TRT lens.. set me straight!

[Jim Fitzgerald]

I have a TRT triple convertible in a Betax #4 shutter that has not seen enough use. I picked up another front component from Jim Galli a while back and I've used that front component to make this lens a 14". The standard configuration is 12",21" 28". The other front component I got from Jim is a 25" if used on the rear. Can one of our lens gurus please explain to me how many possible lens configurations I have? Can you use single components on only the front? Only the back? I'm a little confused and this lens lights up the 11x14 and I want to see what it will do. Curious about the other possibilities. Any help would be greatly appreciated.

Jim

[Curt]

It sounds like you have a 12,14,21,25,28 lens. The 12 is with 21 and 28, the 14 is with 25 and 28. You get 21 alone, 25 alone and 28 alone. What's that add up to 5 possibilities?

[Jim Fitzgerald]

Curt, that is what I kind of thought, but I wasn't sure. Sounds right to me. Thanks.

Jim

[Greg Davis]

What happens if you put the 21 and 25 together?

[Barry S]

What happens if you put the 21 and 25 together?

Ok, add 11.4" to Jim's arsenal.

21" = 0.533 meters = 1.88 diopters

25" = 0.635 meters = 1.57 diopters


1.88 + 1.57 = 3.45 diopters

1/3.45 = 0.290 meters = 11.4"

[Jim Fitzgerald]

Greg, good point! This is why I posed the question. I can't do the math? I hope Ole or maybe Jim will chime in. Thanks.

Jim

[Jim Fitzgerald]

Barry, thanks for the info. I must have been typing at the same time you were.

Jim

[Barry S]

No problem! I just got a 12"/21"/28" TRTC myself and I'm just waiting to get it back after some rehab of the Betax Auto shutter by Carol at Flutots. I like the idea of finding an orphan cell or two to make your own casket set.

[Deckled Edge]

I have a TRT triple convertible in a Betax #4. Can you use single components on only the front? Only the back?

Jim

Jim,
My knowledge of optics is far more limited than most, but I have several TRTCs and was always told that the inner element of each cell must see the light first when used alone. That is, a solitary front element must be reversed to use on the back and the back element cannot be used alone on the front. The geometry of the glass components dictates this mathmatically. This rule is canceled when two cells are used as they correct one another. Additionally a yellow or better filter is required to narrow the wavelengths on single cells, as correction is limited.
You state that this TRTC lights up 11x14. Does the 12" fill the frame? I've only tried it on the 'dorff.

[Jim Fitzgerald]

I thought I read somewhere that you were only to use the single component on the rear of the shutter with a yellow filter in place. I have done this on occasion and the results have been good. I just didn't know if there is something I'm missing with the different components. I have not had a chance to check the coverage on the 12" TRT yet. Sick with the flu. I'll let you know.

Jim