Reversing Diopter Lens

[gregoryzim]

Hi I have a - 1.5 rated diopter - if I turn it back to front - does that give me + 1.5 or something else - anyone know?

Tks

 

[retinax]

No, the power remains the same, but it might simply not work well if it is an asymmetric element. I'm sure you have glasses as you ask a lot about diopters, just look through them from the other side - you still see sharply in the center, less so away from center.

 

[gregoryzim]

Well how about that 🙂 I never realised my glasses could be reversible.

So yes - I have glasses but I am asking suddenly because I moved back to rangefinder film cameras (essentially decided to stop displaying my collection and use them - but also have collected medium format too over the years ) from very modern digital with adjustable view finders and so thought nothing about it all as my eyes were changing over the years .

Suddenly I notice my right eye has changed a fair amount having just had my eyes tested 5 years later - and I cannot focus some of the cameras with my glasses on and none with them off and my prescription shows my own diopter for example in my right eye is +3.5 long distance and +2 short distance (reading , camera etc)

My left eye is less in all cases. Together in normal life - can't tell the change.

So one camera is worse than any other - the finder on an ETRS medium format Bronica fitted with a -1.5 diopter prism behind the eye piece.

I went to the chemist tried on those cheap reading glasses with the camera up to + 3.5 but not much use - and of course the -.1.5 was still in the finder. (I was going to go back with the prism out - but does where it sits in the finder mean a lot ? (IE: If I replace it with wearing a +2 pair of glasses IN FRONT of the eye piece - does that really tell me anything usefull?)

The optician said I need basically a +2 diopter with no glasses on - but the world via the internet at least seems to have run out of Bronica etr AEii +2 finder diopter prisms.

Someone else on here once suggested in another post making your own with one of those cheap pairs of glasses - so my real question and search and education is about that - the prism in the finder though is a lens glued to a lump of clear glass - so I wanted to separate that and re fix a +2 lens to the lump 🙂

Do I need the lump of backing glass (distance??) - anyway - its a new education subject for me and my eyes are not getting stronger - so to really enjoy life on these cameras it would be good to solve this little problem. All help and teaching appreciated tks. I meant to ask - what are minus sizes (-1.5) - are they also "magnifications" - if so where is zero start?

 

[retinax]

Zero is flat glass. Dioptres are the inverse of the focal length in meters or some such. I believe you can imagine it like this: E.g -1 is to bring a shortsighted eye, the furthest focus of which is at 1m, back to infinity focus, -2 is for an eye that furthest focuses at 0.5m and so on, positive powers work the other way around. You can also see that a +1 element will focus an image of e.g. the sun onto a surface one meter from it, if you have a loupe, see at what distance it focuses an image onto a surface, if it's e.g. 12,5 cm you have a +4 element.
I've also replied to your other thread. And again, for most cameras, the correction eyepieces are just add-ons, no need to mess with the cemented doublet. Are you sure that is a correction eyepiece? Many cameras come with something between -0.5 and -1.5 as default because that works for the largest demographic. As you are farsighted, drugstore reading glasses should work. And no, the precise distance doesn't matter, you can try the reading glasses on your face, if you see sharply with them through the camera's finder, it'll work stuck into the eyepiece.

 

[gregoryzim]

Tks much appreciated. It is a standard -1.5 that Bronica puts in all their finders (average across population as you say)

If you want to improve on that they sold a range -2.5 to +1.5 - where optically +.5 is equal to an eye prescription diopter of 2. So I need a +.5 bronica replacement - which I have found on ebay - but will test with a + .5 glasses lens first given they are only $5 at the pharmacy.

 

[retinax]

Tks much appreciated. It is a standard -1.5 that Bronica puts in all their finders (average across population as you say)

If you want to improve on that they sold a range -2.5 to +1.5 - where optically +.5 is equal to an eye prescription diopter of 2. So I need a +.5 bronica replacement - which I have found on ebay - but will test with a + .5 glasses lens first given they are only $5 at the pharmacy.

I hope you have reliable information about the Bronica correction diopter – camera makers differ in their labeling, some write the actual optical power of the thing that you'd need to add to the power of the standard eyepiece, others label them with the power that results when added to the standard eyepiece. Your calculation sounds like neither of these options - how would one arrive at +2 from adding 0.5 to -1.5? I would think that should result in either -1 (which you'll get with +0.5 reading glasses, not enough!) or +0.5...?