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IPD Question (1 Viewer)

D

dug-mac

Member
I just returned from an eye exam in which my optometrist measured my IPD at 63mm. This came somewhat as a surprise as I typically find that my IPD settings on my binoculars fall in the +/- 65mm range. Can one extrapolate their IPD to where they set the IPD on their binoculars, or is there an inherent difference given the mechanics of the optical device (the binocular)? I quickly set one of my binos to 63mm and the field of view appear vignetted and too restrictive - or am I just unused this 'proper' setting?

I would appreciate any experience or feedback.
 
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Hello Dug-Mac;

I have done a little IPD testing and posted about it some time back. There was a lot of good feed back. Look at http://www.birdforum.net/showthread.php?t=156903&highlight=ipd

There is some indication that measuring ones IPD with a ruler tends to be 1 or 2 mm on the wide side. Also, when using large exit pupils and adjusting from the outside in, there is some tendency to set bins wide. Did your examiner use a ruler or machine (pupilometer)?

Try setting IPD while looking at far distant point.

If you don’t find any help in the referenced thread, we may try something else.

Best
 
Mine were measured with a ruler at 65 but I find I tend to manually adjust slightly wider at 66mm (assuming the calibration of the bins is good!). I think part of that is people (well, me) dislike vignetting more at the outer edges of the field than at the inner edges.

The easiest way to measure it accurately yourself (and some of the fancy new pupilometers just the same idea) is to photograph your face with a digital camera with a ruler held to your forehead whilst looking at a distant object. Then you can use the image of the ruler to make a more accurate measure when you view the image on a computer.

The latter is important e.g. a 65mm distant IPD like mine is about 62mm IPD at reading distance. So trying to measure it in a mirror yourself can lead to errors too.

I've seen at least one paper point out the significant errors that can creep in though the ODs seem to think they're all perfect at making an accurate measurement!
 
Thanks for the replies...

It seems like a simple equation to solve - simply merge the barrels until you get a single image - but it was interesting to read your results, Ron, that when setting from the outside most fix on a too wide IPD. This is how I've typically adjusted my binos. My optometrist used a pupilometer and arrived at 63mm, but my best images seem to come from a setting of around 64.2-3mm, although the binocular will perform without eyestrain within probably a good 2mm variation. I would like to be more confident in my setting and not rely merely on what looks right at any one particular time. The thread you gave is helpful. Any additional thoughts since that thread finished out?
 
Me too. That is, I tend to set a little wider than my true IPD (determined by looking into a mirror with the binocular and adjusting to center my pupils in the objectives). There does seem to be greater difficulty seeing the edges at my true setting.

But, overall, the true setting works best. There is less of a tendency to see color fringing, and less of a tendency for the field to "black out" as my eyes wander around.

Try your true value for a few days. If you don't see these advantages, you might as well go back to your "natural" setting.
Ron
 
Dug-mac;

The tendency I have noticed for larger exit pupil binos (you did not say what your were using) is when setting from outside in to stop when your eye pupils get within the inside edges of the exit pupil and conversely, when setting from the inside out, to stop when your pupil get within the outside edges instead of continuing to the center. Although, for some reason, the average is closer to measured IPD when working from the inside out.

Before I started measuring IPD’s, some of the group tried setting from outside in several times and measuring the bino, then setting from inside out and measuring and using the average. This seemed to work well for most. Might be worth a try. Let me know how it comes out with your measured and customary settings.
 
Dug mac,

try this... it's more by feel but works well for me.

Look about 50 yards out at some target. Defocus until you see a blurry field (best when target has lots of sky around it).

Move barrels together until vignetted fields overlap completely.

Somehow the defocusing allows you to see the fields better and center better.

This is just a "feel" thing and works for me.

Cheers
 
Thanks for all the replies...

Ron, I saw in one of your other posts that you recommended someone to set their IPD to 64.25mm as that was the average of tests that you'd conducted - this, you said might be a good place to at least start from. It is interesting that I find this setting to produce some of the best images in my 10x42 Trinovids. I suppose that this makes me an average Joe, but it is still a good 1mm wider than my measured IPD. Closing that 1mm gap feels somewhat unnatural (poor fit with facial structure) and causes vignetting of the image (or perhaps I should say doesn't allow me to view the field edge in the way that I'm used to?). The question I now have is whether to eschew custom and go with the 'right' number under the guise that I will somehow adapt to the change; besides, this approach might satisfy my need for a more scientific calculation. I am now wondering, however, if the intrinsic need for eye movement within a given scene necessitates a slightly wider IPD. I'm sure others have thought along these lines and might have feedback?

Oleaf, it's funny because lately I've taken to viewing out my sliding window at night with the lights turned on inside. You can't see anything outside when looking skyward, but what you do get is the crisp edge of the field - from which you can proceed to merge the images. I'm thinking this may be similar to the approach that you've suggested. I will give yours a try and see what the results are...
 
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Dug-mac

If your eyes are telling you something different than your eye care professionals “official” reading I would defiantly question that 63 mm reading. There are many ways to get erroneous information even from the best instruments. One of the most common is to have the test distance set wrong (TDIS). The ones I use can be set for the test target to merge at 30 cm, 35 cm, 40 cm, 50 cm, 65 cm, 1 m, 2 m and infinity. These targets are seen reflected on the cornea, and that is what you measure to, the corneal reflections, not just the apparent center of the pupil. My versions reset to infinity on power up, but not all do (some use a rotary switch to set distance). Also, it is very easy, while fumbling around looking through the pupilometer for the increase or decrease width buttons, to accidentally hit the TDIS button directly below. I have done this on numerous occasions. Plus, it is possible for these things to get knocked out of alignment; they are two collimators with a single eyepiece for viewing. Since I carry mine in the vehicle all the time and it is subject to more abuse than an office model, I routinely check them by placing a mm scale on the wall and make various width readings with the rule touching the lens back to about 5 inches from the rule, about the limits of acceptable focus. The point being mistakes are not uncommon even when using a pupilometer.

Sometime, when roaming around the malls, drop by an eye glass retailer. A lot of them will have a pupilometer and will check your setting for no charge.

BTW, I meant to mention above, that when setting IPD and measuring bino, you may be surprised at the spread of measurements seen.
 
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Ron,

I assume that one's IPD decreases with near accommodation, because the eyes naturally converge at shorter distances. Since the eye is more comfortable with convergence than divergence, it would seem logical to set the IPD at infinity focus rather than at near focus, — or even to err in the direction of greater IPD rather than less.

What do you think?
Ed
 
Ron, your testing has proved valuable. Inclination says to open the binos up wide and close them until both barrels merge (at least that is how I've always approached it). Your tests indicate that the opposite method - closing the binos and opening until the barrels merge - is more accurate. Like you I feel there must be some ramifications to improper IPD (CA?, less sharpness?, lower image brightness?), but this is speculation and in need of further testing. The news today is that even experienced users might long ago have given up fiddling with IPD because the setting they've been using 'just feels right'. To my mind any adjustment that might lead to a better user experience is worth pursuing. Thanks again, everyone, for your help...
 
Like you I feel there must be some ramifications to improper IPD (CA?, less sharpness?, lower image brightness?), but this is speculation and in need of further testing.
Click to expand...

With a small exit pupil on the bin you can get lower image brightness by really bad alignment but the real problem is usually lateral CA from the eye being off the optical axis of the barrel.

And not being able to "look around" the view as the entrance pupil of your eye moves out of the exit pupil of the bin.

Sharpness won't be affected.
 
Ed;

Yes, my IPD drops to 60-61 mm at 30 cm from 64 and I really have to push (separated targets) to get any measurable divergence.

But, my primary focus is not actually on the relative positions, but rather the off axis aspect of misalignment since the optical axis of the binos is fixed and the eye is not.

For instance, I wonder if the IPD were set too wide where the eyes would be more on axis towards the inside portion of the FOV and further off axis towards the outside if CA would be more apparent towards the outside and if you narrowed the bino IPD setting, if the apparent worst part of the CA might move more to the inside of the FOV. Since I am not bothered by CA, and the experiment is not worth me learning to see it and then put up with it, I will just have to wait until I find someone locally who is really sensitive to it to see and most of the group around here have no idea what CA is, when I mention it, I just get blank looks. I think this may apply to other parameters or aberrations as well.

I may email Brocknroller and get him to look into this.


Dug-mac;

My thoughts would be that as long as your eye pupils are within the exit pupil diameter, the overlap image would be constant.
 
Surveyor said:
Ed;

Yes, my IPD drops to 60-61 mm at 30 cm from 64 and I really have to push (separated targets) to get any measurable divergence.

But, my primary focus is not actually on the relative positions, but rather the off axis aspect of misalignment since the optical axis of the binos is fixed and the eye is not.

For instance, I wonder if the IPD were set too wide where the eyes would be more on axis towards the inside portion of the FOV and further off axis towards the outside if CA would be more apparent towards the outside and if you narrowed the bino IPD setting, if the apparent worst part of the CA might move more to the inside of the FOV. Since I am not bothered by CA, and the experiment is not worth me learning to see it and then put up with it, I will just have to wait until I find someone locally who is really sensitive to it to see and most of the group around here have no idea what CA is, when I mention it, I just get blank looks. I think this may apply to other parameters or aberrations as well....
Click to expand...

Ron,

I've looked for several years to find an insightful analysis of 'off-axis' viewing. Geometrically, it seems to me that with zero convergence (i.e., parallel viewing) one or both eyes might be offset from the optical axes due to an IPD setting error, but still be parallel to them. On the other hand, with positive visual convergence, which probably accompanies instrument myopia anyway, the visual axes necessarily cut cross-axis. I would imagine that these two eye configurations produce different aberration consequences at the retina, but darned if I can figure out what they are. The research this suggests probably can be done by computer models that include the optics of the eye, — but short of that it's anyone's guess.

Ed
 
My apologies for reviving what had become a somewhat dormant thread...

Does anyone else feel that a binocular with less eye relief (say 13mm or so) might need to be set at a wider IPD than a long eye relief model (say 18mm)? I know the difference wouldn't be much, if any. What are your thoughts?
 
dug-mac;

IMO, with a properly collimated binocular, it would not make any difference. The optical axis would be parallel so the distance between principal rays would be the same at 13 or 18 mm.

If the binocular were badly out of collimation it would make a difference. If we assume a 10x were out of collimation 1 degree in object space, or 10 degrees of apparent view, that would be about 0.9 mm difference in IPD from 13 to 18 mm eye relief. It would depend on the direction of the error (convergent or divergent) whether the 13 mm spacing would be wider than the 18 mm spacing or not.

Also, the eyepieces are designed for a single eye relief distance.
 
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Dug-Mac,
I concur with your original contention, that a bit wider than true eye separation makes the full field easier to see. I believe this is due to the particulars of how the eyecups fit into your cranial structure. They nestle in deeper when spread further apart. If eye relief was on the edge, it would be tempting to use the wider adjustment.

But, I find CA and blackouts worse with a too-wide setting. For me the answer has been to kluge the eyepiece height adjustment in some way, as required, so I can see the full field easily, but with the IPD set to my true eye separation.
Ron
 
Ronh, I agree that the true IPD setting makes for a more uncomfortable match with facial structure, at least in my case. This, along with my desire to see the edges of the field, has me cramming the binocular into my eye sockets to the point of strain. I'm now at the place of trying to accept that the field edge may not be entirely visible (and therefore not cram the binos) and be happy with the best image for that part of the field that I can see. I suspect at some point in the future I may be trying to strike a compromise. The situation at night is different - at my true IPD it does appear that the whole field is in view - hours of dealing with minutiae awaits!
 
RonE, you mention that eyepieces are designed for "a single eye relief distance". This has puzzled me somewhat with the infinitely adjustable eyecups (like on my 8x42 FL's). At what measurement should I be setting the eye relief to get the best view? I've experimented a lot to eliminate black outs and yet retain the field edge but have never been entirely happy with my settings...
 
dug-mac;

The eye relief is usually specified in the specs. I think the 8x42 FL is listed as 16 mm. You can search for how to measure this and may need to check it. Some makers measure from the last glass surface, some from the top of the eyecup in the down position.

The reason for the adjustable eyecups is to position the eye at that distance. With the eye cup down and wearing glasses, with the glasses surface touching the eye cup you should have enough distance to allow for the space between the glasses and your eyes. With the cups extended, it should position your eye at about 16 mm from the eyepiece.

In my case, I have deep set eyes and have developed sagging eyelids so I usually have to use an intermediate position, and with some of the shorter eye relief’s, I can even use fully collapsed. With glasses on, most are to short for me.

My usual method of adjustment is to start with cups fully extended (I do not wear glasses) and adjust down to where I can see the full field. I can usually see the full field fully extended, but like to adjust to see slightly wider than the full field (I like to see, or be aware of the edges) as long as I do not get black-outs, crescents or kidney beaning.

Hope this helps.
 
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