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Eyepiece diopter strength -- rule of thumb?

Henry Stanley , Mar 03, 1998; 03:03 p.m.

As age aproaches, most of us could use those handy gadgets called eyepiece correction lenses, measured in "diopters." (Thank goodness for AF cameras when I'm shooting AF 35mm.) Is there a handy rule of thumb for determining what diopter number I need before I order. Is the diopter number different for different cameras/ prisms, etc.? Thanks --Henry

Responses


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Walter Newton , Mar 03, 1998; 03:21 p.m.

Ask your eye doctor. If you are nearsighted and look at the prescription for your eyeglasses/contact lenses it will show something like "-2.5 diopters"; that gives you the strength you need.

Pat Byrnes , Mar 03, 1998; 03:23 p.m.

Ask you optometrist what diopter you should get; it's the same as the diopter correction for your prescription eyeglasses or contact lenses. Make sure you specify to the optometrist which eye you will use on the camera, in case your eyes are of different strengths. If you use both eyes (i.e. one eye when the camera is horizontal, the other when shooting vertically), you should tell the optometrist that, too; they would know the best comprimise.

Some camera diopter lines come in whole units (+2, +1, etc.) while others are more crude and skip units (+2, 0, -2). The +/- sign determines whether the correction is convergent or divergent, and is also critical. A real advantage to on-camera diopter wheels is that the correction is stepless, whereas eyepiece add-ons are less customizeable. And yes, you can use a combination of in-camera and add-on diopter correction, though I don't know if the effects are strictly additive.

By the way, my vision is terrible (-5.25 in both eyes), and the best I can find for my Minolta X-700 is Minolta's own -4 diopter (pretty close, but not ideal). Does anyone know a third-party maker of diopters which might work better?

Bob Meyer , Mar 03, 1998; 04:11 p.m.

Am afraid the prior posters, while trying to be helpful, relayed a potentially serious piece of misinformation. The correct diopter for your eyepiece is NOT necessarily the same as your prescription.

If your eyeglass prescription is for distance vision, it's optimized for a viewing distance of around 20 feet. The apparent viewing distance of the viewfinder image in your camera is typically much closer. In Canon EOS SLRs for example, the viewing distance is 1 meter (about 39 inches). Have your diopter optimized for distance vision and you'll not be helping yourself.

Even if your glasses are optimized for reading, that might not be the optimum eyepiece correction. Most reading glasses are optimized for much closer than 1 meter.

Finally, are you going to use the diopter in addition to your glasses? If you have a distance correction, you may want to go this way so that you don't have to repeatedly take your glasses on or off.

There are a couple of choices to find the proper diopter. First, find out the apparent viewing distance of the viewfinder in your camera, ans ask your eye doctor for a prescription to match. Or, take your camera to a drug or grocery store that sells reading glasses, and try your camera with each available strength, and find which works best for you. And Canon, (and maybe others) provides their reps with variable diopters. If your camera store can borrow one of these, you can try it on your camera and find the best setting, then order that one.

HTH,

Bob

Henry Stanley , Mar 03, 1998; 04:54 p.m.

Bob -- When I go to the store to buy "reading glasses," I get 2.25 strength reading glasses. Is that a +2.5 or -2.5? Recently, I ordered a +2 diopter from B&H for a Nikon N70s and it made the viewing worse and I sent it back. Should I have ordered a -2? And does anyone know the "viewing distance" for Nikons (N70s and N90s)? Thanks! Henry

Pat Byrnes , Mar 03, 1998; 08:27 p.m.

Bob--Thanks for the information on apparent viewing distances. Just last weekend I checked with my local camera shop and optometrist before ordering a diopter eyepiece, and neither made mention of this. I've checked my X-700 manual and find no indication of the viewfinder's apparent viewing distance, so I'll ask Minolta or try your drugstore suggestion.

In the viewfinder section my manual does say, "power: -1D, adjustable with accessory snap-on eyepiece lenses." Now I'm wondering if I should subtract 1 from my prescription need at the viewfinder's apparent focusing distance. (?)

When this thread has reached some closure, I'll print it out and give copies to the camera shop and the optometrist. Again--thanks!

Pat Byrnes , Mar 03, 1998; 08:41 p.m.

Oops ... meant to say "ADD 1 TO my prescription need at the viewfinder's apparent focusing distance" (since the camera MAY already be subtracting 1). Now I really see the value in your suggestion to try several out, if possible.

--Pat

P. Aing , Mar 03, 1998; 09:40 p.m.

Henry : some clarification and "tips" to remember diopter/lense terminology :

First, a normal eye can see clearly at infinity.

*converging lens : has a real focal point. You could measure the focal lenght with a ruler. Dioptric power= 1/focal lenght, is positiove number (+). You (usually old people) need this king of lense to read because the eye's lens cannot converge rays from short distance (books) on the retina anymore. Obviously, rays coming from object at short distance need more "bending" to get to the retina.

*diverging lens : rays bent further from the optical axis, so no real focal point. In fact there is a focal point which is a point where the diverging rays seem to come from, but you need an optical instrument (eye or other lens) to see it. Then focal distance is negative and dioptric power= 1/focal lengh is negative. Near sighted people who can read (if there are not too near-sighted) but can't look at infinity need this kind of diopter to "unbend" the rays coming from infinity.

Your problem with the wrong diopter : maybe as suggested above, if the viewing distance for the SLR is 1m, you don't need such powerful +2 diopter which will put your confortable viewing distance to 30-50cm.(for reading). Maybe you'd need something less powerful, just to bring your normal viewing distance (infinity ?) to 1m.

Lloyd Phillips , Mar 03, 1998; 10:10 p.m.

Your getting a lot of info - some correct and some incorrect. First, each camer mfg has its own correction eyepieces. They are not necessarily the same. For examle a -1 for Nikon is not the same as -1 for Minolto. So, start with which mfg and WHICH model (again some are different. A sharp focus thru the prism is an artificial distance - as previously stated usually about 36 inches (some more - some less). Your optomitrist is frequently uninformed about diopter correction since each mfg has its own system. Minus is for nearsigtedness - plus for far. But, Nikon builds a minus into some (perhaps all) of their prims. First, find out your vision problem. Then take your camera to a well stocked dealer to "try before you buy". This is one time that mail order is not the way to go. Some top of the line cameras have adjustable eyepieces. Finally, be prepared for your eyes to change its ability to focus on a consistant basis which then calls for different correction eyepieces at different times. I went thru all of this and eventually had to have an eyeoperation before it was cleared up - no pun intended. Feel free to respond if you want more specific information.

Bob Meyer , Mar 04, 1998; 10:29 a.m.

I think you've probably had your questions answered. But just in case: The -1 d specification of your viewfinder is equivelent to a viewing distanc of 1 meter. Your reading glass prescription is probably optimized for something closer, so you'd need a less powerful diopter lens (it seems) to focus on the 1 meter apparent distance of the viewfinder image. A + lens if for close-up, a - lens is for distance correction.

Like another poster and I suggested: you need to try them out.

(If, as one poster suggested, a Nikon +1 is different than a Minolta +1, then something's very wrong. Diopter measurements for optics pre-date photography, and there's no reason any optic should not follow the standard).

One other thing to consider: do you have astigmatism? If so, no diopter lens is going to correct that. If no astigmatism, your prescription will read something like +1.5. If astigmatic, you have something like +1.5 +1.25 x 035. The first is the power of the lens, the second is the strength of the barrel curvature necessary to correct the astigmatism, and the 3rd the axis of the barrel curvature.

HTH,

Bob


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