Re: jA Comment on the Lenses

[Chris Lloyd <cl@astro1.bnsc.rl.ac.uk>]

I've consulted my man again and amidst all the conjugate fields or
whatever I have learnt that it ain't trivial. Any lens system will have a
natural pupil size and position, which lies somewhere within the system,
and is an integral part of the design. Any change to the pupil size will
change its optimum position, depending on the complexity of the lens, the
asymmetry of the components etc. If the layout of the lens is reasonably
symmetrical then the pupil will lie close to the physical centre of the
lens system, and this is the best place to put the aperture [my
interpretation of all the caveats], if you don't have access to software
to do the job correctly. Putting the aperture in front of the lens, while
apparently harmless, is definitely second best and could introduce the
additional vignetting that's been seen, and depending on all sorts of
things could introduce other aberrations as well, but I don't think that
we're in that regime.

So, if there's already a pupil in the lens then that is probably the best
place to put the aperture, but if that's where you've put it then we need
further help. If there isn't, and if the lens is fairly symmetrical, and
if there's a way of doing it without disturbing the lens spacings, the
best place is probably near the middle of the lens. Unless someone has the
software...

Cheers,

Chris


On Mon, 4 Nov 2002, Tom Droege wrote:

> As noted earlier, the wide open I lens is down 5% at the edge (on a circle
> tangent to the sides of the CCD) and 10% in the corner.
>
> I note that now I have stopped down the I lens, it is down more like 20% in
> the corner.  This surprised my feeble knowledge of optics.  I thought
> everything got better as you stopped down lenses.
>
> Possibly an optics expert can respond?  In any case, we are still
> considerably superior to camera lenses which are often 50% down in the
> corners of images of our size.
>
> Tom Droege
>
>