From: Lou Boyd <boyd@apt2.sao.arizona.edu>
Newsgroups: rec.guns
Subject: Re: Who knows about scopes?
Date: 27 Jun 1998 21:37:46 -0400

Rob Malkin wrote:

# Why would a larger lens have greater edge distortion than a smaller one?
# While I could see where a high quality large lens might be more expensive to
# make than a high quality smaller lens, I don't see why edge distortion would
# be inherent with size. Also, why would distortion at the edges lower
# contrast in the center of the field?

IF (big if) everything else is the same, a larger diameter lens
will have more distortion.  It's a fact of optical design.  To
keep the same focal length for the objective the same erector
system, the same eyepiece, etc, the larger lens must be thicker and
the light must pass through at steeper angles.  There WILL be more
spherical aberation and chromatic aberation, and more coma assuming
both lenses are made from the same materials and both have optimized
designs.  All of those distortions show up primarily at the edge of
the field.  Is there necessarily enough difference to be noticed?
No.

It's my opinion that the major arguments against the larger
lens are volume, weight, strength.  That is traded against better
low light performance.  At the magnifications used in riflescopes
the performace isn't diffraction limited.  The smaller
objective will (usually) give slightly superior image quality
(at least in the choice between 40 and 50 mm).

Objective size     smaller   larger

brightness                   brighter (in low light or high mag)
depth of field     deeper             (not important at long range)
parallax           better             (focus is more important)
weight             lower
strength           better             (less mass under recoil)
recoil reduction             better   (heavy barrel works better)
image quality      better             (except at extreme mag.)
cost               lower              (if everything else is the same)
looks               ??       ??       (eye of the beholder)

The only real function of a larger objective is better
low light performance.  Unfortunately the human eye looses
lots of resolution as the scene gets dim enought that the
eye's color vision quits.  Increasing the magnification
AND increasing the objective size helps but this gives only
slight improvement going from a 40 and a 50 mm objective.
Only an additional 5 to 15 minutes at sunset and sunrise.
The 50mm objective only gives 56 percent more light than a
40 but the light level is changing very fast with the sun's
position. For real night vision use a gen III image intensifier.

Lou Boyd
FCSA - NRA