42 replies to this topic

[BillC]

Hello again Mark ,

I'm sure you are correct. . . Regards , Kenny

Hi Kenny: Think really hard before casting this in stone . . . at least for daytime use.

Cheers,

Bill

[KennyJ]

Reading back over the series of posts , I now realise that an apology is due on my part.

When I wrote in response to Mark's post " I'm sure you are CORRECT " -- I MEANT to type " INCORRECT ".

In other words , NO -- I do NOT think that the " comfort zone cushion " or " wiggle factor " DOES apply when the exit pupil from a binocular is SMALLER than the dilation of the eye's pupil.

I'm sorry about that misquote -- it just goes to show how two little letters such as " in " can make such a difference and lead to so much misunderstanding ! :-)

Kenny

[Mark9473]

When I wrote in response to Mark's post " I'm sure you are CORRECT " -- I MEANT to type " INCORRECT ".

Kenny, thanks for clarifying this - at least now I'm assured I'm not misreading things.

If you don't mind, I would like to hear your explanation, and that of any other who wants to contribute.

The way I see it is as follows:
If the binocular exit pupil is larger than the eye entrance pupil, you capture a fixed fraction of the light cone even when there is some lateral movement, as long as that movement is less than the difference in radii.
If the binocular exit pupil matches the eye's entrance pupil, any movement causes varying fractions of the light cone to be lost, which is unpleasant.
If the binocular exit pupil is smaller than the eye's entrance pupil, the entire light cone is captured, unaffected by any lateral movement smaller than the difference in the radii.

In repsonse to Bill, I don't (yet) understand how this could be different for night-time versus daylight viewing, other than that of course the eye entrance pupil will be different. But the principle should be the same.

If I'm wrong, then please teach me.

[BillC]

. . . In repsonse to Bill, I don't (yet) understand how this could be different for night-time versus daylight viewing, other than that of course the eye entrance pupil will be different. But the principle should be the same.

If I'm wrong, then please teach me.

Davy Crockett had a mantra. It was: "Be sure you're right, then go ahead." Look what it got him!

I haven't personally done all the cogitation on this one, but, as many on this list, have listened to others. That is why, rather than stamp my feet, I asked Kenny to think hard about it.

I do recall hearing something that sounded quite logical, but, with brain cells dying off, I do not recall the explanation. I will investigate.

As far as day and night, there is no difference when we consider it from a purely mathematical standpoint, where one pupil is compared to another. However, when ONE of those two pupils is a variable with ambient brightness, there must be some effect.

This becomes an issue for optical modeling, and for the forseeable future, I will not have time for that.

I would like to see the contributions of others on the subject.

Cheers,

Bill

[KennyJ]

< That is why, rather than stamp my feet, I asked Kenny to think hard about it. >

Bill and Mark ,

You'll know I didn't START this thread , and this particular topic might be considered something of a diversion from that -- but what's NEW ? :-)

First and foremost ,

I'm delighted that both of you have even given my ideas on the matter some serious thought , albeit only semi - serious in the case of Bill apparantly :-)

As anyone who has not fallen asleep whilst reading most of my posts to this forum will be aware , I probably have more chance of " making it " as a comic script writer than as any kind of " optics expert ".

That said , I have read enough about optical theory to bring on nausea , headaches and insomnia.

I base most of my ideas on more than 40 years of PRACTICAL EXPERIENCES looking through binoculars.

If I thought that little exit -pupils were necessary SO BAD per se , there is no way I would have forked out almost a whole week's wages on a 8 x 20 bino.

I continue to be ASTOUNDED by the sheer quality of images which can be enjoyed through that cigarette packet sized Swarovski , and feel just the same about the ones from Nikon , Leica and Zeiss.

But to attain that pleasure , I've noticed that one simply MUST get the IPD setting absolutely BANG ON , and at the same time develop an oscar - winning impersonation of a person totally paralysed.

If , for any reason ( and it seems there are a few ) the head happens to move just a millimetre out of line or sync with the hands , the visual experience is impeded .

Added to this is the unfortunate truth that in hands as broad as mine , a compact binocular is much more difficult to hold steady than something like a 7 x 42 or 7 x 50 , as a result of induced tremors brought about by the pinching position of the fingertips .

The fact that I have ridiculously long arms doesn't help.

This is true no matter whether my eyes are dilated to 2mm or 7mm.

When I'm using binoculars with 5mm , 6mm or 7mm exit -pupils , as a result of ALL the factors I've mentioned above ( one tends NOT to find 20mm models with such exit -pupils ) , I have never had any such trouble .

I hope this helps to explain what I'm trying to put across.

Regards , Kenny

[Rich V.]

I have been reading this discussion with interest; having a long history of using compact binoculars with <3mm exit pupils I have to agree with Kenny. Small exit pupils do need to be "bang on" or the view suffers.

I surmise that this has to do with our eye needing that tiny light cone to pass mostly ON AXIS with the center of the lens of the eye. The "wiggle factor" moves the smallish exit pupil in and out of the axis of the eye's lens.

A too large exit pupil easily keeps this central axis of the eye illuminated, but not the other way around.

Just a thought.

Rich V

[btschumy]

I have been reading this discussion with interest; having a long history of using compact binoculars with <3mm exit pupils I have to agree with Kenny. Small exit pupils do need to be "bang on" or the view suffers.

Here's a theory, but it's predicated on something I think is true but I don't know for sure. I'm assuming that a small exit pupil illuminates only a correspondingly small part of the retina. True?

So there's the theory... During daylight observations, it is important for the light to land on the retina's fovea where the color and high resolution cone receptors reside. This is why the IPD needs to be "bang on" with small exit pupils. If the IPD is not precise, or if the binos are not exactly centered over the eyes, the fovea is missed and the view is degraded.

Not sure if this is as important when viewing at night. Here it would seem you would want a larger exit pupil in order to hit the non-foveal rods and make use of your scotopic (night) vision.

Any validity to this??

[DJB]

Hi Kenny,

I mentioned this before, but I have to agree with you on a point that you made.

I have a bad back, and I cannot hold light binoculars steady whatsoever, unless tripod mounted--no problem there.

But, I can hand-hold the Fuji 7x50 FMT-SX very well. I don't exactly know why. Perhaps it is the balance of the binocular coupled with the nerve endings in my back. What a diversion. A point well taken, Kenny.

Regards,

Dave.

[Mark9473]

Kenny and Rich,

I think we're coming to the heart of the discussion.

My own experience, but speaking strictly of night time viewing, is that I have substantial tolerance in setting IPD with all my binoculars, with exit pupil ranging from 2.5 to 5.2 mm - my eyes are 7 mm. I'm not saying I'm not a bit more carefull about it for long sessions, but for casual viewing there's nothing immediately disturbing my view.

I have to say Rich's theory about the on-axis requirement sounds elegant; I'm just not sure how it fits in with my experiences.

[BillC]

. . . I'm not sure if this is quite related to what you are thinking of Bill.

You may have noticed that I have an aversion to all the deep techy stuff. I just like to keep it practical. So much of the stuff on these lists is incorrect and the consumer can do very little about the rest.

It’s kind of like the guys over on the ATM side who talk so much about "optimizing" their Newtonian. Those who REALLY want to "optimize" their Newtonians need to concentrate on making a good mirror. The world has more than its fair share of beautifully designed and beautifully constructed "optimized" Newtonians that house 1/2-wave (or worse) mirrors with surfaces that would look like a moonscape under a phase test. [made a few of those myself.]

Cheers,

Bill

[Rich V.]

Bill,

As far as I know, the size of the exit pupil doesn't have a relation to AFOV as you are implying. A small exit pupil from a 70° AFOV eyepiece illuminates the same area of the retina as a large exit pupil from and ep with the same AFOV. The illumination is not as bright but it does cover the same area on the retina.

What's at stake here is IF the exit pupil is small it must pass through the central sharpest area of the eye's lens. Our vision is sharpest through the central axis of the lens just like a camera lens operates best through it's central axis. I know of no camera lenses that produce as sharp an image wide open as they do stopped down. Our eye lens operates in much the same fashion.

When I've been to the eye doctor and been dilated my vision is awful. Too much light is passing through the "less corrected" part of my eye lens. For the same reason, some binoculars are sold "stopped down" (think SV85s) to remove the effects of the less corrected outer edges of the objective lens.

In daytime a small exit pupil produces a sharp image on the retina rather easily as the eye pupil is constricted and all the light must pass through the central axis. You do get a "black out" effect easily since the constricted pupil is a small target.

At night, things are different; now the eye pupil is dilated and it is easier for the small exit pupil to fall off axis in the eye. Careful IPD adjustment is now more important to get the sharpest views. "Black out" is not such a big issue; the dilated pupil is an easy target to hit but if the light cone doesn't pass through the center of the lens image quality suffers.

Anyhow, that's my take on this!

Rich V

[BillC]

Bill,

As far as I know, the size of the exit pupil doesn't have a relation to AFOV as you are implying. Rich V

It was not my intention to imply that, any more than to believe the entrance pupil does the same--a point I address in a cursory way in a post I am currently working on.

Cheers,

Bill

[btschumy]

Bill,
As far as I know, the size of the exit pupil doesn't have a relation to AFOV as you are implying. A small exit pupil from a 70° AFOV eyepiece illuminates the same area of the retina as a large exit pupil from and ep with the same AFOV. The illumination is not as bright but it does cover the same area on the retina.

Rich,
You may be right, but intuitively this doesn't make sense. Suppose you have a very narrow pencil of light like a laser beam. Are you saying that if you shine this in the eye the beam will spread to cover the entire retina? If so, how do they perform laser surgery on the retina?

I'm not implying that the exit pupil is related to the AFOV. I'm saying (and I could be wrong) that a small exit pupil illuminates a correspondingly small area of the retina. If this is wrong, and someone has a reference to straighten me out, I'd be eternally grateful.

[Rich V.]

Hi, Bill,

It's OK. Actually I was referring to the "other" Bill-- Mr. Tschumy!

He said:

"Here's a theory, but it's predicated on something I think is true but I don't know for sure. I'm assuming that a small exit pupil illuminates only a correspondingly small part of the retina. True?"

I was giving my point of view as to why I thought that statement was incorrect, and my reasons why an undersized exit pupil doesn't have the "wiggle room" of an oversized exit pupil.

Rich V

[Rich V.]

You may be right, but intuitively this doesn't make sense. Suppose you have a very narrow pencil of light like a laser beam. Are you saying that if you shine this in the eye the beam will spread to cover the entire retina? If so, how do they perform laser surgery on the retina?

Bill T,

With the laser you're talking about a linear "beam" of light; the exit pupil of an eyepiece is a cone of light which has an angle which is the AFOV. A 70° cone from a 2mm exit pupil illuminates the same area of the retina as a 70° cone from a 7mm exit pupil. Only the brightness is different. Any two eyepieces with a 70° AFOV, regardless of exit pupil will give your brain an image whose apparent diameter is identical.

I thought you were saying that a smaller exit pupil illuminated a smaller area of the retina. This is not true. That relationship depends on AFOV only. AFOV is the angular field projected on the retina and is a function of the lens design not exit pupil.

Rich V

[Swedpat]

What's at stake here is IF the exit pupil is small it must pass through the central sharpest area of the eye's lens. Our vision is sharpest through the central axis of the lens just like a camera lens operates best through it's central axis. I know of no camera lenses that produce as sharp an image wide open as they do stopped down. Our eye lens operates in much the same fashion.

In daytime a small exit pupil produces a sharp image on the retina rather easily as the eye pupil is constricted and all the light must pass through the central axis. You do get a "black out" effect easily since the constricted pupil is a small target.

At night, things are different; now the eye pupil is dilated and it is easier for the small exit pupil to fall off axis in the eye. Careful IPD adjustment is now more important to get the sharpest views. "Black out" is not such a big issue; the dilated pupil is an easy target to hit but if the light cone doesn't pass through the center of the lens image quality suffers.

Rich V

Rich,

I'm trying to understand the reasoning here...
If a smaller exit pupil gives a sharper image because the light will pass on-axis of eye pupil it sounds like it's in opposite to the statement that larger aperture improves the resolution despite same power; a larger aperture will result in larger exit pupil with same power...

But yes, I have experienced small binoculars as sharper at daytime (a 7x20 seems to be sharper than a 7x50) but my thought about the reason of that is simply that the smaller exit pupil cut down the total light amount and therefore increases the contrast. But I may be wrong.

Patric

[btschumy]

Rich,

Thanks for clarifying this. I wish I could find a good reference that explains this. Most of the optics books just show raytracings and also leave out what happens in the eye. This doesn't give me a good understanding of the overall process.