Jostein wrote: a function of how "contrasty" a lens is at open aperture.
I second that. I have notuiced, that if a quite contrasty part of the image is located near but not "in" the fucusing point - the camera (*ist D) tends to focus at the contrasty part in stead of a rather dull object in the exact center of the image. I guess, that due to enlargement of a telephoto lens, the focus point also gets bigger - coceres a larger part of the viewfinder image bigger than the small red square that confirms the focising. Is this the case ?? I guess for concert shots (for which I aim to use my 80-200mm) I'll be fine because of the hard light directly in the face on the performer :-) Regards Jens Bladt http://www.jensbladt.dk -----Oprindelig meddelelse----- Fra: Jostein [mailto:[EMAIL PROTECTED] Sendt: 5. december 2005 11:20 Til: [email protected] Emne: Re: A better 70-200mm F. 2.8 I doubt that operator error applies to Jens...:-) Personally, I suspect "back focus" problems to be a function of how "contrasty" a lens is at open aperture. This is by no means a scientific observation, though. The only lens that has really given me trouble with this is the FA*400/5.6, but I don't have a very broad base of experience. The only other AF telephoto I have is a Sigma EX 70-200/2.8, which is very much more accurate than the 400. The longer focal length makes focus accuracy more critical with the 400, so the margins are smaller. That may account for some part of my observation, but not all I think. IIRC, there was a discussion on lens design waaay back, where someone (possibly Pål) claimed that Pentax tend to define the optimal focal point at the back of the acceptable range (in terms of COC). By that time, this description fit very well with my own observations and frustrations. I would guess that if this goes out of control, it would easily become the kind of problem we're discussing now. Jostein Quoting Paul Stenquist <[EMAIL PROTECTED]>: > "Back focus" is a term used to describe autofocus that misses the mark > and locks in behind the subject. I think that it's most often the > result of operator error. Or at least a product of using autofocus in a > situation where one should be focusing manually. > On Dec 4, 2005, at 6:45 PM, Jack Davis wrote: > > > What is meant by the term "back focus"? > > > > thanks, > > > > Jack > > > > --- Jens Bladt <[EMAIL PROTECTED]> wrote: > > > >> I have been trying out three different lenses in the 70-200mm range. > >> Sigma EX 2.8/70-200mm APO > >> Tokina AT-X Pro 2.8/80-200mm > >> SMCP-FA 2.8/80-200mm ED(IF) > >> > >> All of them seem to suffer from Back Focus. The Sigma not much, > >> though. > >> Had I been offered a used Sigma, I probably would have bought it. > >> > >> I published a small test showing the problem. > >> Due to Back Focus (i BELIEVE), my SMCP F-4-5.6/70-210mm is the > >> sharper one > >> at F. 5.6. > >> > >> Please take a look and feel free to comment: > >> http://www.flickr.com/photos/bladt/sets/1509814/ > >> > >> Regards > >> Jens > >> > >> > >> > >> > > > > > > > > > > __________________________________________ > > Yahoo! DSL Something to write home about. > > Just $16.99/mo. or less. > > dsl.yahoo.com > > > > > ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program.
