I have always assumed that over-all optical performance (resolution,
brightness, contrast, field distortion etc) was more or less a constant
and was not dependent on focus distance.
I'm not so sure of this anymore.
A case in point:
If I compare my Bushnell Excursion 8x28 to my Leica 8x32 at normal
birding distances (over 30-40 feet) I can see that the Leica's are
slightly sharper with more contrast etc. However if I focus close, say
12 feet or so this apparent difference disappears at least to my eyes.
Assuming that my eyes are not deceiving me and that the bins are not
defective does this sound possible? Has anyone else seen a slight
difference in optical performance depending on the focus distance?
If so what could be the cause of this?
[QUOTE=Sout Fork]I have always assumed that over-all optical performance (resolution,
brightness, contrast, field distortion etc) was more or less a constant
and was not dependent on focus distance.
I'm not so sure of this anymore.
A case in point:
If I compare my Bushnell Excursion 8x28 to my Leica 8x32 at normal
birding distances (over 30-40 feet) I can see that the Leica's are
slightly sharper with more contrast etc. However if I focus close, say
12 feet or so this apparent difference disappears at least to my eyes.
Assuming that my eyes are not deceiving me and that the bins are not
defective does this sound possible? Has anyone else seen a slight
difference in optical performance depending on the focus distance?
If so what could be the cause of this?[/QUOTE]
I was comparing my Leica 8x32 BN'S to my Nikon Monarch's 8x42's at close focus (about 6 feet) and I noticed a big difference in contrast and a smaller difference in sharpness with the Leica's being superior in both instances so I don't think these qualities are a function of focus distance.
Dennis
I don't have much experience that seems to relate directly to what you are describing. I do see large differences between close and distant performance when comparing binoculars with different objective spacing for several reasons. Is there any difference at all between the spacing of the objective centers? I've found even a little difference will cause the image in the binocular with the narrower spacing to appear larger at 12', but maybe not at 30' or 40'. I've also seen increasing spherical aberration when star testing binoculars and low focal ratio telescopes at close distances. In one barrel of one binocular I own there is astigmatism at close distances that decreases and then disappears beyond about 50'. But, I don't find the increase in SA or the astigmatism to be visible at binocular magnification. I noticed Ed's answer at CN. That applies only to binoculars that focus by varying the distance between the eyepiece and objective. It doesn't apply to the Leica which uses an internal focusing element. It maintains constant magnification at any distance. I don't know how the Bushnell focuses but the difference in any case is small, maybe 2-3%.
I would suggest that you check this more closely starting with tripod mounting the binoculars if you haven't already done that. It's much easier to see small differences on a tripod. Use some kind of target that allows you to quantify the differences you see in "resolution" if they can be quantified. Also try using one eye with each barrel separately to see if a single barrel has a problem and to determine whether you see the same thing using one eye or only when you use two.
Henry
Optical Gear Head