Bronica ETRS diopter trouble

Discussion in 'Medium Format Cameras and Accessories' started by hadeer, Feb 18, 2007.

  1. hadeer

    hadeer Member

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    Is there anyone who can help me with the following:
    I am nearsighted but wear contact lenses to correct this. So I have sharp vision from about 2 meters to infinity and use reading glasses for the rest.
    However, I prefer viewing the GG of my Bronica ETRSi using the flip-up magnifier without the reading glasses, because this enables me to see the whole area. But that leads to a problem:
    The standard -1.5 diopter magnifier (at least I think it is the standard magnifier, there is no inscription on it) does not permit me to see a sharp image. This simulates a viewing distance of 26.3" (66.8cm) according to an information page of Tamron (http://www.tamron.com/bronica/prod/etrsi_eye.asp) It has a focusing distance of about 7.5 cm. To correct the problem I sought and found on the internet a +3 diopter lens (focusing distance about 6cm) but this only made the problem worse. Apparently I drew the wrong conclusions from the Tamron site.
    Is it possible to calculate the right diopter for my situation?
    I appreciate any help that helps me avoid buying another useless corrective eyepiece.
     
  2. David Henderson

    David Henderson Member

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    I think that without knowing what your prescription is without your glasses you are going to struggle. I have no doubt that your optician could give you this information and help you translate Tamron's table. Ostensibly this last bit looks easy- looks like you need your prescription -1.5 dioptres.

    You then of course have an interesting issue- which is that given that there doesn't appear to be anything inscribed on the lens itself (and I've even unscrewed that on my SQ WLF) then how can you be sure that the existing lens is -1.5 and indeed how can you be sure - without assured original packaging- that any replacement you buy is what you think you're buying. Again you optician may have an easy way of testing this.

    These things do come up pretty cheaply on eBay. So trial & error whilst reselling what you can't use is another possibility. But think on this- on the lens you bought , how do you know its a +3 and how do you account for the fact that there isn't a +3 on the Tamron link you provide? If you were going down the trial & error route it sounds like you might be a very little long-sighted with your lenses but no glasses, and so if you could get a - 0.5 (again not listed) or a zero (again not listed) or even a + 0.5 then this might be the right area to start in. Best of luck
     
  3. hadeer

    hadeer Member

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    Thanks for your suggestions. I forgot to mention that the replacement lens I bought has '+3' inscribed on it. I do have the prescription and that was the basis for buying the +3 lens, but I think I made a miscalculation because when wearing contact lenses the prescription might not have any meaning for an image on a ground glass at an apparent distance of 26.3".
    I agree that the trial and error route might be the easiest way to find the right correction lens. I think minus 2.5 would be my best bet as moving in the plus direction made things worse.
     
  4. Steve Smith

    Steve Smith Member

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    You can work out what the Dioptre of your current lens is by using it to focus something such as a window onto a wall. Move it about until you get a sharp image and measure the distance in metres from the lens to the wall.

    One divided by this focal length will give you its strength in Dioptres.

    e.g. If you measure 0.6m (600mm), strength is 1/0.6 = 1.66D

    To work backwards, if you assume that you have a 1.5D lens, focal length is 1/1.5 = 0.666m or 666mm So if you get an image with the lens about 660mm from the wall then you do have a 1.5D lens as you suspect.

    Steve.
     
  5. David Henderson

    David Henderson Member

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    "I think minus 2.5 would be my best bet as moving in the plus direction made things worse."

    Look, I'm no optician but I think that what happened is that moving in that direction -and to that degree- made things worse and that doesn't mean that you don't need a "plus" something - it means that +3 is further from what you need than -1.5. If the right answer was zero then you'd see worse with +3 than with -1.5.

    To opt for a -2.5 now would I think imply that your prescription is about -1 with lenses and no glasses. Which would mean a little short sighted. However you've told us that you can see well from 2 metres to infinity like this- which implies a little long sighted. Still if its trial and error its your trial and of course ----. Just trying to help.
     
  6. hadeer

    hadeer Member

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    Steve, David,
    Thanks for your help. I tried to figure out the diopter of the original lens using the hint from Steve. The lens projects a sharp image at 6.5 cm. That would mean its diopter is 1/0.065=15.4D. The new lens has a sharp image at 6 cm which means its diopter would be 1/0.06=16.6D. On the lens it says +1.5D. Now I am confused.....
    Hans
     
  7. Steve Smith

    Steve Smith Member

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    Yes, that does seem confusing. Perhaps what is happening here is that the standard lens has a focal length equal to its height above the focussing screen i.e. 6.5cm which would be correct for viewing with your eye focussed for distance (infinity). It could be that the lens marked +1.5D is actually whatever strength the standard lens is plus an extra 1.5D which would reduce its focal length a bit as your results show.

    This would make sense if the +1.5D marking is seen as a correction rather than the overall strength of the lens.

    Steve.
     
  8. hadeer

    hadeer Member

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    I think this must be the explanation. It fits with the description on the Tamron site where they talk of corrective viewfinder eyepieces. I'll go with the -2.5 lens if I can find one.
    Hans
     
  9. dancqu

    dancqu Member

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    I've the AEIII prism finder, the waist level, and the Rotary.
    The first is, for myself using a tripod, a pain in the neck.
    The waist level is good only for landscape mode. Some
    real relief came when I got the Rotary; bright, clear,
    and sharp. Makes the outfit very usable whenever
    tripod mounted and off-camera metering OK.
    A new bright screen was also a big +. Dan
     
  10. hadeer

    hadeer Member

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    Does it have a variable diopterlens? I would like to give it a try if I can find one.
     
  11. dancqu

    dancqu Member

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    Reading from instructions: The standard eyepiece
    is -1.50 diopters. Minus 4.50, 3.50, 2.50, 0.50, and
    plus 0.50, and 1.50 are available. I see the waist
    level finder has the same selection. They may
    be one and the same.

    You are farsighted with contacts. Years ago I wore
    contacts and IIRC some time must pass after they are
    removed before the eye adjusts; is back to normal. I'm
    about 1 meter + farsighted and use the likely minus 1.5;
    will check. I wouldn't bet on it but the minus 2.5 may be
    what you need being more farsighted with contacts.

    Plus 3 is a very strong reading glass and a world apart from
    minus 1.5. I use a drugstore pair for close examinations.
    BTW perhaps a nearby drugstore has a selection. Dan
     
  12. Richard Harris

    Richard Harris Member

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    When i needed diopter adjustment for my bronica I used the following method.
    I just added the diopter adjustment of the reading glasses that i found best for viewing to the diopter adjustment of the fitted lens, normally -1.5. I found that I got a better view and focus ability with +1 diopter reading glasses so fitted a -0.5 lens to my prism finder; perfect.
     
  13. Richard Harris

    Richard Harris Member

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    just a post script. your try with a +3 dipter would have given you a correction of +4.5. You would need to be as blind as the proverbial to need that much help.
    By the way if you need reading glasses you do not want to be movin in the minus direction
     
  14. hadeer

    hadeer Member

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    As it happens, I ordered a -2.5 diopter some weeks ago. It must be in customs somewhere which takes a long time sometimes, they are not very service minded to say the least. I'll keep you posted about its effects once I receive it.
     
  15. dancqu

    dancqu Member

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    Here's an update. The waist level and rotary do
    not share the same eyepiece. The rotary's is more
    substantial. If you go for a rotary some day you may
    wish for the non-metering eye level prism. On occasion
    I could use a little more heighth. May purchase. Dan