• True, von Hoegh, and I sometimes get VERY annoyed when a wide angle ALMOST (but not quite) focuses to infinity. I usually take apart said lens and make a minor adjustment.

But, all, don't forget this. As bernard_L infers, there is, indeed, a difference oftentimes between apparent (viewfinder) focus and real (film plane) focus, and such is usually caused by the mirror angle being a tiny bit off. Do this: take a picture of a 'picket fence' at 45 degrees with the lens wide open and camera rigidly mounted. Focus PRECISELY at a certain point, VERY carefully. Then process the somewhat underexposed negative and achieve a bit more contrast with extra development. Inspect the negative carefully in the enlarger. Find out exactly WHERE the REAL focus is. If it is EXACTLY where it was in the viewfinder, you are OK. But if it is off, that means that the mirror has to be a tiny bit raised or lowered. Some cameras let the mirror rest upon a set screw (Fuji ST series, Soviet Zenit) and some let it rest upon a metal prong (K1000, SRT). That front part of the mirror has to be either raised or lowered. Remember ALWAYS, that mirror has NOTHING to do with REAL focus and exists ONLY for 'telling' the viewer what is in focus. We do not want that informational feedback to be faulty!

Make the slight adjustment so that the viewfinder and film plane are precisely aligned. Rule of thumb: If the viewfinder focus is BEHIND the film plane focus, the mirror needs to be slightly lowered. If the viewfinder focus is BEFORE the film plane focus, the mirror needs to be slightly raised.

Easiest way to do this without going nuts: find, precisely, the exact location on the lens's distance scale that a random point on the picket fence is in precise focus ON THE FILM PLANE. Now you know the precise focus for that particular point. (Make certain that you can repeat this, including the precise position of the camera.) Then raise or lower the mirror so that, at that camera location, that point on the fence is in precise focus IN THE VIEWFINDER. You will have matched viewfinder with film plane. Of course, again, keep the lens WIDE OPEN so that you limit depth of field.

Finally, for those brave souls who find this 'fantasy' I have posited too unbelievable to believe: take sliver of paper (about the thickness of a postcard) and carefully place it under the mirror at the point where the mirror rests upon (ie, set screw or prong). NOW, put the lens on the camera and focus. You will think that you are focusing at infinity when you will REALLY be focusing at about 15 feet. In other words, if you took a picture, you would THINK that focus should be at infinity but it would REALLY be at about 15 feet.) In this case the mirror needs to be lowered (ie, lowered, because the paper raises it too high!). In other words, (see FIRST 'rule of thumb, above) the mirror (with paper underneath) is causing the viewfinder to seem to focus at infinity when the REAL (film plane focus) would be at only about 15 feet.

Moral of the story: tiny differences in mirror height make quite a difference in matching viewfinder information with actual information. - David Lyga