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Effect of glasses on magnification and apparent field of view (1 Viewer)

mpeace

Well-known member
Here I present the results of a quick analysis of the effect of glasses on magnification and apparent field of view.

I used a Swarovski 8.5x42 EL SV FP and an iPhone 5 mounted on tripods taking photos of the image through the binoculars with no glasses, -2.75 prescription glasses and +3.50 reading glasses. The images are attached. Measuring distances between the same points for each image taken enables a comparison of magnification using the "no glasses" image as a reference.

Results (magnification)

No glasses - 8.50x
-2.75 - 8.37x
+3.50 - 8.59x

For me where my eyes are naturally +2.75 and I need the -2.75 prescription, the base line is the -2.75 glasses and the true magnification for me is:

No glasses - 8.63x
-2.75 - 8.50x
+3.50 - 8.72x

We can also relate this somewhat to the change in apparent field of view by scaling the specified 60 degree apparent field of view by the proportional change in magnification:

Results (apparent field of view):

No glasses - 60.0 deg
-2.75 - 59.1 deg
+3.50 - 60.7 deg

For me with my +2.75 eyes making the -2.75 glasses the base line:

No glasses - 60.9 deg
-2.75 - 60.0 deg
+3.50 - 61.6 deg

Discussion

Positive prescription glasses (reading glasses) increase binocular magnification and apparent field of view and negative prescription glasses reduce magnification and apparent field of view. By removing negative prescription glasses you increase the apparent field of view and magnification slightly. If you normally don't wear glasses for distance viewing, but have some reading glasses you may find the slight increase in apparent field of view and magnification from wearing them whilst using binoculars interesting to try.

I did this little analysis because having always used glasses (-2.75) with my binoculars I recently have been taking them off more and felt the view was more immersive with a slight gain in magnification. This is born out in the results above. The changes though slight are noticeable in practice, though for practical reasons I'll probably only continue removing my glasses in stationary locations like hides or in my garden. The +3.50 reading glasses I used are low cost £5 glasses purchased to experiment with. I shan't be using these much as while the image is still useable there is a drop in quality to some aspects of the view. If you have higher quality reading glasses you may find the quality is maintained.

Hopefully some of view may find this interesting or even useful.
 

Attachments

  • No glasses.JPG
    No glasses.JPG
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  • -2.75.JPG
    -2.75.JPG
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  • +3.50.JPG
    +3.50.JPG
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Rico70

Well-known member
In my opinion, technical curiosity is always appreciable, in order to better understand how things are.

Having said that, I can add that in general, the contribution of eyeglasses (within a value considered normal of +/- 4 diopters) is on average much lower than the tolerance between the real and nominal magnification values of the binoculars. Consequently, the difference in apparent amplitude of the observation window (what is erroneously called AFOV) is theoretically of minimal degree.

However, I believe and I can also accept that, although the theory is error-free, the practical response due to personal feeling can make things differ and also accentuate the phenomenon (a bit like mirages).


I analyzed the photos you posted and I noticed a certain degree of distortion and blurring that could have compromised the accuracy of the test.
 

Binastro

Well-known member
What is not included here is the distance between the glasses and the eye.
If this distance is varied, either by the spectacle frame or facial figure then the magnification changes.

In fact, I get quite useful magnification gains.
With the VisionKing 5x25 and glasses I can get 5.5x magnification and still see the full 15 degree field.
The simple measure AFOV increases from 75 degrees to 82 degrees, although I haven't measured the AFOV properly.

In addition, I can get a 2x magnification just by holding my distance glasses 20cm in front of my face with quite a reasonable image. No other optics involved. I do this often to read street names etc.

In another context using a 2,200mm focal length 135mm observatory refractor we got a reasonable magnification, maybe about 4x or 5X with no eyepiece at all. Just using our eyes at age late 20s. No glasses involved. I can't remember the actual magnification from so long ago, but it was interesting that we didn't need an eyepiece.

Taking photos with the glasses and a binocular I think that the magnification depends on the separation of the components. Camera, glasses and binocular.
This may not be the same as actually wearing the glasses.
Also the accommodation of the eye might influence the result.

The nominal and real magnification of the binocular is not part of the equation. The binocular is what it is.

B.
 
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MandoBear

Well-known member
Surely, also - if one needs prescription glasses of say, -2.75 dpt, then this implies that one's eyes are already at +2.75 dpt - so there could be a magnification factor to cancel out...? And as Binastro aptly adds, the spacing of the optics makes a very important contribution to any magnification factor.
 

mpeace

Well-known member
Surely, also - if one needs prescription glasses of say, -2.75 dpt, then this implies that one's eyes are already at +2.75 dpt - so there could be a magnification factor to cancel out...? And as Binastro aptly adds, the spacing of the optics makes a very important contribution to any magnification factor.

Exactly. That's why I've given two sets of figures. So for me wearing -2.75 glasses cancels out the +2.75 of my eyes (I presume) hence in my second set of figures I take this as the baseline. Then removing the glasses puts me at +2.75 and I get the magnification effect.

In each case the glasses were in direct contact with the eye cup and the iPhone camera distance behind the binocular eye piece was fixed at ~3cm.

I'm quite sure Binastro is correct about the effect the position of the optics has, but this wasn't a study of that, just a quick comparison to try and quantify in a relative way the effect of different glasses that is easily observable in use. The figures shouldn't be taken as absolute, just evidence of the effect. You can find equations to calculate the change in magnification and they're mind-bogglingly complex taking in a huge amount of factors. So as I say this was a rough and ready approach to try and capture what I see and get a ball-park figure of the level of the effect.
 
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Rico70

Well-known member
What is not included here is the distance between the glasses and the eye.
If this distance is varied, either by the spectacle frame or facial figure then the magnification changes.
The distance between the glasses and the eye it is the common one for which the ophthalmologist calculates the prescription. And the changes are minimal.
So it is completely useless and misleading to talk about of 20-25cm distance, when the ultimate goal is to observe binoculars correctly with your glasses.

In fact, I honestly don't understand what the real purpose of these speeches is.


In my opinion it is much more important to discuss whether or not you can observe your binoculars with glasses (prescriptions). On the possibility of using contact lenses, or also on the possibility of being able to mount the corrective lenses directly on the eyepieces of the binoculars (e.g.).

A displacement of +/- 5% about the magnification is rather irrelevant on the width of the apparent window (but also +/- 10%).
 

mpeace

Well-known member
A displacement of +/- 5% about the magnification is rather irrelevant on the width of the apparent window (but also +/- 10%).

It's not irrelevant at all. From my observations the actual field of view through binoculars dosen't change with wearing glasses, but glasses affect the magnification of the image through the eye-piece, so a change in magnification has a direct linear correlation to the apparent width of the field of view.

For me, between not wearing glasses and wearing reading glasses, there was a 1.6 degree change in apparent field of view. So going from 60 degrees to 61.6 degrees (2.7% change). It's not massive, but noticeable to me and worthwhile (removing more of the black field stop ring from around the image, giving more of an SF feel to the view). If someone has a pair of binoculars that feels a bit tunnel like at say 59 degrees and can change the apparent field of view with glasses to say 61 degrees that for them may be worthwhile also.

I find it odd that you think a +/- 5% or even +/- 10% change in a parameter is irrelevant. I hope you don't think this is within manufacturing tolerances as your first response suggests. I'm only guessing, but I imagine parameter tolerances for the Swarovski 8.5x42 EL SV (or any other binocular for that matter) are well within 10%.
 

Rico70

Well-known member
For me, between not wearing glasses and wearing reading glasses, there was a 1.6 degree change in apparent field of view.
It is not correct to look at binoculars with reading glasses. You understand that right?
But other than that, 1.6 ° difference over 60 °, it can't cause anyone's tunnel effect. The tunnel effect can be seen between 60 ° and 45 ° (33%).

However, I don't want to contradict you, I don't want to ruin your belief and I don't want to discuss your opinion. Feel 100% free to believe anything. :t:
 

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