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07-25-2013, 07:40 AM   #1
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What do rear elements in a lens do?

I have been experimenting with longer lenses used some for photography but more so converted to a telescope. So I am finding that it is quite common to convert camera lenses to a telescope. Apparently some people just rip the whole rear group of elements out of the lens claiming better image quality with less glass. I have 300mm and 500mm lenses (cheapo t mount) where the rear section including the rear group can be screwed off. Handheld they seem to work fine without the rear group. So what does the rear group do? Is it just for some sort of correction? Fyi, some of the lenses I have seen used have been your average dozen or more element zooms, not just simple t mount ones, and they still ripped the rear group out.

Fyi the 300mm t mount I have was given to me and has fungus in the rear group. It seems glued together. I was contemplating just breaking the elements out or trying more aggressively to break the glued rings free (its of no use in its current condition).

07-25-2013, 08:53 AM   #2
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Hi Ripit,
I read that photographic lenses need multiple elements to correct the various distortions on the photographic focal plane, and to work closer than infinity.
Telescopes might not need all that.
The first mathematically designed photographic lens with rear elements was by J Petzval in 1840.
It is an interesting subject for me because in early stages of engineering school,
we had a physics lecturer who was crazy about optics and we were force fed an unusual dose of geometric optics, both theory and experimental.

So, having more time these days to re-visit that, I purchased last year two photographic lens books from an LFer.

Arthur Cox "Photographic Optics"
Brandt, H-M " The Photographic Lens"

These are older books, about the design of photographic lenses pre computer, and I can recommend both.

Last edited by wombat2go; 07-25-2013 at 08:59 AM.
07-25-2013, 08:59 AM   #3
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I just see that the Petzval is being re-born for DSLR and video
http://microsites.lomography.com/petzval-lens/

Looks like it works rather well !
07-25-2013, 09:24 AM   #4
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Photography requires greater optical precision than visual use. For visual use, we only focus on a small portion of the frame at a time and can get away with using less glass. As you move around the eyepiece your eyes can do minor adjustments for focus. A camera needs everything in sharp focus at the same time.

In addition to focus, chromatic aberrations happen because different wavelengths of light bend differently as they pass through a lens. This effect is more noticeable in a saved photo than through a telescope. Extra elements added to a design can decrease distortion.

2 lenses are adequate for visual astronomy but 3 look better on bright objects. Astrophotography uses 3 to avoid false color, and 4+ to achieve a true flat field. That's all assuming infinity focus. Camera lenses use more elements because you want good color, flat field, ability to focus at different distances, and then we get to zooms that need to do it at different focal lengths. Lens designers have a lot of work to do.

07-25-2013, 10:46 AM   #5
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Original Poster
Thanks for the info. I thought it might have something to do with things like chromatic aberration but didn't even think about things like flat field. I would think while not necessary, having the rear elements would be of benefit, even to a telescope? I would think less aberrations and a flatter field would be easier on the eyes?
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