Re: Lens with fixed focal

From: Robin Hull (robin_at_davinoptronics.com)
Date: 09/29/04 

Date: 29 Sep 2004 02:40:58 -0700

Helge Nareid <hn.ii04@nareid.demon.co.uk> wrote in message news:<g8ajl0tufnm3cfn56porka816l80fnaeid@4ax.com>...
<snip>
> As a side issue I'm somewhat intrigued by the assertion that the
> Nikkor 1.8/50mm changes focal length by 0.2mm over its focusing range.
> Where does that number come from and is that figure really larger than
> the measurement error?

If none of the lens separations change, as I believe is the case for
this lens, then of course true paraxial focal length does not change.
Measuring focal length of a wide-ish field lens with significant
uncorrected aberrations is a non-trivial exercise.

I would guess that the original poster is measuring a calibration
value which factors in distortion and defocus to balance spherical
aberration, field curvature etc. In this case a 0.2mm variation with
focus is perfectly believable.

Robin.

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