Diopter Adjustment Question (1 Viewer)

Information about the diopter adjustment wheel always states that it is intended to compensate for differences between the eyes. But I have been unable to find what differences it actually compensates for. Does it compensate for differences in distance vision? If both eyes are corrected to 20/20, should the wheel be at the center mark (assuming it was put on correctly by the manufacturer)? Or, if one eye has a problem that makes vision in that eye less clear than the other, does the diopter adjustment affect that? Finally, if the diopter number is + (where the diopter wheel is on the right barrel), does that mean that the right eye is weaker or stronger than the left?

Thanks,

Ken

Hi Ken,
I am nearsighted enough, and I hope, optically inclined enough to take a stab at your questions. I have also used an individual-focus binocular, which has each eyepiece focus setting marked in diopter units, which has increased my awareness of these issues. Not quite an MD in opthamology, but those are my credentials and here goes.

Does it compensate for differences in distance vision?
Yes. It compensates if the two eyes are not equally farsighted or nearsighted.

If both eyes are corrected to 20/20, should the wheel be at the center mark (assuming it was put on correctly by the manufacturer)?
Yes, if both eyes are 20/20, or also if both eyes are equally nearsighted, or equally farsighted, it should be at the center mark.

Or, if one eye has a problem that makes vision in that eye less clear than the other, does the diopter adjustment affect that?
If the problem is a difference in farsightedness or nearsightedness, for example one eye perfect and the other simply nearsighted, it will. But if the difference is some more pathological problem or to the extent that either or both eyes are astigmatic (focusing error due to a more arbitrary misshaping of the eye's optics, ie, the eye's lens is lumpy), it won't.

Finally, if the diopter number is + (where the diopter wheel is on the right barrel), does that mean that the right eye is weaker or stronger than the left?
I don't know which you mean by strong, farsighted or nearsighted. But, a right-barrel + adjustment means the right eye is more farsighted than the left, analogous to a presciption for eyeglasses. Just like if you were farsighted, your glasses prescription would require convex, converging lenses, which are said to have "+" power. If your right eye was more strongly farsighted by one diopter, then, if the right-barrel adjustment was marked in actual diopter units, it should be set at +1.

The diopter is the unit if lens strength. It is the inverse of the lens's focal length in meters, and taken to be positive if the lens is converging. This inversion is done so that more strongly focusing lenses will have higher diopter values. This makes better commons sense to most folks, but also carries a handy mathematical simplification: "diopters add". For example, two converging lenses, strengths 2D and 3D, placed close together will converge the same as a single 5D lens.

End of my knowledge, so end of lesson! Hope that helps.
Ron

I am 20/15 in my left eye and 20/25 in my right eye. My right eye diopter adjustment is minus 1, give or take a little bit depending on the binocular I am using. Hope that helps a bit.
Bob

And keep in mind that manufacturers often make little effort to zero the diopter setting. Of my half dozen bins only half are truly zeroed (Zeiss FL, Leica UV, Nikon SE). The others (Zen ED2, Pentax ED, Nikon compact, Olympus compact) are off a little, some pos some neg.

Oops that's seven isn't it.

Not surprisingly, cost evidently has something to do with whether or not it's zeroed.

Thanks, everybody.

Ron, I have a followup question. When one focuses the central focus. it moves the internal lens. As you look at greater distances, in which direction does the lens move (i.e. toward the objective lenses or away)? Also, if you are setting the focus, after the left eye has been focused, in which direction does the diopter wheel move the right internal lens relative to the left internal lens if the diopter is positive? or if it is negative?

Thanks again,

Ken

Ken:
Just to chime in here, it really does not matter how the lenses move, as they are different in various kinds.
What matters is for you to simply adjust them to your eyes. Focus on a somewhat midrange target, then start usually your left eye to a sharp focus with the right eye closed, them close the left eye, and focus the right with the diopter setting. Then you are good to go.
If your vision is the same in both eyes, most quality optics are very close to O on the diopter.

Jerry

Ken,
Jerry is right, thank goodness the user never has to think about this stuff or only very rarely would a bird be IDd! But I enjoy trying to understand these things in detail for some strange reason, and it appears that you suffer from a similar sickness.

In my roof prism binos, I can look inside and see that the direction of motion of the internal focusing lens corresponding to a more distant view is towards the objective lens. For one of these, a Leica Trinovid, I happen to have a good cutaway view:
http://www.birdforum.net/attachment.php?attachmentid=132587&d=1207050612
You can see that the moving focusing lens, the single lens back from the objective a ways, is concave. Moving that lens more towards the objective moves the focal point backwards, towards the eyepieces. That makes sense. There may very well be other makes that use a convex focusing lens, and that would move oppositely.

It is similar for the one-barreled "diopter setting" adjustment--a more negative setting compensates for that eye being more nearsighted than the other. It is analogous to a nearsighted eye requiring a negative, or concave eyeglass lens to see clearly beyond its natural range, all the way to infinity. Everything reversed if the right eye is farsighted, naturally!

So, I hope that totally useless answer answers your totally useless question!
Ron

Hi Ron, That was nice of you to cut up your Leica binocular just for this thread BTW This fellow is new on here and I am sure he knows you are just kidding about all this being totally useless.
Regards,Steve

ceasar said:

I am 20/15 in my left eye and 20/25 in my right eye. My right eye diopter adjustment is minus 1, give or take a little bit depending on the binocular I am using. Hope that helps a bit.
Bob

Click to expand...

Hi Bob, My eyes are close but my right eye is a little better. Have you ever bought binoculars that the left side was a bad side? I had a Pentax 20x60 WP that the right side was bad and kept wondering what was going on until I looked through the left side. I am right handed and right eyed.

Regards,Steve

mooreorless said:

Hi Bob, My eyes are close but my right eye is a little better. Have you ever bought binoculars that the left side was a bad side? I had a Pentax 20x60 WP that the right side was bad and kept wondering what was going on until I looked through the left side. I am right handed and right eyed.

Regards,Steve

Click to expand...

Hi Steve,
Yes I did. I had to return an (unnamed) 6 x 32 Roof because the left barrel was off. I couldn't figure out the problem until I turned them upside down and looked through them.
Bob

Hi Bob, Thanks for your reply. You might not of noticed it as much if it had been the right side. For the longest time my eye Rx for left eye was -1.5 diopter and I kept adjusting my binoculars to 2.0 diopters and found now with -2.0 diopters my left eye is almost as good as my right eye a very easy 20/15 but not quite 20/10.
Regards,Steve

ronh said:

So, I hope that totally useless answer answers your totally useless question!
Ron

Click to expand...

It does, indeed. Many thanks.

Ken