Re: OT lens design Was :spotting scope question-specific:

jonisaacs@xxxxxxx wrote:
Jerry and whomever else is still bored enough to be reading this....

Some thoughts and comments:


Yes. The 2" f.l. Wray lens we used for making semiconductor masks for a

step-and-repeat machine gave diffraction-limited images at its
wide-open
f/4 and could only be worsened by being stopped down.
----

I am sure there are designs that are not improved by stopping them
down, but I think there are plenty of designs that are . Photographic
lens one example. Is the Wray lens similar in design to the typical
binocular objective? I would guess a semi-conducotor lens probably
optimized for a very close focus and maybe even for a narrow range, and
maybe even for a narrow range of wavelengths, quite unlike the typical
spherical optics used in reasonably priced binocular objectives.

That lens produced images on a 2x2 glass plate that resolved 1000 line pairs per millimeter in a circle about 1-5/8" dia. It was hardly a doublet, but a casual look at the glass diagram didn't distinguish it from a Tessar or an Ektar to my not very experienced eye.

Merely stopping down the fast design doesn't take

advantage of the optimization that can be had by designing for the
smaller aperture.
===

So your claim is that the spherical aberration in an F12.5 lens
probably optimized for F5 would not be superior to an F5 lens of the
same effective aperture operating at F5? I would seem that while as
you say "it does not take advantage of the optimization" available at
F5, these being spherical lenses and all, it would seem the spherical
aberration is probably significantly reduced anyway.

A 100 mm f.l. lens of 20 mm diameter is f/5. An 8 mm lens of the same focal length is f/12.5. If each is designed to provide the best image (by whatever criteria are thought important) at full aperture, the image of the f/12.5 lens will be superior to that of the f/5 lens stopped down to f/12.5.

Coma and astigmatism are also worse in faster doublets (and wider fields.)


These would seem to be improved by stopping down an objective.

I thought that's what I implied.

But the 2" lens isn't designed on the assumption that it will be stopped down, so the actual tolerance that the glass must be worked to is tighter.

-----

Coming from the Astronomy field where 2 inch objectivesseem quite tiny
and larger 100mm F5 and F6 achromats are expected to provide
diffraction limited performance, it doesn't seem that producing high
quality 42mm objectives should be a problem, at least glass wise. I
have always thought that the difference was not in the optical
tolerances but rather in the quality of the coatings. Top notch
binoculars use expensive coatings....

You can expect diffraction-limited performance of a 200 mm f.5 only in a small central region. That's all that's needed with high-power oculars, and at lower powers the less-than-diffraction-limited image in wider views won't be resolved by the eye. The image of a star or other point object is undimmed by high magnification. Sufficient magnification to make diffraction effects apparent is had only with exit pupils of a millimeter or less.

Do you think run of the mill $100-$200 8x42's have objectives that are
compromised by the tolerances ??

Not nowadays. I have a pair of reading glasses from the dollar store that is a model on a Foucault tester. It used to be that I could find hyperboloids and ellipsoids in spectacle stock from AO, but now they're all good spheres.

o, my point here is really to consider the effects of the fact that

eye itself stops down a larger binocular objective during the day and
consider whether the improved view that one often sees in a larger
binocular might be at least in part to the fact that it is being
operated at this longer effective <focal ratio>



sure the effect is there, but one needs to know how to take

advantage of it.

I suspect that the effect is there but I am not sure how significant it
might be. Certainly when it comes to false color, masking the
aperture certainly has the potential to reduce it. But exactly how
much of the false color one sometimes sees is due to the objective and
how much due to other factors is hard to say.

I have done a small amount of experimentation by making aperture masks
for binoculars, this forces the eye to center itself much like your
suggestion but allows one to compare the view at different effective
apertures. I have done this mostly just looking at brightness issues
and never in a situation where false color is apparent. I think it
would be interesting to see if some of the false color one sees during
the bright of the day might be due to not having the eye well centered
and so one is looking through a part of the lens that does exhibit
chromatic aberration...

Another interesting experiment, now on my long to-do list, is a comparison of the images produced by centered and eccentric stops.

One thing to consider, when designing a pair of binoculars, it is
possible that the designer considers the fact that they will most often
be used at smaller exit pupils and may optimize the correction for some
in-between value. My guess would be that the spherical correction of a
42mm objective operating at 16mm effective aperture with a effective
focal ratio of F12.5 is probably minimal anyway....

One good thing about birding, the birds don't go away just because it
cloudy outside.

Another is that the beauty of the subjects makes it easy to overlook small deficiencies in the images. Consider http://users.rcn.com/jyavins/grosbeak.jpg. It is a crop of an image taken through a cheap Baytronix 80 mm f/5 refractor with a WW II surplus Kelner eyepiece with a vest-pocket Fujifilm camera. The most obvious defect is operator error: soft focus. (The setup is shown in http://users.rcn.com/jyavins/photoadapter.htm. I posted that link earlier, but you may not have seen it.)

Jerry
--
Engineering is the art of making what you want from things you can get.
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.

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