Triplet apo barlow / telenegative comparison (1 Viewer)

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Paul Corfield

Well-known member

• Dec 12, 2012

• #1

Example of how a triplet apochromatic barlow lens compares to the arrangement of glass in a telenegative group from a zoom camera lens. I've greyed out all the other glass on the zoom lens image just to highlight the telenegative. The barlow is quite a similar design, the main difference is that the glass is cemented differently. Telenegatives also function very nicely when used with an eyepiece as a cheap alternative to triplet barlows.

All my telenegatives are very similar to the zoom lens example I post here. I've seen some other 5 element designs but as yet I haven't found a lens cheap enough.

Paul.

 

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Last edited: Dec 12, 2012

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Paul Corfield

Well-known member

• Dec 12, 2012

• #2

Here's a lens diagram for a Tamron SP 80-200mm Adaptall lens. This is an interesting one which features a 5 element design in the telenegative that I haven't come across in other lenses. I might be able to make a copy of this from all the odds and ends I have collected.

Paul.

 

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Paul Corfield

Well-known member

• Dec 12, 2012

• #3

Tried making a 5 element design which resembles the one in the last post. Mag is quite high in the ones I made. I'd say mine varied from around 2.5X up to over 4X depending on which elements I used.

Here's a couple of photos from 35m (115 feet) and these are both uncropped.

First image is a reference taken with a 1.4X TC and second image is with the 5 element telenegative.

Paul.

 

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cango

Well-known member

• Dec 12, 2012

• #4

pretty good considering the magnification.

You should soon open shop, selling ready mounted tn's!

 

DanC.Licks

AKA Daniel Bradley

• Dec 12, 2012

• #5

Paul,
It is interesting that you can get away with making your own 5 element TN with bits and pieces. I would have thought that the lenses would have to be made specifically for each other. Very interesting and the result is not at all bad!

 

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Paul Corfield

Well-known member

• Dec 12, 2012

• #6

cango said:

pretty good considering the magnification.

You should soon open shop, selling ready mounted tn's!

Click to expand...

If I ever get time then adapting and mounting them to sell as apochromatic barlow lenses for astronomers would probably bring in some good income.

Paul.

 

Last edited: Dec 12, 2012

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Paul Corfield

Well-known member

• Dec 12, 2012

• #7

DanC.Licks said:

Paul,
It is interesting that you can get away with making your own 5 element TN with bits and pieces. I would have thought that the lenses would have to be made specifically for each other. Very interesting and the result is not at all bad!

Click to expand...

Yeah, I was surprised it worked so well and didn't introduce a load of aberrations. I made another one late on which I will try on Thursday. I had time to try it as the light was failing and I'd estimate it was getting on for around 5X mag with pretty good quality. The main lens I'm lacking at the minute is the middle one, a simple double concave negative. I had loads of them laying around up until I had a good clear out a few months back. Usually they aren't much use, until now that is. :-O

Paul.

 

DanC.Licks

AKA Daniel Bradley

• Dec 13, 2012

• #8

If there is one in the Canon or the Sunagor 100-200, then I have one. Send me your address in a PM.

 

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Paul Corfield

Well-known member

• Dec 13, 2012

• #9

DanC.Licks said:

If there is one in the Canon or the Sunagor 100-200, then I have one. Send me your address in a PM.

Click to expand...

You don't get them in telenegatives usually, I've not had any yet. All my old ones came out of old teleconverters. I've still got all the rear ends off of the zoom lenses I've took apart and I might be able to find one in them. I've got one nice one here but it's quite large in comparison to the other lenses I'm mating it with and that came out of an old 6 element ED eyepiece.

Paul.

 

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Paul Corfield

Well-known member

• Dec 13, 2012

• #10

In the telenegative you get a half negative lens that is flat one side and concave on the other. I found that getting two of them and putting them back to back makes a nice negative group.

Paul.