Unfortunately, some lenses (particularly cheaper ones) suffer from focus shift when the aperture is changed. Focus correctly at the maximum aperture, adjust the aperture, and the focus shifts. I've done some tests with my lenses to verify that this effect is real, and it is real with the 4-element lenses I tested. IIRC, the effect was undetectable with my 6-element Nikon el-Nikkor f/2.8, though.Originally Posted by Ed Sukach
Personally, I prefer focusing at the aperture I use for exposing the print simply because when I use the maximum aperture for focusing, I often forget to reset the aperture before exposing the print. That makes for a lot of wasted paper (and chemistry, since I usually don't discover my mistake until I've at least begun to process the print).