A unified theory: Exit pupil aberrations and Eye position, types of image blackout (1 Viewer)

[kimmik]

With some experimentation, calculations, this is a breakdown of different types of image black out that seems to me most commonly experienced when roaming the eye around looking through binoculars. SAEP being spherical aberration of exit pupil, prism vignette being the "almond" shape of the light beam arising from the periphery.

Eyepiece without SAEP:
Closer eye position - crescent
On spec - shutter
Further - counter crescent

Eyepiece with SAEP and low prism vignette:
Closer eye position - kidney
On spec - kidney + counter crescent
Further - counter curtain

Eyepiece with SAEP and stronger prism vignette:
Closer eye position - curtain
On spec - double crescent
Further - counter curtain

Best tested in bright conditions that shrink the anatomical pupil and exacerbate blackout effects. Larger exit pupil binoculars are more forgiving.

 

[WJC]

The image IS NOT formed IN SPACE! 'Sorry about that. The rest, however, is accurate.

 

[kimmik]

Bright daytime use, I prefer settings 1, 6 and 8.

Some crescent blackout is better than shutter blackout for me, where you suddenly lose the whole image. Crescent is a gradual loss of image starting on the side you are looking away from.

Round pupil design is best for long eye relief but limits eye roaming a bit.

Overcast/low light softens the blackouts substantially as the anatomic pupil is more dilated.

 

[kimmik]

Putting my theory to practice, here is a monocular that has mild SAEP related kidney bean blackout, can be photographed with a phone camera.

1. well centered camera
2. off centered camera and too close to eyepiece, kidney bean
3. putting a small washer over the prism to simulate a smaller prism, partially vignetting the exit pupil of the peripheral image
4. no more kidney bean if too close to eyepiece (more crescent rather than curtain in this example). this is the leica noctivid design in a nutshell

 

[tenex]

This is basically about eye relief and roaming, i.e. vertical positioning of the eye. What about horizontal alignment -- why are some eyepiece designs prone to blackouts at the slightest deviation from a centered pupil, when most are more forgiving? (A small exit pupil can exacerbate this but isn't normally the underlying cause.)

 

[kimmik]

The effect of horizontal misalignment feels pretty similar to eye rotation related misalignment but I haven’t investigated it thoroughly, what do you think?

Besides SAEP and prism vignette, there are other uncommon or minor pupil aberrations which I don’t know enough about to evaluate. Eg i am yet to see chromatic aberration of exit pupil, possibly because it is so disturbing that designers absolutely avoid it?

 

[tenex]

Maybe the effect is similar, and a design with a small "eye box" will be as finicky about horizontal alignment as vertical? I don't have much experience with this, but some NL users complain that eye placement is tricky, and I recently was unable to avoid blackouts with a Meopta wide-angle scope eyepiece.

 

[kimmik]

For comparison I have a 75ish degree AFOV vintage bino with 3.75mm exit pupil. And while it has no SAEP on measurement, the blackout/pupil vignette were noticeable during use, crescent and shutter types. That was how I decided to focus on larger exit pupil when choosing wide angle binoculars.

Test the eyepiece for SAEP, prism vignette, and in conjunction with knowing the AFOV, exit pupil size and eye pupil size, are the 5 parameters that about fully predicts “eye box” behaviour.

 

[tenex]

Test the eyepiece for SAEP, prism vignette, and in conjunction with knowing the AFOV, exit pupil size and eye pupil size, are the 5 parameters that about fully predicts “eye box” behaviour.

Where have you found instructions for this? I'd like to understand it better.

 

[Troubador]

This is basically about eye relief and roaming, i.e. vertical positioning of the eye. What about horizontal alignment -- why are some eyepiece designs prone to blackouts at the slightest deviation from a centered pupil, when most are more forgiving? (A small exit pupil can exacerbate this but isn't normally the underlying cause.)

I am perfectly willing to be corrected but my experience suggests this: if your eyes are too far away from the binos' eyepieces you cannot see the full field of view, and if your eyes are too close to the eyepieces then you experience 'blackouts'.

This puts a considerable responsibility on the dimensions of the eyecups (and its available positions) and of your eyes/eye sockets/spectacles as to where your eyes end up in relation to the eyepieces and therefore whether you can see the full field of view without blackouts.

If the available positions of the eyecups do not provide viewing without blackouts I have had success in slipping rubber o-rings over the eyecups and down towards the optical tubes in order to secure the eyecups in positions not provided by their 'click-stops'.

Lee

 

[kimmik]

Where have you found instructions for this? I'd like to understand it better.

No comprehensive instructions anywhere as far as I know, but cloudy nights has a thread with many photos demonstrating the severity of SAEP of some astro eyepieces. Looking at the photos will allow one to devise their own means of replicating it.

 

[MiddleRiver]

I am perfectly willing to be corrected but my experience suggests this: if your eyes are too far away from the binos' eyepieces you cannot see the full field of view, and if your eyes are too close to the eyepieces then you experience 'blackouts'.

This puts a considerable responsibility on the dimensions of the eyecups (and its available positions) and of your eyes/eye sockets/spectacles as to where your eyes end up in relation to the eyepieces and therefore whether you can see the full field of view without blackouts.

If the available positions of the eyecups do not provide viewing without blackouts I have had success in slipping rubber o-rings over the eyecups and down towards the optical tubes in order to secure the eyecups in positions not provided by their 'click-stops'.

Lee

Entirely consistent with my real-world experience. I wear thin wire-framed specs. With Trinovid 8x32HD (17mm) and UV 8x42HD+(16mm) I need to have my specs pretty well glued to the ocular and I have no blackout issues. I tried NV 10x42's (19mm) in store once and much to my surprise had blackout issues. Pulling eyecups out one step took care of it and I could still see full field. With 8x40SFL's (17mm) eyebox is perfect and actually better than the UV as it feels a little less finicky, while not causing any blackouts.

 

[tenex]

If the available positions of the eyecups do not provide viewing without blackouts I have had success in slipping rubber o-rings over the eyecups and down towards the optical tubes in order to secure the eyecups in positions not provided by their 'click-stops'.

Unfortunately I needed not an intermediate position, but a taller eyecup. I wasn't going to glue something onto the rim, so the WA eyepiece went back.

Here's a nice illustration from handprint.com. Being too close produced kidney beans as on the right here, classic sign of too short an eyecup. But losing contact with it created difficulty staying centered, leading to crescent blackouts shown on the left, as lateral motion would in any eyepiece. (I was too hasty in imagining the Meopta specifically more sensitive to this; it was all an eyecup problem.)

 

[Troubador]

Unfortunately I needed not an intermediate position, but a taller eyecup. I wasn't going to glue something onto the rim, so the WA eyepiece went back.

Here's a nice illustration from handprint.com. Being too close produced kidney beans as on the right here, classic sign of too short an eyecup. But losing contact with it created difficulty staying centered, leading to crescent blackouts shown on the left, as lateral motion would in any eyepiece. (I was too hasty in imagining the Meopta specifically more sensitive to this; it was all an eyecup problem.)

OK, here is a suggestion that depends on the design of the eyecup and the port into which it screws.
You could try putting a rubber o-ring underneath the eyecup so that when it is screwed down it is in a higher position.

Lee

 

[kimmik]

Another phenomenon is chromatic aberration of the exit pupil. Even if the image is perfectly corrected for chromatic aberration (seen in below simulation of an extreme case), the exit pupil may have different eye relief for red part of image vs blue part of image.

This can lead to flashes of blue fringe or red fringe, depending on eye position. The red fringe isn't as noticeable as the blue fringe.



Zeiss SF 8x32 pupil in the far periphery, optimised to show the effect. In person it is very subtle.

 

[Thotmosis]

With some experimentation, calculations, this is a breakdown of different types of image black out that seems to me most commonly experienced when roaming the eye around looking through binoculars. SAEP being spherical aberration of exit pupil, prism vignette being the "almond" shape of the light beam arising from the periphery.

Eyepiece without SAEP:
Closer eye position - crescent
On spec - shutter
Further - counter crescent

Eyepiece with SAEP and low prism vignette:
Closer eye position - kidney
On spec - kidney + counter crescent
Further - counter curtain

Eyepiece with SAEP and stronger prism vignette:
Closer eye position - curtain
On spec - double crescent
Further - counter curtain

Best tested in bright conditions that shrink the anatomical pupil and exacerbate blackout effects. Larger exit pupil binoculars are more forgiving.

View attachment 1474775

This kind of posts is what makes the binoculars & spotting scopes section of birdforum one of fmy avorite hangouts on the World Wide Web. Though i understand maybe just 25% of it ;-)

 

[kimmik]

This kind of posts is what makes the binoculars & spotting scopes section of birdforum one of fmy avorite hangouts on the World Wide Web. Though i understand maybe just 25% of it ;-)

Its the synergy of physics and biology that I enjoy around here! The nature side keeps theoretical stuff in perspective and relevant. In an astro forum its usually just a case of bigger = better.

 

[Thotmosis]

Its the synergy of physics and biology that I enjoy around here! The nature side keeps theoretical stuff in perspective and relevant. In an astro forum its usually just a case of bigger = better.

True but e.g. cloudynights.com has also very interesting treads on vintage binoculars. It’s not so much about the latest and greatest as i sometimes feel it’s a bit too much about that here, but i enjoy reading it nevertheless

 

[kimmik]

Oh yes, latest is greatest hype here is a bit testing, a lot if filtering required. Almost second nature now lol.

 

[Trinovid]

...if your eyes are too close to the eyepieces then you experience 'blackouts'.

This could have saved me so much frustration and explains now why I'm not having nearly so much trouble with blackouts any more, as I've gradually adopted a different hold. Wish I'd understood this sooner!