jaapv Posted March 5, 2011 Share #21 Posted March 5, 2011 Advertisement (gone after registration) ummm- two meters actually. Link to post Share on other sites More sharing options...
Advertisement Posted March 5, 2011 Posted March 5, 2011 Hi jaapv, Take a look here Diopter Assistance. I'm sure you'll find what you were looking for!
tdtaylor Posted March 6, 2011 Author Share #22 Posted March 6, 2011 ummm- two meters actually. About now I am getting really confused. Actually ordered a couple from B&H, and will ferret out what works. So, inquiring minds what to know- why 2 meters? Link to post Share on other sites More sharing options...
hoppyman Posted March 6, 2011 Share #23 Posted March 6, 2011 (edited) As best as I understand it, the comment taken from the Leica website does not cover every situation. Possible complications (some noted earlier in the thread). Progressive or bifocal spectacle lenses (varying prescription in different parts of spectacle lenses for near/far correction. Astigmatism Aspherical spectacle lens design, if any (eg. for a thinner/ lighter design, improved aesthetics with same optical performance). Where the optical axis (centre) of the spectacles lens is vs. how you position your eyeball in relation to the viewfinder eyepiece. Contrast (more imperfect lenses/surfaces will also reduce that) Eye relief The degree of accommodation that your eyes have. This declines with age. Sound familiar? I did some searching to try to improve my understanding. Informed corrections/comment welcome of course and maybe we can then add to a FAQ Maybe these summaries help. ................................................................. Accommodation Ability to change focus from far to near objects. Declines with age. Acuity Clearness. your ability to see small detail. Measured by your ability to distinguish smallest letters on a chart. Normal is considered to be 20/20 or 6/6 (feet/metres). What you see at 6 metres is the same as a person with normal vision sees at 6 metres. It is possible to have vision superior to 20/20: the maximum acuity of the human eye (without visual aids) is generally thought to be around 20/10. Astigmatism That is an aspherical cornea surface. Light image focuses on two separate points in eye. The amount and angle of that error is the Cylinder and Axis part of your spectacles prescription. You cannot correct for astigmatism (the cylinder and axis part) with a standard screw in (spherical) correction lens. Depth of Focus The range of distance that objects are in (acceptable) focus. Depth of focus is directly affected by accommodation as well as aperture (pupil size) of course. Diopter (dioptre) Measure of the refractive power (focus, not magnification) of lens. It is a recriprocal of 1 metre. Plus 1.0 diopter is a convex lens that will focus (converge) parallel neutral light rays as a visible image 1 metre past the lens. 2.0 diopter to one half metre etc 0 is flat (Plane) Neither converging nor diverging. Minus 1.0 diopter is a concave lens that diverges the rays. So it produces a virtual image that appears to be one metre in front of the minus lens ( if you extend the diverging rays forward. It would not actually focus as a visible image on an optics table). Minus 0.5 (standard fixed eyepiece) gives a virtual image at two metres. So if you want to duplicate this condition, you need the sum of the diopter values of the camera eyepiece, screw-in correction lens and any spectacles to match that. A range of values may still work. There should be a most comfortable point though. However that may not be at that theoretical best value? The Sphere (spherical) part of your prescription is the value to calculate with. Remember that the Cylinder and Axis values (if any) are related to astigmatism. Someone else will need to chime in to explain why a virtual image at two metres is used in the camera. I cannot visualise how that works personally. Best suits the Leica demographic of mature eyeballs? Eye relief Maximum distance from last surface of eyepiece that still gives full viewing angle (no vignetting). Affected by magnification (0.68x in case of M9) and diameter of the eyepiece. Too far back means you can not see the whole field of view, eg. framelines visibiilty. Exit Pupil The width of the cone of light that is available to the viewer at the exact eye relief distance. Larger than eye pupil allows for imperfect pupil positioning to avoid vignetting. Magnification Enlarging the image, not refocusing it. M9 magnification is 0.68x to allow visibility of whole field within diameter of eye piece. This is relevant to using 1.25x or 1.4x magnifier with or without a correction lens. Critique and correct away if you are better informed than I:) .............................................................. Here's my experiment. I am assuming that the frame lines and focus patch are in the same plane as the virtual image. From memory with an SLR camera, varying the diopter showed different positions where finder image and finder graphics were sharper for me. I tried to keep each look short to avoid accommodation and tiredness influence. My left (shooting) eye prescription looks like this: Sphere -0.25, Cylinder -0.50, Axis 60 (a little nearsighted plus astigmatism). Right eye sphere is -0.75 I have two correction lenses got cheaply on impulse, a -0.5 (smallest increment) and a Plus 3.0. Sum of left eye and fixed eyepiece is -0.75 for a virtual image at 1.33 metres (works best for me) Sum of right eye and fixed eyepiece is -1.25 for a virtual image at 0.8 metres (second best for me) Left eye, fixed eyepiece and -0.5 is -1.25 (same as right eye with no correction lens). Right eye, fixed eyepiece and -0.5 is -1.75 for a virtual image at 0.57 metres (third best for me) Left eye, fixed eyepiece and +3.0 is +2.25 for an image (actual not virtual) at 0.44 metres Right eye, fixed eyepiece and +3.0 is -1.75 for an image (actual not virtual) at 0.57 metres. Both hopeless but objects 150mm or so in front of the viewfinder window are sharp. Adding diopters is how reading glasses work. My reading and computer glasses are also progressives and add +2.25 to each eye somewhere in their range and work best the range from about 0.3 metres to 0.8 metres . The least worst result with my spectacles is with the most distant vision sections. In any event the blurring is progressively worse the higher the diopter sum value for me. My best result is actually with my master eye (-0.25) and no correction lens nor spectacles. I think that the theoretical correction for my left eye should be plus 0.25. It would be interesting to see if Plus 0.5 (smallest positive increment) would work better than none. Actually slightly too high. I suspect that it is better to be below than above the ideal. Anyone want to exchange to try this out?? I would add that any spectacles cause eye relief problems for me anyway. Jaap mentioned a possible diopter correction that is also included in the magnifier formulations. I cannot identify a change in acuity of the framelines/rangefinder patch/virtual image with either but the larger patch/image does assist with detecting focus error with the bigger patch so it must. Reducing field of view is the trade off. With the 1.4x my impression is lower brightness and clarity around the edges of the field with my example. Does your brain hurt yet ? All of that only reinforces for me what others have also said in this thread. It's too hard to work out for yourself. Seek professional practical assistance from an optometrist or opthamologist. Even if you need to pay for a new visit (and you may not), surely worth the investment when measured against the camera cost and even the cost of buying correction lenses when you can't be confident that they will do what you want. Edited March 6, 2011 by hoppyman 2 Link to post Share on other sites More sharing options...
jaapv Posted March 6, 2011 Share #24 Posted March 6, 2011 About the magnifier in this context: Leica explains that adding a magnifier should not change the diopter correction needed. However, quite a few users, myself included, have reported a shift in correction of -0.5 diopter. It suggests to me that the change in eye relief by the magnifier somehow influences the subjective correction needed, especially in a situation where we have a slight overcorrection, like the 0.25x you mention. Link to post Share on other sites More sharing options...
myshkine Posted March 6, 2011 Share #25 Posted March 6, 2011 I got my diopter correction lens at my shop in downtown DC (Pro Photo). The dealer had a very useful instrument. A sort of round visor with all the gradations of correction lenses. i just had to look through the glass with different degrees and was able to quicky choose the level of correction I needed (-0.5). Link to post Share on other sites More sharing options...
jaapv Posted March 6, 2011 Share #26 Posted March 6, 2011 That must have been an old-established dealer. I haven't seen those very practical gizmos for ages. Link to post Share on other sites More sharing options...
jaapv Posted March 6, 2011 Share #27 Posted March 6, 2011 Advertisement (gone after registration) About now I am getting really confused. Actually ordered a couple from B&H, and will ferret out what works. So, inquiring minds what to know- why 2 meters? Because that is the virtual distance of the framelines and rangefinder patch. Link to post Share on other sites More sharing options...
juergen Posted March 6, 2011 Share #28 Posted March 6, 2011 I got my diopter correction lens at my shop in downtown DC (Pro Photo). The dealer had a very useful instrument. A sort of round visor with all the gradations of correction lenses. i just had to look through the glass with different degrees and was able to quicky choose the level of correction I needed (-0.5). If you now would let us know what the correction factor of your specs were at that time, would be helpful. Juergen Link to post Share on other sites More sharing options...
myshkine Posted March 6, 2011 Share #29 Posted March 6, 2011 If you now would let us know what the correction factor of your specs were at that time, would be helpful. Juergen 1.25 I believe. Link to post Share on other sites More sharing options...
tdtaylor Posted March 6, 2011 Author Share #30 Posted March 6, 2011 I got my diopter correction lens at my shop in downtown DC (Pro Photo). The dealer had a very useful instrument. A sort of round visor with all the gradations of correction lenses. i just had to look through the glass with different degrees and was able to quicky choose the level of correction I needed (-0.5). Fabulous, and thank you. I have to be down at 17th and M on Tuesday and will stop by. I also see they are a Authorized Leica dealer. Link to post Share on other sites More sharing options...
myshkine Posted March 6, 2011 Share #31 Posted March 6, 2011 Fabulous, and thank you. I have to be down at 17th and M on Tuesday and will stop by. I also see they are a Authorized Leica dealer. They're a long-established shop. I also had my 50 Summicron repaired there. They did an excellent work. Link to post Share on other sites More sharing options...
Jeff S Posted March 7, 2011 Share #32 Posted March 7, 2011 (edited) I also see they are a Authorized Leica dealer. Friendly and knowledgeable staff, too. Small shop that's been around several decades. Jeff Edited March 7, 2011 by Jeff S Link to post Share on other sites More sharing options...
hoppyman Posted March 9, 2011 Share #33 Posted March 9, 2011 Well my last post was a bit of an encyclopedia in this thread. Everyone must have been impressed or amused, I can't decide which I wanted to follow up on one point as I was very interested to read Jaap's comments that the Leica 1.25x and 1.4x magnifiers also include a diopter component. Of course this required that I extend my experiment. Remember that my left eye uncorrected is -0.25 and my right is -0.75 so ideally I need -.025 and +0.25 correction respectively (to match what the fixed eyepiece does). I have a -0.5 correction lens, the 1.25x magnifier and the 1.4x magnifier. So I could look through each magnifier on and off camera as well as with and without the -0.5 lens (and that correction lens off the camera duplicates the fixed eyepiece uncorrected) I can see a clear diopter effect through both magnifiers off and on the camera in addition to the magnification. In other words, off the camera, with both of my mildly nearsighted eyes detail at distance, for example leaf edges, is sharply defined as well as larger. It approximates the definition that I get in the bare viewfinder. Remember that bare viewfinder is 0.68x magnification (so net magnification is 0.85 and 0.95 (sounds like an M3!) . Without measuring equipment I estimate the magnifiers' additional diopter to be -0.25~ -0.5 So I agree with what Jaap has pointed out. This is even more reason to try your equipment and with any magnifier with your friendly optical guru. . About the magnifier in this context: Leica explains that adding a magnifier should not change the diopter correction needed. However, quite a few users, myself included, have reported a shift in correction of -0.5 diopter. It suggests to me that the change in eye relief by the magnifier somehow influences the subjective correction needed, especially in a situation where we have a slight overcorrection, like the 0.25x you mention. 2 Link to post Share on other sites More sharing options...
tdtaylor Posted March 12, 2011 Author Share #34 Posted March 12, 2011 Just to attempt to bring this to a close, I ended up ordering the -0.5 and -1.0 and trying them out. It appeared the -0.5 should be the correct one (my Dr. told me -1.0 @ 1 meter), but the -1.0 is just a tad sharper- just a tad. May be that my eyes have changed a little in 6 months. Bottom line, the correction appeared to be just about as predicted. I would heavily encourage, as Hoppy mentioned, that a FAQ be set up for this topic- it appears to be one of those topic mired in differing opinion. Thanks to everyone for the help- nice to be able to see clearly through the viewfinder again! Link to post Share on other sites More sharing options...
Jeff S Posted March 12, 2011 Share #35 Posted March 12, 2011 Glad things worked out. If there's a FAQ, it short be short and sweet, i.e., "try them out on the camera, pick the one that works (if any), and return the rest." Your experience supports this approach. The theory may yield a close result, but trying trumps all theory. There could, however, be some explanation on pros and cons of using a diopter in the first place. For instance, my glasses allow me to see clearly through the finder, around the frame lines, and everywhere else, whereas a diopter doesn't address my astigmatism, nor my need to wear prescription sunglasses outdoors anyway. Everyone is different...different eyes, different needs and different preferences. In fact, the "just try it" approach could serve well for many FAQ subjects IMO. Jeff Link to post Share on other sites More sharing options...
jaapv Posted March 12, 2011 Share #36 Posted March 12, 2011 Hey Geoff! Could you do the FAQ please? Link to post Share on other sites More sharing options...
Jeff S Posted March 12, 2011 Share #37 Posted March 12, 2011 No offense, Jaap. I love the FAQ. I just think that most people are lazy and prefer to listen to others rather than trying things out for themselves. They would learn a lot more IMO with a little effort...supplemented with facts, of course. Jeff Link to post Share on other sites More sharing options...
jaapv Posted March 12, 2011 Share #38 Posted March 12, 2011 No- I meant Geoff should put his excellent post in the FAQ. Link to post Share on other sites More sharing options...
hoppyman Posted March 12, 2011 Share #39 Posted March 12, 2011 I shall put up some term explanation summaries after checking in with the Mods . I had hoped that there might have been comments/corrections on the information that I summarised since I have no formal expertise on the subject at all. 1 Link to post Share on other sites More sharing options...
Jeff S Posted March 12, 2011 Share #40 Posted March 12, 2011 I gave my thoughts in post #35. While your two posts, Geoff, were quite erudite (as usual), much of it was technical, and while technical is often good and useful, in this instance there is no substitute for trial and error IMO. All the technical info in the world didn't get Terry to the right solution; trying it out did. Eyes are funny that way. Just my 2 cents. Jeff Link to post Share on other sites More sharing options...
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