Eye Relief range (1 Viewer)

ER specs are always given as an exact number, meaning (as I understand it)the proper distance from the eye to the eyepiece for viewing the full FOV. In reality though, there is a window (or range) in which the view is, let's say, full. Under a given set of conditions, this full view window has a definite close limit and far limit, which would seem to constitute an actual range of ER. (It's easy to detect these limits by simply moving the bins from too close to too far from one's eyes. Nearer than the close limit or farther than the far limit both result in loss of field and/or "blackout".) In terms of "ease of view", the width of this ER range would seem to be quite significant. I suspect that it could come into play on a regular basis in field use for most viewers.

To what extent does this "ER range" vary from model to model? If so, is it inherently linked to magnification, ER, objective diameter, etc?

Also, would it vary due to differences in eye-pupil diameter?

-APS

The ER is a precise number that comes out of the optics design of the eyepiece (it's the distance from the back surface of the final lens (or perhaps it center in some cases) to the position of the entrance pupil.

Depending on the eyepiece design only it can depend on several parameters. Generally as the eyepiece focal length decreases the ER decreases too (so 10x bins have a lower ER than the 8x bins of the same design).

The best description of how the rays converge (afocally) through the exit pupil is in Wikipedia

http://en.wikipedia.org/wiki/Exit_pupil said:

The exit pupil can be visualized by focusing the instrument on a bright, nondescript field, and holding a white card up to the eyepiece. This projects a disc of light onto the card. By moving the card closer to or further away from the eyepiece, the disc of light will be minimized when the card is at the exit pupil, and the bright disc then shows the diameter of the pupil. A clear vial of milky fluid can also be used to visualize the light rays, which appear as an hourglass shape converging and diverging as they exit the eyepiece, with the smallest cross-section (the waist of the hourglass shape) representing the exit pupil.

Click to expand...

The "blackouts" you see most of the time are just vignetting of this bundle of rays so the "range" is not a precise one (it depends on the angle of bundle and the size of your entrance pupil and the positioning of your eye on the axis (large exit pupils are preferred becaise they allow imprecise positioning of the binocular). Some blackouts ("kindney bean blackouts") appear to be due to curvature/spherical aberation in the exit pupil though I don't have any good reference for that but it seems to crop up in some designs making them less usable for "some" people (but I'm not sure how to define that population).

A part of the problem comes from a lot the manufactures just taking the measurement from the optical designer and quoting it in the specification. And not taking into account the thickness of the eyepiece or eyecup hardware. A classic exampe of this is the Swift 820 porro with a speced ER of 16mm (enough for eyeglasses you would think) but when you account for the hardware the usable ER is 12mm (measured by edz) which is not enough for a whole field for majority of eyeglass wearers.

Of course some do take account of this hardware and quote an (honest) effective ER but then it's difficult to tell who is doing what.

The other problem comes with the vertex distance of your glasses (the distance from the front of the cornea to the back side of the eyeglass lens). Though in opthalmic optics this is usually taken to be a "fixed number" of around 12mm if one makes the measurement (I've actually read a paper where they did that) they found a range of vertex distances from 6mm to 15mm (or more). This is where most of the problems come from: glasses worn down the nose, thick glasses (it matters!).

Add that to the shape of your face: how deep set are your eye's from the bridge.

You can see the number of variables mount up. So once again the only way to really know is to try it.

e.g. I recently got a Yosemite 8x30 with speced ER of 14mm. I can see pretty much the whole field in my close-fitting eyeglasses (and even only miss a very small amount in by bigger less close fitting glases). So I rather suspect Leupold are giving a "usable ER". Or I'm just getting lucky.

The only thing in the eye that could affect the position of the entrace pupil (which has to match up with the exit pupil of the optics) is if the curvature of the cornea, which itself is a lens which a large strength, is rather difference than they expect. As the cornea images the entrace pupil of the eye it moves it's apparent position from that actual physical position of the iris. But the iris itself is rather flat so ER doesn't change with diameter of the aperture in the iris.

Thanks Kevin for the extensive reply. If the "ray bundle" converges at a specific distance, that makes sense for a precise distance spec. But ...

There is a perceivable "range" though, right? So then, what causes it? Does the "range" possibly represent the distance in which the "ray bundle" is of a diameter less than one's eye pupil diameter (at a given moment)? Would that also imply that bins with larger EP have a smaller range? Or, would it make sense that the "ray bundles" of various bins converge on different angles depending on the eyepiece design (a gradual angle would result in a wider ER range), meaning that there are perhaps other over-riding factors?

Regardless of the actual cause(s), I'm primarily interested in knowing how this perceivable range varies from one binocular to another. Is it perhaps better (in general) in some bins than in others? And to what extent? And are there inherent bin design factors (other than, or in addition to, eyepiece design)? Most importantly, is this possibly a factor in the "ease of view" so often mentioned in field comparisons?

Thanks, APS

If the entrance pupil is not at the exit pupil (either too close or too far) not all the rays enter the eye so you get vignetting which eventually gets bad enough to blackout i.e. fully vignetted.

But with "small" amounts of vignetting the eye is actually not very good at seeing the vignetting (e.g. I've seen it mentioned that some bins have up to 50% vignetting at the edge of field that just goes unnoticed by the viewer) so you get a usable range.

The same is true for placing the the entrance pupil of the eye off axis at the exit pupil. This is one effect (large exit pupil) that gives rise to a "relaxed" placement of the eye.

How bad this effect is depends on the ER and the angle of the bundle as they come through the exit pupil. As you say smaller the divergence the greater the range.

Bins with a smaller ER generally have a smaller range (for a given divergence in the bundle) so as well as needing to get your eye closers you have a smaller range to keep it in without excessive vignetting.

As I said in the first post: it depends only on eyepiece design. Other features of the bin may affect that eyepiece design choice but they don't directly affect the ER. Change the eyepiece and you change the ER.

But i could see making an argument (say on apparent FOV that depends on magnification and FOV) that would effect the divergence of the light coming out of the eyepiece that would affect the "range" of the ER. I guess that would make the range smaller as AFOV increases.

Thanks Kevin; I appreciate the detailed explanation.

APS

APSmith said:

ER specs are always given as an exact number, meaning (as I understand it)the proper distance from the eye to the eyepiece for viewing the full FOV. In reality though, there is a window (or range) in which the view is, let's say, full. Under a given set of conditions, this full view window has a definite close limit and far limit, which would seem to constitute an actual range of ER. (It's easy to detect these limits by simply moving the bins from too close to too far from one's eyes. Nearer than the close limit or farther than the far limit both result in loss of field and/or "blackout".) In terms of "ease of view", the width of this ER range would seem to be quite significant. I suspect that it could come into play on a regular basis in field use for most viewers.

To what extent does this "ER range" vary from model to model? If so, is it inherently linked to magnification, ER, objective diameter, etc?

Also, would it vary due to differences in eye-pupil diameter?

-APS

Click to expand...

APS,

I think your suspicions may be right about a range, but it would be hard to come up with a generalization that necessarily applies to every instrument. In part designers are at liberty to set up eye-ocular interface models differently, and in part observers are at liberty to vary instrument placement and eye movements during use.* I've attached an interesting paper that gets into some of the coupling issues, although I have no idea how much of this shows up in real designs. Ever since reading it I've been unwilling to draw strong conclusions, however, other than to say that in practice some designs work better for me than others. I'm also not sure but that these aspects are more of an art than a science.

It's an interesting question.
Ed
* Your comment refers to the observer's freedom to move the instrument in and out along the line of sight. This will be accompanied by a change in field, as Kevin discussed, but with the eye centered I'm not sure the observer would be all that concerned. More important to me are differences in the amount of movement possible before blackout or vignetting occurs. I have a strong sense that the typical observing task requires axial and lateral adjustments on a continuous basis, which relate to the eye-head movement strategy. All of this adds up to some instruments seeming more or less natural in use, and others being quite unforgiving.

Thanks for posting the interesting link, Ed.

Fig.2, c and d look like good representations of the blackout problem some have with the Nikon 8x32 SE. A combination of very long eye relief and an eyecup without incremental adjustments can place the exit pupil deep inside the eyeball for some people.

Henry

(Henry, check your mail at CN, please---Ron)