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Re: [OM] focus offset

Subject: Re: [OM] focus offset
From: Gary Reese <pcacala@xxxxxxxxxxxxx>
Date: Wed, 24 May 2000 21:12:17 -0700
Dirk writes:

<< So, are macro lenses "flat field"? >>

Depends. The term "macro" got used a bit fast and loose by marketing
folks. A macro focusing zoom isn't likely to be flat field. Perhaps
cheap macro lenses aren't, either. But there is only so much flatness
that can be incorporated into a macro lens before it won't precisely
focus on a body with focus offset as part of its design. Joel W. pointed
out a posting on the Nikon list where a Micro-Nikkor required a bit of
focus shift to get the focus right on the film (as opposed to the
viewfinder). That sounds like a perfect example of making the projected
image too flat.

<< I assume this is because the depth of field under macro conditions is
extremely small, and any significant curvature of the projected image
will seriously degrade the resolution? >>

Yup.

<< Are macro lenses typically also "flat field" at infinity focus, or
just 
at a predetermined reproduction ratio? >>

I don't know. I presume that distance focusing aberration correction
(floating elements) keeps the image flatter than it otherwise might be
at infinity. But there are lots of other faults to correct and maybe
that isn't one of them. Curvature of field would be less important an
attribute to correct for when shooting 3D subjects, although one could
argue that at infinity it is very important. I think we can assume that
at the optimum magnification ratio, flatness of field is optimized.

Gary Reese
Las Vegas, NV

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