Odd result. Lens focus and diopter adjustment don't interact quite that
way. The lens creates an image on the focusing screen, the sharpness of
which is entirely dependent on lens focus. The prism and eyepiece lens
system then inverts that image and presents it to the eye. Think of the
focus screen as like a slide and the eyepiece system as like an
adjustable focus loupe. If the slide is out of focus, there is no
adjustment of the eyepiece dipoter correction that will yield a sharp
image.
In an SLR, if the diopter is out of focus, even perfect focus on the
screen will still apperar unsharp to the eye. However, the sharpest
image resulting from focusing the lens, even if still somewhat unfocused
appearing, will still be at the correct focal distance. If an object is
at 20 ft., it will not appear to be in focus at 30 ft. because of
diopter misadjustment. Rather, it will appear even more out of focus
than at the correct lens focal point.
With split image and/or microprism screen, focus on some nice, clear
line, like the edge of a window, door or building. The method these
focusing aids use, aligning images split by prisms, is unaffected by the
diopter setting. Then set the diopter wheel by focusing critically with
it on the matte part of the screen on the adjacent part of the building
which is at the same distance.
If that doesn't work, there is something out of whack in the camera.
None of this applies to the special clear screens that use aerial images
rather than matte surfaces.
Moose
IanG wrote:
Does anybody know of a quick and simple way to set the dioptic adjustment
accurately? I've just got back a test film on a 100/2.8 to find that I
thought I had focused on a target 20ft away but was actually focused closer
to the 30ft mark <gulp>.
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