointing. William Robb - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
