I recently did a quick resolution check on my 28-105USM lens with my EOS 300 body - doing succesive exposures at 28, 50 and 105 focal lengths at wide open (effectively worst optical theoretical performance notwithstanding very small apertures where difraction plays a big factor) and F8 (arguably the theoretical optimum aperture) I used the single center AF sensor and all of the shots were of distant city scenes (across the Thames) where the camera should have been focusing at infinity. When I got my trannys back I was concerned to see that all the pictures at 28mm were very out of focus!!! - 50mm were better and 105 generaly spot on. I also noticed that when you focus at 105mm and zoom out to 28 the camera refocuses to much less than infinity on the lens scale and vice versa. Now this may be a function of the lens not keeping the same focus at different lengths but I may have a faulty AF or the AF sensor in the' budget' bodies is not up to doing the very fine focusing delineation that is required with wide lenses (I know you get more DOF with wide lenses but the actual focusing is more critical). Regards

John

-- John Griffin (john.griffin@millerhare.com), February 01, 2002

Answers

>When I got my trannys back I was concerned to see that all the pictures at 28mm were very out of focus!!! - 50mm were better and 105 generaly spot on.

Did you refocus the lens each time you adjusted the zoom setting?

Every zoom setting has a different optimal focus point - you can't simply focus at one zoom setting and then adjust the zoom at will without compensating for variances in focal length.

-- NK Guy (tela@tela.bc.ca), February 01, 2002.

another possible reason for out-of-focus shots is a shutter speed slower than 1/focal length.

-- legnum (legnum@email.com), February 01, 2002.

I just tried this with my 28-105 and Elan IIE. At 28mm and 105mm, it focused at approximately the same place (not exactly at the same place). At 105, with repeated tries, it focuses at the same spot much more consistently than at 28mm, which tends to have some variability (around the infinity mark). None of my AF attempts at distant objects came anywhere close to the last marked distance (20 feet) on the scale. These results seems to be consistent with focusing results using manual focus. Focusing with a 105mm (or a fast 50mm) is very repeatable and sure, since the focusing aids, like a micro prism, snap into clear focus with great precision. Focusing a 28 or 24mm (2.8 or 3.5), is more difficult to determine exact focus because the micro prism is less discerning, and you rely on the ground glass and spit image focusing aid. Repeated focusing with WA results in slight discrepancies. Of course in the old days, if the subject was far away, you just racked the lens back to infinity and shot (focusing ring could not go any further).

I have never noticed any problem with sharpness in slides or prints, probably because such focusing discrepancies are well within the lenses DOF. In fact the 28-105 is certainly sharper at 28 or 50mm, than at 105.

Using an EOS 3 or 1, or 1v would not help because the "high precision" AF sensors revert back to normal precision AF sensors when slow lenses (like the 28-105) are attached.

-- kenneth katz (socks@bestweb.net), February 02, 2002.