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Knowledge of Taylor Hobson Projection Lenses?
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PostPosted: Thu Jan 16, 2014 9:41 pm    Post subject: Knowledge of Taylor Hobson Projection Lenses? Reply with quote
I have acquired some TH lenses at good price.





I have heard about the silver ones having short BFL, but what about the painted brass ones?

Anyway if I am unlucky I was thinking about a relay lens to increase BFL. A 10mm achromat should increase BFL by 40mm without image deterioration but with Tstop penalty and upside down image.

Has anyone tried a relay lens solution?


PostPosted: Fri Jan 17, 2014 7:57 am    Post subject: Re: Knowledge of Taylor Hobson Projection Lenses? Reply with quote
JohnBar wrote:
....
I have heard about the silver ones having short BFL, but what about the painted brass ones?

Anyway if I am unlucky I was thinking about a relay lens to increase BFL. A 10mm achromat should increase BFL by 40mm without image deterioration but with Tstop penalty and upside down image.

Has anyone tried a relay lens solution?


I have not tried relay lenses with projection lenses up to now. I experiment with relay lenses with CCTV / Cine taking lenses or to use the Zenitar 16mm fisheye as fisheye on a crop camera.

I work at the moment a lot with Shapley lenses / focal reducters with positive focal lengths - but those shorthen BFL. The other way round would be with teleconverters to get more BFL works with negative focal lengths, a diverging lens.

Cine projection lenses for 35mm film have sometimes long BFL - but not all.


PostPosted: Fri Jan 17, 2014 8:33 am    Post subject: Reply with quote
I've tried to increase the focal length and register distance of a projector lens (Angénieux 50mm f/1.2) using diopters (-2 glasses lens) mounted in front of the lens and it works but the resolution decreases a little and the CA is more pronounced.
I was not satisfied with the result and I gave up.
Probably with negative achromats it works a little better but they are hard to find and expensive. Surplus Shed probably still has some cheaper ones.

ZoneV wrote:
I work at the moment a lot with Shapley lenses / focal reducters with positive focal lengths - but those shorthen BFL.

On the other hand, using positive achromats as focal reducers works quite well in some circumstances.


PostPosted: Fri Jan 17, 2014 1:00 pm    Post subject: Reply with quote
dan_ wrote:
On the other hand, using positive achromats as focal reducers works quite well in some circumstances.


I suppose for not to extreme reduction factor or low speed systems achromats work well. At the moment I try to get a reasonable good high speed medium tele ~ 135/1.4. There I need better optical elements. Have a way, but this would work much better with short register samera like the Sony 7.


PostPosted: Fri Jan 17, 2014 1:32 pm    Post subject: Reply with quote
These lenses are old, so may have a decent BFL. When they arrive I will know.

http://en.wikipedia.org/wiki/Relay_lens

Placing an achromat at 2 x focal length just relays the image without much effect on quality, but light loss.

Bad news is a new relay lens 4/15mm costs about $250 Sad


PostPosted: Mon Jan 20, 2014 2:20 pm    Post subject: Reply with quote
The lenses arrived today and the gods have smiled on me. It seems both can be adapted to NEX etc.

On the 3 inch F1.6 the BFL is 30+mm


It seems they are of Petzval type design see Patent GB376025
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