In article <1995Dec18.214226.21945@schbbs.mot.com>, bobc@ecg.csg.mot.com says...
>In article
>>Wayne Anderson (corky@serv01.net-link.net) wrote:
>>: I'm a photojournalist at a medium sized daily in the midwest. i have >>:a Nikon 300mm/f4 and it's soft at f4. Dissipointing for a $900. lens.
>>Besides Bob Neuman's advice, check that your camera mirror box
>>and focusing screen are properly adjusted. Many a camera have
>>this problem. Focus error is a big killer of sharpness.
>How do we check it? I have always wondered about this. If it sharp at >the viewfinder, how can I be sure it is sharp at the film plane?
I thought of this with the original posting I made, but did not add it,
since you would have noticed a camera focus error more with short
lenses, which I presumed you had used with the same body. FYI, there
are two ways to check, though the second will tell you more about
focus accuracy, and the first will tell you more about alignment (a
camera that fails the first test will probably fail the second, but
a camera that passes the first may fail the second).
-1) Take almost any fixed focus lens (preferably a fast normal to short tele Nikkor), aim it at distant detail (like a tree-edged horizon, or
distant buildings), focus the same distant subject carefully at the
four edges of the frame several times (observe the focus marks on the barrel each time you focus). If the focus is almost always the same in the center of all four edges of the frame, the lens is probably forming an image of the subject that is parallel to the film plane. If there is a difference in focus, try another lens (lenses can form images that
are not parallel to the film plane, and be otherwise sharp).
-2) Put a Nikkor 50mm f1.8 or f1.4 lens on the camera and shoot a newspaper at about 45 degrees at about 2' at f2. Focus carefully on
one letter in a paragraph that you can identify easily on the processed film. Shoot the target 5 or 6 times, refocusing each time. Look at the film with a GOOD 10x magnifier. If you hit focus right-on more than 1/2 the time, and the other focus points are randomly in front and behind the correct point, the focusing screen almost certainly agrees with the film.
As to lens checking, that is another (much longer) story for later.
Hope this helps.