sweet spots, chromatic abberations, and eye position

[Alexis Powell]

I noticed something recently that really surprised me, and I wonder if it relates in some way to the discrepancies between different observers' reports of edge sharpness in a given binocular model. For the record, I am an eyeglasses wearer and a person who easily notices chromatic abberations in binoculars when I look for it, though it usually doesn't bother me otherwise. In comparing sharpness outside the very center of the field of view (I was trying to tease out the seperate effects of distortion and field curvature) and off-axis chromatic abberation in my beloved 8x32 Leica BA Utra/Trinovid and my new 8x32 Zeiss FL, I noticed that eye position can have a big effect on edge sharpness and off-axis CA in a way that I have never before noticed. For me, in normal use, the Zeiss has extremely little off-axis CA, while there is quite a bit in the Leica, but the Leica retains much better sharpness outside the center of the field. Generally, when thinking about eye position, one assumes that it is important to have one's eyes alligned and centered with the oculars and exit pupil, or else the view will be compromised--and this is generally true--but what I found, for example when looking at the edge of the field at the top of the view, was that if I shift my eye to look more through the bottom of the exit pupil (rather than being centered on it), that chromatic abberation is reduced in the Leica, and if I look through the top of the exit pupil, that edge sharpness improves considerably in the Zeiss. This is an uncomfortable manuever to perform, and is of no utility or significance when it comes to actually using these binoculars, but again, I wonder if it relates in some way to the different impressions different observers have of the same binos. Let me make it clear that I'm not suggesting that some observers are not properly alligning their eyes with the ocular and exit pupil, but that perhaps our eyes match up with it or sample it differently when we are using the same bino correctly.

--Alexis Powell

[Pileatus]

I noticed something recently that really surprised me, and I wonder if it relates in some way to the discrepancies between different observers' reports of edge sharpness in a given binocular model. For the record, I am an eyeglasses wearer and a person who easily notices chromatic abberations in binoculars when I look for it, though it usually doesn't bother me otherwise. In comparing sharpness outside the very center of the field of view (I was trying to tease out the seperate effects of distortion and field curvature) and off-axis chromatic abberation in my beloved 8x32 Leica BA Utra/Trinovid and my new 8x32 Zeiss FL, I noticed that eye position can have a big effect on edge sharpness and off-axis CA in a way that I have never before noticed. For me, in normal use, the Zeiss has extremely little off-axis CA, while there is quite a bit in the Leica, but the Leica retains much better sharpness outside the center of the field. Generally, when thinking about eye position, one assumes that it is important to have one's eyes alligned and centered with the oculars and exit pupil, or else the view will be compromised--and this is generally true--but what I found, for example when looking at the edge of the field at the top of the view, was that if I shift my eye to look more through the bottom of the exit pupil (rather than being centered on it), that chromatic abberation is reduced in the Leica, and if I look through the top of the exit pupil, that edge sharpness improves considerably in the Zeiss. This is an uncomfortable manuever to perform, and is of no utility or significance when it comes to actually using these binoculars, but again, I wonder if it relates in some way to the different impressions different observers have of the same binos. Let me make it clear that I'm not suggesting that some observers are not properly alligning their eyes with the ocular and exit pupil, but that perhaps our eyes match up with it or sample it differently when we are using the same bino correctly.

--Alexis Powell

Alexis,

I can affirm your observations. I recently discovered that I could improve contrast in the normal Ultravid image by repositioning my eye in an unnatural manner...at the expense of increased CA, I might add.

I believe eyeglasses can seriously degrade the "normal" image delivered by any binocular. There are many reasons for this, but it should be obvious that eyeglasses increase the complexity of the optical system, often at the expense of image quality.


John

[Atomic Chicken]

it should be obvious that eyeglasses increase the complexity of the optical system, often at the expense of image quality.

John

John,

Just out of curiousity... can you think of any optical phenomenon that is actually improved by the addition of eyeglasses? (other than the obvious corrective purposes of eyeglasses in the first place, of course! ;) )

Best wishes,
Bawko

[Robert Ellis]

My specs add noticeable CA to streetlamps at night, so I imagine that they add even a little in any light.

[Pileatus]

John,

Just out of curiousity... can you think of any optical phenomenon that is actually improved by the addition of eyeglasses? (other than the obvious corrective purposes of eyeglasses in the first place, of course! ;) )

Best wishes,
Bawko

Bawko,

The Hubble space telescope.

John

[marcus]

Well, could it be that maybe you would not recognize someone who wears eyeglasses if they did not have them on? And so wearing them, they are recogizable. Is that possible?
OK, I know, that was silly. I'm sorry.

[Rich N]

I noticed something recently that really surprised me, and I wonder if it relates in some way to the discrepancies between different observers' reports of edge sharpness in a given binocular model. For the record, I am an eyeglasses wearer and a person who easily notices chromatic abberations in binoculars when I look for it, though it usually doesn't bother me otherwise. In comparing sharpness outside the very center of the field of view (I was trying to tease out the seperate effects of distortion and field curvature) and off-axis chromatic abberation in my beloved 8x32 Leica BA Utra/Trinovid and my new 8x32 Zeiss FL, I noticed that eye position can have a big effect on edge sharpness and off-axis CA in a way that I have never before noticed. For me, in normal use, the Zeiss has extremely little off-axis CA, while there is quite a bit in the Leica, but the Leica retains much better sharpness outside the center of the field. Generally, when thinking about eye position, one assumes that it is important to have one's eyes alligned and centered with the oculars and exit pupil, or else the view will be compromised--and this is generally true--but what I found, for example when looking at the edge of the field at the top of the view, was that if I shift my eye to look more through the bottom of the exit pupil (rather than being centered on it), that chromatic abberation is reduced in the Leica, and if I look through the top of the exit pupil, that edge sharpness improves considerably in the Zeiss. This is an uncomfortable manuever to perform, and is of no utility or significance when it comes to actually using these binoculars, but again, I wonder if it relates in some way to the different impressions different observers have of the same binos. Let me make it clear that I'm not suggesting that some observers are not properly alligning their eyes with the ocular and exit pupil, but that perhaps our eyes match up with it or sample it differently when we are using the same bino correctly.

--Alexis Powell

It's not unusual to see that effect in eyepieces for astronomical telescopes. I also wear eye glasses when using binoculars and telescopes.

Rich

[Andrew Rowlands]

As this is a general optics Thread, I've moved it somehere more appropriate ;)

Cheers,

Andy.

[AlanFrench]

Having the proper IPD and proper placement of your eyes is important. Misplacement or the wrong IPD can certainly add noticeable chromatic aberrations to the image. At night, with a telescope, I find this very obvious (the misplacement, not the wrong IPD, generally <g>).

Last summer my wife and I were hiking and we had along a pair of Canon 10x30 IS binox. She commented on a yellow fringe she saw in the image. I suggested changing the IPD. She did, and the fringe went away.

If you have astigmatism in your eyes, GET glasses to correct it. Astigmatism is very harmful to the image falling on your retina - more so than other aberrations. Vanilla near-sighted or far-sightedness can be corrected by simply refocussing, but that won't do anything about astigmatism.

Clear skies, Alan

[Rico]

My explanation of the sharpening effect at the edge (which I also observe) is the improvement of certain optical aberrations when the aperture is stopped down. (Lateral color isn't one of them, sadly.) Those of us who are near- or far-sighted can sharpen a small portion of a view by observing just along a ruler's edge, or even our hand. Hold the edge about one inch from the eye, and view through the penumbra. Not very useful for birdwatching!