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

 

[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

 

[darinwc]

"1.88 + 1.57 = 3.45 diopters

1/3.45 = 0.290 meters = 11.4""

Barry, I think the spacing adds some complexity to the math but I think its mostly right.

 

[Barry S]

Yeah, I know that these kind of calculations apply to simple thin lenses, but if you apply it to the existing TRTC lenses, they work out exactly as labeled. So I figure it's got to be pretty close.

 

[df cardwell]

Random stuff

• Gundlach designed the lens to step around the B&L license from Zeiss,
splitting the 4th cell into a 5th. When the lens is assembled properly, it will perform like a Protar.
Few TRs, however, are correctly centered, which might limit you to a 3x or 4x enlargement !

• Each component of a Protar (or TR) is the equivalent as a Rapid Rectilinear lens, high quality image of a narrow field, at f/12.5.

• You MUST use a single convertible cell BEHIND the aperture.

• Using two cells, it seldom matters which cell is in the front or in the rear. Try it, see which performs best for you.

• With two cells, you should have a fully color corrected lens.

• With a single cell, you may want to use a strong yellow or orange filter for highest definition. If you are not obsessed by counting pine needles on a tree a mile away, you can probably get along without it.

• With the 21 & 25, you might not quite cover 8x10 wide open. Stopped down, 11x14 with a little movement. Test drive it, check the corners of the groundglass. If it corner detail is important, stop down until you can see a full, round diaphragm.

• With a single cell, each cell covers 8x10 and then some, but mind the corners: if detail is important, look through the cut corners of the GG !

•••••• HAVE FUN ! Expect fine results, not something quaint and blurzy. They were high quality optics a hundred years ago, and they can be today.

 

[Jim Fitzgerald]

Random stuff

• Gundlach designed the lens to step around the B&L license from Zeiss,
splitting the 4th cell into a 5th. When the lens is assembled properly, it will perform like a Protar.
Few TRs, however, are correctly centered, which might limit you to a 3x or 4x enlargement !

• Each component of a Protar (or TR) is the equivalent as a Rapid Rectilinear lens, high quality image of a narrow field, at f/12.5.

• You MUST use a single convertible cell BEHIND the aperture.

• Using two cells, it seldom matters which cell is in the front or in the rear. Try it, see which performs best for you.

• With two cells, you should have a fully color corrected lens.

• With a single cell, you may want to use a strong yellow or orange filter for highest definition. If you are not obsessed by counting pine needles on a tree a mile away, you can probably get along without it.

• With the 21 & 25, you might not quite cover 8x10 wide open. Stopped down, 11x14 with a little movement. Test drive it, check the corners of the groundglass. If it corner detail is important, stop down until you can see a full, round diaphragm.

• With a single cell, each cell covers 8x10 and then some, but mind the corners: if detail is important, look through the cut corners of the GG !

•••••• HAVE FUN ! Expect fine results, not something quaint and blurzy. They were high quality optics a hundred years ago, and they can be today.

Thanks for all of the very useful information. I'll certainly play with the different components and see what I get. If I can use this lens on 8x10 and 11x14 it would be great. I'm only contact printing so ultimate sharpness is not an issue. I am aware of the filter needed on the single component. Thanks for the reminder on using it on the rear only. I usually stop this lens down to 32-45 at least.

Jim

 

[Ole]

...
• You MUST use a single convertible cell BEHIND the aperture.

• Using two cells, it seldom matters which cell is in the front or in the rear. Try it, see which performs best for you.
...

I would replace MUST with SHOULD in the above - as long as you have enough bellows, use a single cell behind. If you run out of bellows before you get to focus, put it in front.

Also, and for the same reason, the shorter focal length should be behind the shutter when using two cells. Since the difference in needed extension in this case is minimal, that could even be said to be a must - unless you're shooting macro.