Bronica GS-1 and SQA prism finder diopter compatibility

[brianentz]

I have a Bronica GS-1 system I've just put together, but the diopter in the prism finder is really not the right one for me. And I'm having a hard time finding other options on Ebay or wherever. I think I need a -1.5 strength diopter for my prism finder (not rotating type) and can't find one anywhere. However, I have found one but for the SQA prism finder S. They look identical and I'm guessing that the -1.5 for the SQA is identical in strength to the -1.5 for the GS-1 prism finder?? I'm considering buying that one and giving it a try even if I have to shave off some of the plastic mount or build it up a bit.

Any input on this?? Thanks in advance.

 

[brianentz]

Actually, they don't appear identical. The SQA is more squarish, as to be expected. Still, maybe acceptable?

 

[reddesert]

I don't know the answer to this and don't have a GS-1 prism. However, I have examined some Bronica eyepiece diopters, so here is some information that may or may not be helpful.

- It's fairly easy to extract/unglue the eyepiece from its frame, so if you needed to swap a different piece of glass into the frame you might be able to. The SQ and GS eyepieces are likely not exactly the same, but you might make one fit, or get an optician to make you an appropriate piece of glass.

- I did measure the focal lengths of a few ETR and SQ eyepieces. They are all positive lenses. -1.5 diopter is the default eyepiece. The focal lengths of ETR +0.5, -1.5, -2.5 eyepieces are about +9, +10.5, +13 cm. The focal length of the SQ -1.5 eyepiece is about +14 cm. This makes sense because the SQ prism is bigger and the focusing screen is further away, but by extension it also means that an SQ -1.5 eyepiece might translate to about a -1 to -0.5 eyepiece on the GS.

- The way this works is that the focal length of the eyepiece combines with the actual optical distance from eyepiece to focusing screen (including the reflections in the prism) to produce the labeled diopter. The focusing screen is only about 8-10 cm from the eyepiece, but the eyepiece makes an apparent image that is further away so your eye can focus on it.

Diopters are 1/meters, thus a -1.5 diopter is intended to produce an image of the focusing screen at an apparent distance of ~0.67 meters. If you are nearsighted you need a more negative diopter (like -2.5), and if you are farsighted or need reading glasses, you need a more positive diopter, which translates to a shorter focal length, stronger positive lens in the eyepiece.

It is easy to measure the focal length of the eyepiece lens you have. Hold it a short distance away from a wall so that it projects an image of a window on the far side of the room onto the wall. Move it until the image is in focus, then the distance from lens to wall is the focal length. You can flip it around, measure again, and take the average for a tiny bit more accuracy.

 

[brianentz]

Thanks for your extensive response. Very knowledgeable. I did buy a -1.5 for an ETRSi and it fits loosely in my prism for my GS1. It is a big improvement over the mystery one that came in the prism. Then, luckily, I came across a -1.5 for the GS1 prism that had been mis-labeled by the seller as a +1.5 for the GS1. The -1.5 is virtually impossible to find and sells immediately when it comes available- I was told by a dealer in Japan. I’m waiting for it to arrive.
I talked to my optometrist about having a small lens ground but I’m not sure something that small is possible.