40 replies to this topic

[Vertigo]

Hello,

 

I'd like to try a Dioptrx on my 31mm Nagler.

 

I've read several times now that astigmatism worsens slightly at night due to pupil dilation.

I have -2.00 in my observation eye. Would a -2.50 diopter be appropriate here, or should I use the "correct" -2.00?

What are your general experiences?

Thanks in advance...

 

Andreas

[buckingham_hoo]

I would give the 2.50 a try. I have 0.75 diopters in my observing eye. I have multiple of both 1.0 and 1.25 dioptrx. Both work great for me, but I feel like the 1.25 is ever so slightly sharper. I find them easy to use with the 31T5.

[TOMDEY]

The expresses theory should not change the best Dioptrx that you use. The units of Diopter (inverse meter) are by its very nature pupil size independent, What confuses many (even most?) users is that your sensitivity to astigmatism is proportional to pupil size, but not the needed amount of correction itself. That's a subtle difference in interpretation. Which is to say >>> if your optometric correction is -2.00D, that is the Dioptrx that you would best benefit using.

[Jon Isaacs]

The expresses theory should not change the best Dioptrx that you use. The units of Diopter (inverse meter) are by its very nature pupil size independent, What confuses many (even most?) users is that your sensitivity to astigmatism is proportional to pupil size, but not the needed amount of correction itself. That's a subtle difference in interpretation. Which is to say >>> if your optometric correction is -2.00D, that is the Dioptrx that you would best benefit using.

 

As they say, in theory, there is no difference between practice and theory. In practice there is.

 

That's why the eye doctor tries different combinations to determine your prescription..

 

Would you be surprised if someone with a prescription for 2.0 diopters of astigmatism found the 31 mm worked best with 2.5 diopter Dioptrix?  I wouldn't be 

 

Jon

[davidgmd]

I think you’re both right. A refraction done with the pupil at 6 mm may be different than one done at 4 or 3 mm. But in my experience, rarely more than 0.25 diopters (D) different. Also one reason why refraction is performed in a dim room is so that the pupil is dilated to somewhere in the mesopic to scotopic range. That makes it easier for the patient to “see” the effects of the astigmatism during the refraction and accurately choose which is better, lens 1 or 2.

  
Everyone’s eyes are unique, but based on typical tendencies my advice would be to get a Dioptrx that matches your spectacle prescription. If your prescription lies between 2 Dioptrx powers (i.e. X.25 or X.75 diopters), I’d recommend rounding up 0.25 D to the next highest power.

  
If you think 0.25 D of uncorrected astigmatism will really bother you, most can use the 31 mm Nagler and see the whole field with glasses.

 

Edit: In my personal experience with 2.00 D of astigmatism, I see the same with a 2.00 D Dioptrx on the 31 mm Nagler as I do with my glasses.

Edited by davidgmd, 13 April 2025 - 03:27 PM.

[T-rav82]

I have 2.00 D of astigmatism and having also read that many observers favour a dioptrix that is slightly over their prescription, I decided to purchase two: a 2.0 and a 2.25.

I am inclined, solely based on experience, to say I favour the 2.25. However, there are nights that I cannot tell the difference or feel the reverse is true. I only use them with tv delites

I find that the following factors most affect my findings; magnification (exit pupil size), how well dark adapted my eyes are, and tiredness.

I have been using them for months and with many variables to consider, I am still not sure.

With a widefield ep like the 31 Nagler, I believe it may be difficult to adjust the dioptrix to achieve pin sharp images right across the field at the same time. Having said that, I sometimes have a similar issue with glasses and I feel I have to roll my head slightly to focus on stars at the edge of field.

No science behind this, just my experience

[TOMDEY]

As they say, in theory, there is no difference between practice and theory. In practice there is.

That's why the eye doctor tries different combinations to determine your prescription..

Would you be surprised if someone with a prescription for 2.0 diopters of astigmatism found the 31 mm worked best with 2.5 diopter Dioptrix?  I wouldn't be 

Jon

Good point --- but a best 1st guess is that it would be the same as the doctor measured in the office.    Tom

[Jon Isaacs]

Good point --- but a best 1st guess is that it would be the same as the doctor measured in the office.    Tom

 

I recommend following David's advice .

 

TeleVue doesn't make a Dioptix to match my eye.. 

 

Jon

[TOMDEY]

Yeah, the ~residual astigmatism~ presented your retina will vary between the difference and the sum of your eye's and the Dioptrx... depending on the rotation of the Dioptrx relative to your head. What we of course do is to seek out the best correction, which is the difference. The sensitivity to "perfect rotation" is proportional to the magnitude of the sum. So someone needing e.g. 1D correction will find it much more forgiving than someone needing 3D.

 

Oh! Something I just remembered --- regarding the ~clocking of the Dioptrx~ The A B C D E F letters around the edge have no fixed phase angle relationship to the astigmatic lens element; it's arbitrary. So if you're keying into that, it will be different for each Dioptrx that you use.   Tom

Edited by TOMDEY, 13 April 2025 - 07:31 PM.

[thecelloronin]

Heads up: For some, the 31 Nagler + DioptRx might not have enough eye relief. I found it borderline, but I've got thick plastic frames.

[Jon Isaacs]

Heads up: For some, the 31 Nagler + DioptRx might not have enough eye relief. I found it borderline, but I've got thick plastic frames.

 

So it seems you're using a 31 mm Nagler + a DioptRx and eyeglasses? That seems redundant.

 

Jon

[Jon Isaacs]

 

Yeah, the ~residual astigmatism~ presented your retina will vary between the difference and the sum of your eye's and the Dioptrx.

 

In case you were responding to my comment that TeleVue doesn't make a DioptRx to match my eye, my prescription, that's kind of a joke.

 

I have 0.0 cylinder in both eyes.. zero astigmatism, that's what the doctor measured, that's what I see.  That's one hypothesis.

 

The other hypothesis is my eyes are so crappy the astigmatism cannot be determined. 

 

That is also kind of a joke. 

 

Jon

[davidgmd]

I have 2.00 D of astigmatism and having also read that many observers favour a dioptrix that is slightly over their prescription, I decided to purchase two: a 2.0 and a 2.25.

I am inclined, solely based on experience, to say I favour the 2.25. However, there are nights that I cannot tell the difference or feel the reverse is true. I only use them with tv delites

I find that the following factors most affect my findings; magnification (exit pupil size), how well dark adapted my eyes are, and tiredness.

I have been using them for months and with many variables to consider, I am still not sure.

With a widefield ep like the 31 Nagler, I believe it may be difficult to adjust the dioptrix to achieve pin sharp images right across the field at the same time. Having said that, I sometimes have a similar issue with glasses and I feel I have to roll my head slightly to focus on stars at the edge of field.

No science behind this, just my experience

  
The 2.25D Dioptrx is the only one currently marketed in a quarter-diopter step. It may also be the only one where it might be logical to order up one step and buy the 2.25D when your prescription says you have 2.00D of astigmatism. It will be off by 0.25D at most.

 

If you really need only 2.00D, as Tom Dey alluded to, 2.00D is what you’ll get as you rotate the 2.25D Dioptrx for maximum sharpness. When the axis of the Dioptrx is not aligned with your axis of astigmatism, the effective astigmatic correction drops off.

  
So why not just order 0.50D more power than your prescription in every case and simply rotate to maximum sharpness? Because the axis misalignment needed to give the correct astigmatic power induces a bit of blur. The greater the mismatch between your prescription and the Dioptrx power, the more off-axis you’ll end up for maximum sharpness. Resulting in more residual blur. With a 0.25D mismatch, it’s undetectable by most people.

  
Incidentally, glasses are made in 0.25D steps because most people under most lighting conditions can’t consistently tell the difference between smaller increments. Though as I'm fond of saying, astronomers aren’t most people, and we don’t operate under typical lighting conditions.

[Starman1]

In my case, the sharpest DioptRx was the same correction as my glasses, though glasses were better when looking at the edge of an 82° field because of the angle.

[TOMDEY]

In case you were responding to my comment that TeleVue doesn't make a DioptRx to match my eye, my prescription, that's kind of a joke.

 

I have 0.0 cylinder in both eyes.. zero astigmatism, that's what the doctor measured, that's what I see.  That's one hypothesis.

 

The other hypothesis is my eyes are so crappy the astigmatism cannot be determined. 

 

That is also kind of a joke. 

 

Jon

A whole other dimension is that our corrections vary during the day and with (alas) head position. I remember when the surgeon was working on my eyes (patient supine, head looking straight up relative to gravity) that he mentioned a "position correction" that they account for when measuring and doing the corneal topology for PRK. That way, when I stand up (or whatever their nominal assumption is) --- my correction would more likely gravitate/relax to targeted best. Whatever he was doing --- it indeed corrected me to a reliable 20/15, which has held stable for many years now. When I wake up in the morning... my acuity is deficient for seems like an hour and then gets better for the rest of the day.   Tom

[davidgmd]

A whole other dimension is that our corrections vary during the day and with (alas) head position. I remember when the surgeon was working on my eyes (patient supine, head looking straight up relative to gravity) that he mentioned a "position correction" that they account for when measuring and doing the corneal topology for PRK. That way, when I stand up (or whatever their nominal assumption is) --- my correction would more likely gravitate/relax to targeted best. Whatever he was doing --- it indeed corrected me to a reliable 20/15, which has held stable for many years now. When I wake up in the morning... my acuity is deficient for seems like an hour and then gets better for the rest of the day.   Tom

    
There is a bit of cyclotorsion when we lay flat. The eyes rotate a bit within the orbit. Our astigmatism axis doesn’t actually change in reference to the eyeball, but it does shift a few degrees relative to the rest of the world. Uncorrected vision is not affected by this, but it will throw off any external correction a tiny amount. Whenever an astigmatism correcting procedure is done, the horizontal axis is marked for reference in the upright position, prior to lying flat for the surgery.

   
The cyclotorsion reverses immediately on sitting upright. Blurring lasting an hour or so on awakening is more likely due to tear film or corneal surface irregularity or corneal edema that resolves with the eye open and resumption of normal blinking.

[TOMDEY]

Cyclotorsion --- I think I get a bit of that when going around a curve on my motorcycle.    Tom

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[davidgmd]

Since you no longer wear any correction for astigmatism, you don’t have to worry about your vision when taking those 80 mph turns at a 60 degree lean.

[TOMDEY]

Since you no longer wear any correction for astigmatism, you don’t have to worry about your vision when taking those 80 mph turns at a 60 degree lean.

Unless I hit a pothole.    Tom

[davidgmd]

Unless I hit a pothole.    Tom

  
If that happens someone else may be using your corneas.

[Vertigo]

Thanks guys, lots of input!  It seems to vary from person to person.

 

The thing is, I don't like using the 31mm with eyeglasses anymore. As a farsighted glasses wearer, I find the view extremely uncomfortable. I've often considered selling the eyepiece, but on the other hand, the optical performance is very good; using it with a dioptrx would be the last chance.

 

Apparently, with dioptrx, 7-8mm of eye relief is lost, so the 31mm would then effectively only have 12mm of eye relief. I often use orthos and Co. and the like with even closer eye relief, but they naturally have a much smaller field of view. With the Nagler I would have to see if the entire field of view is even available. In practice, however, a more humane view would be more important to me, otherwise the Nagler would no longer make sense in the long run.

 

I'll probably order a dioptrx lens for my prescription and then test whether it works.

Thanks again, everyone...

 

Andreas

[Mike W]

Thanks guys, lots of input!  It seems to vary from person to person.

 

The thing is, I don't like using the 31mm with eyeglasses anymore. As a farsighted glasses wearer, I find the view extremely uncomfortable. I've often considered selling the eyepiece, but on the other hand, the optical performance is very good; using it with a dioptrx would be the last chance.

 

Apparently, with dioptrx, 7-8mm of eye relief is lost, so the 31mm would then effectively only have 12mm of eye relief. I often use orthos and Co. and the like with even closer eye relief, but they naturally have a much smaller field of view. With the Nagler I would have to see if the entire field of view is even available. In practice, however, a more humane view would be more important to me, otherwise the Nagler would no longer make sense in the long run.

 

I'll probably order a dioptrx lens for my prescription and then test whether it works.

Thanks again, everyone...

 

Andreas

Dioptrix is only for astigmatism not farsightedness.

https://www.televue...._page.asp?id=54

Edited by Mike W, 14 April 2025 - 07:59 PM.

[Vertigo]

Dioptrix is only for astigmatism not farsightedness.

https://www.televue...._page.asp?id=54

Of course, long-sightedness or short-sightedness is corrected with the focuser.

 

Andreas

 

P.S. Farsighted wearers usually need a larger eye relief because the lenses are convex, which doesn't exactly make using the Nagler 31mm easier.

Edited by Vertigo, 14 April 2025 - 08:18 PM.

[davidgmd]

Thanks guys, lots of input!  It seems to vary from person to person.

 

The thing is, I don't like using the 31mm with eyeglasses anymore. As a farsighted glasses wearer, I find the view extremely uncomfortable. I've often considered selling the eyepiece, but on the other hand, the optical performance is very good; using it with a dioptrx would be the last chance.

 

Apparently, with dioptrx, 7-8mm of eye relief is lost, so the 31mm would then effectively only have 12mm of eye relief. I often use orthos and Co. and the like with even closer eye relief, but they naturally have a much smaller field of view. With the Nagler I would have to see if the entire field of view is even available. In practice, however, a more humane view would be more important to me, otherwise the Nagler would no longer make sense in the long run.

 

I'll probably order a dioptrx lens for my prescription and then test whether it works.

Thanks again, everyone...

 

Andreas

  
The bolded portion is only partly correct. Dioptrx eats up 7-8 mm of eye relief, but it gives back the vertex distance between your eye and the back of your glasses plus the thickness of your glasses. In most cases that’s more than 7-8 mm so the net effect on eye relief is to increase it.

 
That said, everyone’s anatomy and eyeglass fit differs. If your glasses sit unusually close to your eyes, you’ll gain less of an increase in effective eye relief or possibly even lose some with Dioptrx.

[davidgmd]

For perspective, the typical spectacle vertex distance is 12-14 mm. I've seen anywhere from 5 mm to more than 20 mm. 

     
The center thickness of a typical eyeglass lens is 2-3 mm, with a range of around 1.5 to 6 mm. Larger frames and stronger prescriptions tend to the thicker end of the spectrum. High index materials tend to the shorter end of the spectrum.

   
Adding these all together, the “typical” eyeglass wearer would gain 13 + 2.5 = 15.5 mm from ditching their spectacles and lose 7.5 mm to the Dioptrx, for a net eye relief gain of 8 mm.
    
YMMV. The optical shop where you buy your glasses can tell you your vertex distance and the central thickness of your lenses.
 
Edit: Of course, if you squish your glasses closer to your eye while observing, the optical shop measurement associated with normal wear will be greater than what you experience while observing. A little at-the-eyepiece experimentation with how much squishing you do to see the entire FOV with your glasses on might be helpful.

   
Worst case scenario, with a 1.5 mm thick lens and a 5 mm vertex distance, you would lose 1 mm of effective eye relief with a Dioptrx when compared to wearing glasses. Probably not too many people in that category.

Edited by davidgmd, 15 April 2025 - 11:31 AM.