Eyepiece, lens and 32/42mm questions (1 Viewer)

cheeseontoast · Jan 15, 2020

Hi folks

As a beginner, and with a lot of sometimes too detailed info out there and living nowhere near a good bino shop, in trying to decide between buying an 8x32 and an 8x42 bino I would like to ask, in theory;

1) Do all the rays of light coming through an (one) eyepiece come out parallel?

2) So if my pupil is 5mm, the exit pupil is 5mm and both are aligned then everything the objective gathers will be on my retina?

If so, considering the quite sizeable difference in area between a 32 and 42mm objective, why does nearly every (like for like) comparative review between such sizes describe the difference in brightness as being so small (usually), to the extent it nearly makes no worthwhile difference carrying the bigger pair just from a brightness point of view?

Where does all that extra light the 42mm gathers go? Is it equally spread over the wider field of view most likely apparent?

So in reality the trade off is between portage and ease (rather than quality) of view? (Quality?!? :eek!: Why did he use that word?!)

If someone asks which bino for carrying on a walk or trip 32's are often suggested, but reading generally on this forum most seem to actually use bigger. Is general advice at any given budget so ingrained to ignore what most with experience actually end up doing? Reinforced in the Zeiss UK interview as one specific instance https://www.birdforum.net/showthread.php?p=3925387#post3925387

A 2-300 gram saving is nice, but that degree of saving could easily be lost depending how much spare fruit cake etc is also carried.

jring · Jan 15, 2020

cheeseontoast said:
1) Do all the rays of light coming through an (one) eyepiece come out parallel?

2) So if my pupil is 5mm, the exit pupil is 5mm and both are aligned then everything the objective gathers will be on my retina?

Hi,

answers to the numbered questions are:

1) no, if they were parallel, you would not get an image in a finite distance

2) yes, but during daylight your pupil will most probably be smaller than 5mm.

The area approach is ok for calculating the amount of energy ruining your eyesight when looking at the sun through an instrument without filter, but not so much for determining how much detail you will see with different instruments... Others have given nicer explanations in the past, if I find them, I'll add a link.

As for what is used - 8x32 and 8x42 are fairly evenly distributed... see

https://www.birdforum.net/showthread.php?t=376666

Joachim

AlanFrench · Jan 15, 2020

As you probably know, the exit pupil's diameter is equal to the objective diameter divided by the magnification. An 8x32 binocular will have a 4mm exit pupil and an 8x40 will have an exit pupil 5mm in diameter.

The exit pupil is a tiny image of the illuminated objective. There is an exact correspondence between the objective and the exit pupil. Light passing through the outer part of the objective passes through the out part of the exit pupil. Light passing through the center of the objective passed through the center of the exit pupil. (If you place a transparent plastic ruler just in front of the objective, you can see it in the exit pupil. A magnifier helps.)

On a bright sunny day your eye's pupil gets small. For discussion we'll say it's 2mm in diameter. When you put your eye up to the binocular's exit pupil it will admit only the center 2mm of light. Your eye is stopping down the binocular. Both the 32mm and the 40mm binocular are onlly using the center 2mm of the exit pupil, so each is stopped down to 16mm.

People often call this stopping down "wasted light," but that's not really true. With your eye open to 2mm there is no way to get more light into your eye. You are getting the brightest possible view you could have under those conditions. Yes, you are carrying a larger, heavier binocular than you need, but you have to potential to get more light when the landscape is poorly illuminated and your eye opens up to let more light in.

Clear skies, Alan

Pileatus · Jan 15, 2020

cheeseontoast said:
Hi folks

As a beginner, and with a lot of sometimes too detailed info out there and living nowhere near a good bino shop, in trying to decide between buying an 8x32 and an 8x42 bino I would like to ask, in theory;

1) Do all the rays of light coming through an (one) eyepiece come out parallel?

2) So if my pupil is 5mm, the exit pupil is 5mm and both are aligned then everything the objective gathers will be on my retina?

If so, considering the quite sizeable difference in area between a 32 and 42mm objective, why does nearly every (like for like) comparative review between such sizes describe the difference in brightness as being so small (usually), to the extent it nearly makes no worthwhile difference carrying the bigger pair just from a brightness point of view?

Where does all that extra light the 42mm gathers go? Is it equally spread over the wider field of view most likely apparent?

So in reality the trade off is between portage and ease (rather than quality) of view? (Quality?!? :eek!: Why did he use that word?!)

If someone asks which bino for carrying on a walk or trip 32's are often suggested, but reading generally on this forum most seem to actually use bigger. Is general advice at any given budget so ingrained to ignore what most with experience actually end up doing? Reinforced in the Zeiss UK interview as one specific instance https://www.birdforum.net/showthread.php?p=3925387#post3925387

A 2-300 gram saving is nice, but that degree of saving could easily be lost depending how much spare fruit cake etc is also carried.

Here's a good reference...
https://imaging.nikon.com/lineup/sportoptics/how_to/guide/binoculars/basic/index.htm

Exit pupils: 8X32 = 4.0mm; 8X42 = 5.25mm.
All other things being equal your pupil must exceed 4mm to perceive any difference in "brightness".

Note: The overall quality of a model is paramount. A great 32mm can make an average 42mm look bad and vice versa.

PS...pay attention to posts 2 & 3

cheeseontoast · Jan 15, 2020

jring said:
Hi,

answers to the numbered questions are:

1) no, if they were parallel, you would not get an image in a finite distance

2) yes, but during daylight your pupil will most probably be smaller than 5mm.

The area approach is ok for calculating the amount of energy ruining your eyesight when looking at the sun through an instrument without filter, but not so much for determining how much detail you will see with different instruments... Others have given nicer explanations in the past, if I find them, I'll add a link.

As for what is used - 8x32 and 8x42 are fairly evenly distributed... see

https://www.birdforum.net/showthread.php?t=376666

Joachim

Thank you.

1) I had thought the eye lens did that? Pileatus' Nikon link diagrammatically suggests the rays leave the eye piece in parallel here
https://imaging.nikon.com/lineup/sportoptics/how_to/guide/binoculars/basic/basic_05.htm

but then not here
https://imaging.nikon.com/lineup/sportoptics/how_to/guide/binoculars/basic/basic_07.htm

If they do not does that mean that the variation in eye relief is used to get every bodies eye lens the same distance from the bino eye piece, since the light rays maybe converge at the same point? Where, at the eye lens centre, which then adjusts to focus it on the retina?

And although that useful survey suggests an evenness between 8x32 and 8x42 if you exclude 8.5x, it doesn't between those objectives at 7x and 10x so overall bigger is more common perhaps?

cheeseontoast · Jan 15, 2020

AlanFrench said:
As you probably know, the exit pupil's diameter is equal to the objective diameter divided by the magnification. An 8x32 binocular will have a 4mm exit pupil and an 8x40 will have an exit pupil 5mm in diameter.

The exit pupil is a tiny image of the illuminated objective. There is an exact correspondence between the objective and the exit pupil. Light passing through the outer part of the objective passes through the out part of the exit pupil. Light passing through the center of the objective passed through the center of the exit pupil. (If you place a transparent plastic ruler just in front of the objective, you can see it in the exit pupil. A magnifier helps.)

On a bright sunny day your eye's pupil gets small. For discussion we'll say it's 2mm in diameter. When you put your eye up to the binocular's exit pupil it will admit only the center 2mm of light. Your eye is stopping down the binocular. Both the 32mm and the 40mm binocular are onlly using the center 2mm of the exit pupil, so each is stopped down to 16mm.

People often call this stopping down "wasted light," but that's not really true. With your eye open to 2mm there is no way to get more light into your eye. You are getting the brightest possible view you could have under those conditions. Yes, you are carrying a larger, heavier binocular than you need, but you have to potential to get more light when the landscape is poorly illuminated and your eye opens up to let more light in.

Clear skies, Alan

Thank you.

Your second paragraph I hadn't thought of in that way before, very helpful.

At 58 and a bit degrees north a better view on dim days is what I'm interested in achieving and why I am keen on an 8x42 perhaps. But apart from discussion on models where large price tags are involved to gain a few percentage points over the competition which is then compromised with the "but most people can't tell that degree of difference", nearly everyone says a 32 will be fine and a 42 is only very marginally better, but how many saying that are as far from the Equator as I am?

cheeseontoast · Jan 15, 2020

Pileatus said:
Here's a good reference...
https://imaging.nikon.com/lineup/sportoptics/how_to/guide/binoculars/basic/index.htm

Exit pupils: 8X32 = 4.0mm; 8X42 = 5.25mm.
All other things being equal your pupil must exceed 4mm to perceive any difference in "brightness".

Note: The overall quality of a model is paramount. A great 32mm can make an average 42mm look bad and vice versa.

PS...pay attention to posts 2 & 3

Thank you.

It really does seem I should try before I buy, but either I wait months until I'm near a shop, or it will cost me the price of an Opticron Countryman to get there as a special trip! I was in such a shop in the summer too buying a camera! Doh!

cheeseontoast · Jan 15, 2020

And shown parallel exiting the eyepiece here
https://www.bestbinocularsreviews.com/blog/how-binoculars-work-04/

jring · Jan 15, 2020

Hi,

I prefer this one...

https://www.telescope-optics.net/eyepiece_aberrations_1.htm

If the rays were parallel, you could position your eye at any distance from the eyelens and that is obviously not the case...

Joachim

Rico70 · Jan 15, 2020

cheeseontoast said:
nearly everyone says a 32 will be fine and a 42 is only very marginally better, but how many saying that are as far from the Equator as I am?

You can buy an 8x56 directly, so you will have no more doubts

Or, go to the ophthalmologist to do your pupil tests in necessary conditions (which you will have to measure first, in "candles/square meter" or similar) and so you can know for sure which binoculars you really need for your needs.

Everyone is different, lives in another place and nobody can tell you what you need!

WJC · Jan 15, 2020

cheeseontoast said:
And shown parallel exiting the eyepiece here
https://www.bestbinocularsreviews.com/blog/how-binoculars-work-04/

200115

One of the most blatant distortions used to teach people about optics is the myriad drawings showing light passing through a system and coming to a point at the focal point. This, however, shows only the position of the focal plane and not how the system works. If it really came to a point, there would be no image, only an infinitely small dot and nothing relating to an image.

In the first image attached, we see that kind of image, concerning my 6-inch Houghton Cassegrain telescope. In the second, we see the telescope produces an infinite number of focal points capable of producing an image.:cat:

Cheers,

Bill

Tringa45 · Jan 16, 2020

jring said:
If the rays were parallel, you could position your eye at any distance from the eyelens and that is obviously not the case...

Assuming the binocular were directed perpendicularly at a plane surface (wall) and focussed by a normally-sighted user, then rays from any point on the wall would emerge parallel from the eyepiece.

A short-sighted viewer would focus so that the the rays diverged from the eyepiece and a far-sighted viewer woud focus so that the rays converged. These light bundles merge at the exit pupil.

If the eye's pupils were placed inside the exit pupils, the viewer would experience blackouts and if they were outside the exit pupils then there would be a loss in field of view.

The advantage of larger objectives in bright light is that with centred diminished 2 mm eye pupils, in say an 8x binocular, one is only using the central 16 mm of the objectives, where spherical, and chromatic aberration and astigmatism are lower.

If one assumed a focal ratio of f/4 for most binoculars, then under these conditions an 8x32 would effectively become an f/8 and an 8x56 an f/14.

John

cheeseontoast · Jan 22, 2020

Thank you all.

Although jring your link I am sure is very helpful that is a little over my head! I will start at it's page one and see how I get on. An amazing resource!

Eyepiece, lens and 32/42mm questions (1 Viewer)

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