Astigmatism and exit pupil (1 Viewer)

[John Russell]

I followed a recent dicussion on the binoculars forum on contact lenses and there were some interesting contributions from looksharp65, who is an optometrist. I should be grateful if he or anyone else could answer the following questions.

Firstly:-
Assuming the eye does not need correction for astigmatism, at what pupil size (or exit pupil diameter) does it perform at its best? I recall that Holger Merlitz once mentioned some old military research on terrestrial scopes, in which an exit pupil diameter of 2,5 mm was considered optimal and 30x magnification the ideal compromise for the majority of conditions. The latter would, of course depend on climate and terrain but would place the ideal scope for purely visual use at 30x75.

Secondly:-
At what exit pupil diameters does the depth of field of the eye obscure astigmatism of, say -0,5 diopters or -0,25 diopters? I believe some amateur astronomers use magnifications, which result in exit pupils of around 0,5 mm, which would make cylindrical correction unnecessary for most. On the other hand, one manufacturer of astronomical eyepieces offers cylindrical correction lenses for the longer focal length eyepieces. For birders the practical lower limit of exit pupil would be around 1,5 mm, beyond which things start to get rather dim and it interests me whether this could obscure some degree of astigmatism.

John

 

[looksharp65]

I followed a recent dicussion on the binoculars forum on contact lenses and there were some interesting contributions from looksharp65, who is an optometrist. I should be grateful if he or anyone else could answer the following questions.

Firstly:-
Assuming the eye does not need correction for astigmatism, at what pupil size (or exit pupil diameter) does it perform at its best? I recall that Holger Merlitz once mentioned some old military research on terrestrial scopes, in which an exit pupil diameter of 2,5 mm was considered optimal and 30x magnification the ideal compromise for the majority of conditions. The latter would, of course depend on climate and terrain but would place the ideal scope for purely visual use at 30x75.

Secondly:-
At what exit pupil diameters does the depth of field of the eye obscure astigmatism of, say -0,5 diopters or -0,25 diopters? I believe some amateur astronomers use magnifications, which result in exit pupils of around 0,5 mm, which would make cylindrical correction unnecessary for most. On the other hand, one manufacturer of astronomical eyepieces offers cylindrical correction lenses for the longer focal length eyepieces. For birders the practical lower limit of exit pupil would be around 1,5 mm, beyond which things start to get rather dim and it interests me whether this could obscure some degree of astigmatism.

John

Holy cow, John, you're really pushing me beyond the limits!
I am not sure there is such information available, and I am not clever enough to compute it for you.

So I have to take another point of view, merely based on assumptions, though qualified ones.

When I make visual examinations, I put considerable effort in reaching refractive balance (when both eyes are focused to infinity), and to correct the astigmatism as perfect as could be done.
(in reality, the results often have to be tweaked to make the spectacles comfortable in real use).
However, refractive balance is not necessary for binocular users as they can change the diopter setting, and for scope users not necessary for obvious reasons.

After finishing with the phoroptor ("motorized trial lens set") I put on the trial frame with the appropriate correction. Very often, if the patient only needs a cross cylinder (+0.25 -0.50) they still react to the difference in clarity.

A person with this refractive state would easily be able to read the smaller lines. So, what does really happen when they admit that the correction lenses really make a difference?

If anyone has a refractive error, the light beams don't converge to a perfectly point-shaped focal point on the retina. The circle of confusion will have different size depending on the magnitude of the refractive error, and the size of the entrance pupil.
A smaller entrance pupil will cut off much of the defocused light - when the "beam bundle" gets slimmer, the circle of confusion will get smaller.

But within the beam bundle, as opposed to those cut-off rays, the refractive error will still be present. While some of the blur will go away, perfect sharpness will not be there.

So we can diminish the entrance pupil a lot using artificial devices as stenopeic (pin-) holes, but all that happens is that the diminished circle of confusion will mask some blur. When the pinhole gets too small, diffraction will occur and the image quality decreases again.
This is valid for myopia, astigmatism, presbyopia and also hyperopia, if its magnitude is great enough not to be compensated for by accommodation.

To be continued...

 

[looksharp65]

Before writing my reply above, I tried my scope with three different eyepieces, with 3 mm exit pupil, with 2 mm exit pupil, and finally with 1.3 mm exit pupil.

I used my right eye, that has an astigmatism of about -0.75 D, thus slightly more than the rate you mentioned in the OP.
All three were significantly less sharp without my spectacles than with them. The e/p with 1.3 mm exit pupil was possibly a little worse, although I avoid using it with my spectacles because of its eye relief.

So, in the end I feel a little puzzled about your question. My recommendation is always to use spectacles or contacts. The latter is not suitable for very low grade refractive errors.
If anybody is aware he has an astigmatism, but is content with his everyday long range vision, and only uses ready-made reading glasses, the need for a birding correction may not be so urgent - of course depending on how hardcore-a-birder he/she is.

Larger exit pupils have so many advantages, except for the size and weight of the instrument. So besides using spectacles, I recommend using as large exit pupils as possible. And all optical instruments should have sufficient eye relief to allow the use of spectacles, then any refractive error would be no problem for birders.

//L

 

[mooreorless]

http://www.telescope-optics.net/eye_aberrations.htm

 

[looksharp65]

Wow Steve, that's a great link! I bookmarked it.
And it does answer the first of John's original questions.
An entrance pupil (eye's pupil) of 2-2.5 mm will provide the best compromise.

At present there is no really effective way of controlling higher order aberrations of the human eye. So in daily life, me and my patients are stuck with sphero-cylindrical corrections.
I really mean that what could be done to enhance the sharpness with these simple means, also should be done (see my posts above).

A few years ago, there was some rave about special wavefront-correcting contact lenses, that were supposed to double or triple the maximum human visual acuity.
AFAIK, they never actually hit the market, but another one did. The Orto-K.

Orto-K is a custom made rigid contact lens that you sleep with, only to remove it in the morning. The ambition is to correct myopia without wearing spectacles or contacts during the day, but the means used to achieve this also provide another advantage.
The corneal roughness is greatly reduced, and the users often reach remarkable visual acuity.

http://www.ortho-k.net/

 

[mooreorless]

I forgot to bookmark it, so it took a while to find it. Some more information that might help.
http://www.cloudynights.com/ubbthre...32/page/262/view/collapsed/sb/5/o/all/fpart/1

If you want to search on there.
http://www.google.com/custom?q=&cof...ghts.com&sa=Search+Cloudy+Nights+Using+Google
An interesting site.;-)

http://www.michaelbach.de/ot/col_lilacChaser/index.html

 

[mooreorless]

The 30x75 terrestrial scope with the 2.5 exit pupil is probably a compromise for all lighting conditions and new lens coatings could make a difference. A lot of people use the 30x eyepiece in the Nikon 82Fieldscope for good reason going by this and wide field.

"I recall that Holger Merlitz once mentioned some old military research on terrestrial scopes, in which an exit pupil diameter of 2,5 mm was considered optimal and 30x magnification the ideal compromise for the majority of conditions."

 

[John Russell]

Many thanks Lars and Steve for all this information. I think it's going to take a while to digest all that .
The first question is answered then with 2 mm-2,5 mm exit pupil. For wide angle eyepieces, where 30x magnification is probably a sensible upper limit for general use, not only in respect of heat haze but also in finding the bird, there is probably not a lot to be gained by 85 mm objectives as opposed to 65 mm. Jan Meijerink's tests of various scopes would tend to support this.

Regards,
John

 

[RJM]

Yes, perhaps for visual use on average. But there are times when bigger is better and certainly so if you want to digiscope.

 

[John Russell]

Lars and Steve,

I'm sorry that I might have been inadvertently winding you up with a rather confused idea :-(.
I had somehow assumed that (human) astigmatism could become insignificant at small pupil or exit pupil diameters, just as the remaining CA of an apochromat disappears within the Airy disc, but I guess it's always there.
At least I have learned that I should keep my glasses on when using my scope!

Regards,
John

 

[mooreorless]

John I think it is boils down to how bad your astigmatism is. I know Brock on here talks about exit pupil and his astigmatism, but I forget at what EP helps him and how much astigmatism he has. -.25 would not be very much.

 

[looksharp65]

No problem, John! It has been a pleasure to reply.

So, when does astigmatism need to be corrected? My qualified assumption says that a corrected astigmatism of between -0.25D to -0.50D would make a similar impression on the view as phase-coating has.
I know that these things aren't comparable, I am just trying to roughly quantify the impact on the image quality.

That is, if you get yourself a phase-coated bin but don't use spectacles, the general image quality would be similar to whan you use a non-phase-coated bin with spectacles, everything else being equal.

This is an assumption. I didn't try it, so I may be far out.

 

[pshute]

Orto-K is a custom made rigid contact lens that you sleep with, only to remove it in the morning. The ambition is to correct myopia without wearing spectacles or contacts during the day, but the means used to achieve this also provide another advantage.
The corneal roughness is greatly reduced, and the users often reach remarkable visual acuity.

http://www.ortho-k.net/

Interesting. I assume these things take a while to mould the cornea to the right shape - the FAQ mentioned it can take 3 to 6 months. Does that mean that for that whole time, the subject's vision is different every day? They must go through a few pairs of glasses.

 

[looksharp65]

Interesting. I assume these things take a while to mould the cornea to the right shape - the FAQ mentioned it can take 3 to 6 months. Does that mean that for that whole time, the subject's vision is different every day? They must go through a few pairs of glasses.

Quote from Ortho-K.net: If your doctor prescribes overnight Ortho-K, expect results immediately. Your vision should be markedly improved after the first night.During the first few days or weeks your vision may fade later in the day. Improvement should be noticed each day.

The cornea is not permanently altered, and basically this is a good thing, as are the immediate results. The downside would be that if you want to desert from the therapy, previous spectacles cannot be used until the cornea has regained its former shape (that it had before the therapy).

I interpret their information as follows:
Good vision can be had immediately, but equilibrium may not be achieved until at least three months have passed.

 

[kabsetz]

Lars,

You have piqued my interest in the Ortho-K process. Happens that I'm about the get my first multifocals, now that my presbyopia and hyperopia together conspire to make even infinity focus impossible. My present prescription for reading glasses is ca. +2.25 and, for best focus at infinity I need ca. +0.8 (haven't had my eye exam yet). What is your educated guess concerning the suitability of this option for my 53-year-old eyes?

Kimmo

 

[pshute]

Quote from Ortho-K.net: If your doctor prescribes overnight Ortho-K, expect results immediately. Your vision should be markedly improved after the first night.During the first few days or weeks your vision may fade later in the day. Improvement should be noticed each day.

I wonder how much this fading is, and whether my comments about periods of uncorrectable vision apply in the first evenings, or even afternoons. I assume they recommend not driving at night for the first week.

The cornea is not permanently altered, and basically this is a good thing, as are the immediate results. The downside would be that if you want to desert from the therapy, previous spectacles cannot be used until the cornea has regained its former shape (that it had before the therapy).

The same thing applies there. I wonder how long it takes to stabilise. I guess they might be able to provide you with lenses, or whatever these things are called, to get the shape baack how it was more quickly.

 

[pshute]

Orto-K is a custom made rigid contact lens that you sleep with, only to remove it in the morning. The ambition is to correct myopia without wearing spectacles or contacts during the day, but the means used to achieve this also provide another advantage.
The corneal roughness is greatly reduced, and the users often reach remarkable visual acuity.

This bit about reducing the roughness, does that mean it might be possible to achieve acuity better than glasses?

 

[looksharp65]

Lars,

You have piqued my interest in the Ortho-K process. Happens that I'm about the get my first multifocals, now that my presbyopia and hyperopia together conspire to make even infinity focus impossible. My present prescription for reading glasses is ca. +2.25 and, for best focus at infinity I need ca. +0.8 (haven't had my eye exam yet). What is your educated guess concerning the suitability of this option for my 53-year-old eyes?

Kimmo

Kimmo,

I don't practice this method, but I felt the urge to mention it, especially because it sometimes enhances the visual acuity beyond what can be done with spectacles or contacts.
The website states that it is for all refractive errors (distance errors, as opposed to prebyopia) so technically it may be done. But your refractive error isn't too gross, your reading prescription holds the lion's share of the power needed. You can't do that with Ortho-K, I'm afraid.

1) If you want the "cleanest" view I recommend bifocals. They will restore your visual performance when looking far and near.

2) Or you could try contacts for distance vision only, and go on with reading glasses as before.

3) One birder I know uses one contact lens for reading (monovision), but has set her binocular's diopter to compensate for that. You would need one for distance and one for near.

4) For everyday normal life that not includes extensive optics testing, I recommend a good pair of progressive spectacles.

If you have further questions, don't hesitate to ask again!

 

[looksharp65]

I wonder how much this fading is, and whether my comments about periods of uncorrectable vision apply in the first evenings, or even afternoons. I assume they recommend not driving at night for the first week.

The same thing applies there. I wonder how long it takes to stabilise. I guess they might be able to provide you with lenses, or whatever these things are called, to get the shape baack how it was more quickly.

I guess it will take some patience to deal with the initial side effects. But I am sure that their website can answer your questions much better than I can.

This bit about reducing the roughness, does that mean it might be possible to achieve acuity better than glasses?

Yep, and that is why I bothered to mention it at all. Remarkable visual acuity has been reported from some individuals unable to achieve it with spectacles.

 

[mooreorless]

Lars, My right eye [oculus dexter] has always been no correction for far distance, about three years ago, OD prescribed +.5 for my right eye. When I received my new glasses they seemed ok inside the building, but when I drove home I had a hard time with these new glasses. I found they worked ok at shorter distances inside the house but not for distance.In fact I sort of like them for in the house. I think my presbyopia is moving further out in my right eye. BTW we compromised and he made it +.25 and I still have problems sometimes when I am tired. BTW I am 59.
Sorry to even bring this up, it seems we are asking you a lot of questions. Send John the bill.

Another ?" Before writing my reply above, I tried my scope with three different eyepieces, with 3 mm exit pupil, with 2 mm exit pupil, and finally with 1.3 mm exit pupil."
Lars was this looking at stars?

Thanks for any reply!!