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Optic size verses exit pupil..?

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/02/2007 at 09:46
windstrings View Drop Down
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Given the quality of lenses are the same  and your comparing equal for equal as far as brand etc.....

Will a 10 X 42 with an exit pupil of 4.2 have better night vision than a 8 X 32 which also has an exit pupil of 4.2?

Another example would be 10 X 50 which as an exit pupil of 5.0 or 8.5 X 42 which has an exit pupil of 4.94?

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/02/2007 at 11:02
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Theoretically, yes, because the increased magnification helps you resolve more detail, even though the exit pupil is the same.  This generally holds true up to a point until the higher magnification optic's reduced field of view and greater difficulty in steadying the image starts to work against you in terms of acquiring the object being viewed. This is the "twilight factor" theory, which holds that magnification and objective diameter both equally improve low light vision through an optic, which though valid to a point, I think is a b.s. measure.  Twilight factor is the square root of magnification X obj dia.  So, the 8X32 would has a twilight factor of 16, while the 10X42 has a TF of 20.5.  While the general idea is sound, using this as a measure of low light performance is severely flawed, because it doesn't take into account differences in optics and coating quality, which has much more influence on low light performance.  It also doesn't take into account the fact that as you increase magnification, you also increase optical aberrations, and the FOV and depth of field both narrow.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/02/2007 at 11:48
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Assuming all else is the same.... I do understand more magnification brings you "closer" to the lighted object you are viewing and thus better resolution,  however,  like you said, unless your binocs are on a tripod, the movement causes the brain to loose the information for processing as it keeps having to rewrite the map "so to speak".

For instance, a person can read much tinier detail if the binoc is kept immovable verses free holding in the hand... unfortunately in real use, it is free held in the hand and so the lower power often helps final resolution or at least "interpretation" of the object.

I guess what I'm trying to figure out is, if you look into the darkness of trees, will both allow you to "step" into that darkness to see as well as the other.... Keep in mind, generally as magnification increases, so do the F stops that creates the telescopic "tunnel" and brings in less Ambient light to the total picture or view.

If a person is debating over 8.5 X 42 verses 10 X 50... I'm wondering if its a "wash" as far as night vision... realizing that by going with the 8.5 X 42...., FOV increases dramatically, as does the weight decrease as well as better depth of field and more stabilization and in some cases, less cost.... many advantages are acquired by giving up the 1.5 power of magnification.

I'm wondering... am I also giving up night vision too as compared with the 10 X 50 to gain all those other advantages?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/02/2007 at 12:50
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stabalization is HUGE, in my opinion - i would never have a bino over 7x because of that.

 

i don't elk hunt and don't have the need to scan long distances.

 

for in the trees and small fields (300 yards or less) 6-7 power is ideal AND stable for me.

 

and brighter, to boot.

 

my 2 cents.

 

J

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/02/2007 at 21:12
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Originally posted by jonbravado jonbravado wrote:

stabalization is HUGE, in my opinion - i would never have a bino over 7x because of that.

 

i don't elk hunt and don't have the need to scan long distances.

 

for in the trees and small fields (300 yards or less) 6-7 power is ideal AND stable for me.

 

and brighter, to boot.

 

my 2 cents.

 

J

 

That's what I used to think until I got those 15x80 Steiners. They are balanced so well with their bulk that they are very steady. I will post more on them later after more use. I don't use them for hunting, though.

I am not a great fan of twilight factor as a metric, but Ted does have a point. A lot of people use 10x40's, too. 

My favorite for general hunting is 7x42.



Edited by tahqua
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/02/2007 at 21:36
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tahqua,

 

Looking forward to reading your review on the 15x80mm.

 

We all know that 15x is not the recommended magnification for handholding, however, it's nice to see that some people have figured out that they can still get some enjoyment out of binoculars handheld at this power.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/02/2007 at 22:14
tahqua View Drop Down
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They are quite large and somewhat bulky but not excessively heavy. I hold them at the ends near the objective lenses and I can watch the ships go by. I can even do this when standing. When sitting it is really easy. I thought I would need a tripod, for sure.

I'll be ship and sky watching during the Michigan deer season and will have more observations.

I think they are better for my purpose than a spotter and they bring the ships in a lot closer than my 7x42's. I am really happy so far and have ILya Koshkin to thank for these. Great glasses and a great guy.



Edited by tahqua
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/03/2007 at 11:05
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It is fun to be able to see "up close" one specific target... .I love looking at the moon with the 10 X 50's, but the 8.5 X 42's are good too..... but when focusing on "one" target, the magnification is nice.


However it works against you for depth of field and Field of vision when scanning the terrain for game.

The Clarity of the moon is astounding with either.

I think the larger powers tend to have "heavier" frames and so help stability.... if you can stand putting up with the weight....but if you try and get high power with no weight, your  FOV stinks.

I do notice a difference in stability just dropping from 10X to 8.5 and the general overall experience is better for me.... my only exception is when I have the luxury of leaning against something for stability and really want to pull in an object such as the moon etc..... too bad the variable binocs all seem poor compared to fixed.

But the answer about the exit pupil must be elusive.
I am now playing around with the EL Swarovski 8.5 X 42 and based on my memory by scanning the same areas as before with the 10 X 50's, I see no difference in night vision.

It would be fun to have them side by side to compare.

I just wonder if someone has done that comparison or knows the real scoop.

Seems big objective lenses don't matter as compared to smaller for night vision as long as you end up with the same exit pupil in the equation?....

I've noticed that as you go up in MM size on objective lens, you also tend to add length or height to the binocular thus killing your FOV....

I never realized all the complex considerations before my original purchase.



Edited by windstrings
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/03/2007 at 12:02
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Originally posted by windstrings windstrings wrote:



But the answer about the exit pupil must be elusive.
I am now playing around with the EL Swarovski 8.5 X 42 and based on my memory by scanning the same areas as before with the 10 X 50's, I see no difference in night vision.

Seems big objective lenses don't matter as compared to smaller for night vision as long as you end up with the same exit pupil in the equation?....

 

The exit pupil of the 8.5x42 is 4.94mm and the exit pupil of the 10x50mm is 5mm.  That is not enough of a difference between the two E.P.'s for your eyes to distinguish any variation.

 

It is my understanding that as magnification is increased, the aperture must also be increased, to assist in maintaining a similar level of brightness, as you have already observed, during your low light comparisons. Of course, there are other factors involved besides E.P.  The quality of the optical glass and the optical coatings plays a major role in better low light transmission.

 

 

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/03/2007 at 13:40
windstrings View Drop Down
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I agree the differences should not even be mentioned as they are basically the same number.

I guess I really still don't understand how the light actually gets focused inside the binoculars....

Just playing around with a magnifying glass burning ants and leaves as a kid, I see the bigger the lens, the more power and light when focused to a given mm of focal point.

I understand why the exit pupil needs to be at least as big as the viewers pupil to obtain full use of the pupils ability to catch and draw in light, but I'm still not sure if a bigger lens with the same exit pupil has the same effect of not.....

Lets exaggerate the situation a bit more for clarities sake....

what if you had a 1000mm lens being viewed at 200X... that would still give an exit pupil of 5.0.... would that still give the same intensity of brightness per square/mm in its 5.0 of exit pupil as the binocular that was only viewing with a 50mm lens at 10X?

Thats an awful large amount of light being caught with a 1000mm lens being focused down to an exit pupil of only 5?... where is all that extra light going if its supposed to only be as bright as the 50mm at 10X, or for that matter as the 8.5X at 42mm?

Surely I'm missing something obvious here....


Edit...... I may have answered my own question.... as the mm of the main lens increases, so the power has to increase to maintain the same exit pupil.... so in the example of the 1000mm lens, but having to to to 200X power to maintain the same exitpupil, you have in effect "missed allot of the light that was captured before as would be with a less powered lens.
You are looking more and more through a tiny tunnel or straw as you increase the power as compared to lower powers with the same size objective lens.

In a camera, the Fstops increase with magnification and the picture gets darker because the picture gets bigger and has less light to work with from the target area since the field of view is now less.

Its not so much that the camera, telescope or binocular is wasting any light gathered by the lens as much as it is the fact that a smaller area of the target is being looked at so less light is available to work with.

As I ramble in my logic.... is there anyone can that can either confirm or deny?


Edited by windstrings
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/03/2007 at 14:58
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We try to expalian it from another view.

 

If you look at something when it's fairly dark withouth any kinds of optics at all you can see somewhat decent at 2 yards, at 10 yards you can still recognize that person as  you'r brother, but at 50 yards you are hardly able to see him.

If this was in daylight you would be able to see him well over 1000 yards, and probably recognize him at 50 yards?

If I am right then we can agree on that the distance when looking at an object in the darkness is highly important.

 

The magnification on a binocular moves the image close to eye, so a 10X binocular moves the deer at 100 yards down to 10 yards, and the 6X down to 16,5 yards.

So if the glass, coatings and the exit pupil is the same, the higher magnification will give a far better "nightview" than the lower magnification.

 

If someone disagrees with me here then you problably have to belive that the "nightview" in a 1-4X24 set at 1X is better than a 10x40 as the exit pupil is much larger.

But no, the 10x40 gives of course a better picture than the 1x24......

 

Of the total lens surface it's only a part of that in the middle that is really "top quality"

That is regardless of make.

So when using a 8x56 daytime in good light, that will give a better picture than an 8x32.

 

Another factor is weight.

A heavy binocular is far more stable and easy to hold steady than a low weight binocular.

So even if the low weight binocular is as sharp as the heavyer one, the eye will not see it that way.

 

 

 

Regards Technika

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/05/2007 at 11:04
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I followed you and agreed with you completely until you made this statement:

So when using a 8x56 daytime in good light, that will give a better picture than an 8x32.

In the above examples.... the powers are the same?..... only the mm of the main lens is different.

Plus in daylight, the pupil is far too constricted to take advantage of the larger exit pupil of the 8 X 56 as the 8 X 32 with an exit pupil of "4" would serve as good as it gets?

Please elaborate as I may have misunderstood you.

----------------------------

I agree when given a constant ambient light "as in broad daylight", its much easier to see someone and recognize them if they can be brought closer to you and still maintain that same brightness...Remember, magnification "tunnels" and so you loose light unless the exit pupil can accomodate the loss.......
When  using a camera and a telephoto lens is placed to view the same target, the camera accomodates by opening the aperature a few F-stops to make up for the loss of light.

Everyone knows that when viewing with more magnification, the total viewing "area" of the scene is smaller so the total available light is less as the target is brought up bigger.

I agree magnification is nice and the very same principles applies at night as long as both binoculars being used are totally equal in glass quality and all other factors, and the exit pupil on both are large enough to accommodate the maximum dilation of the viewer.


Edited by windstrings
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/05/2007 at 16:10
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I believe what he is trying to relate to you is that the 56mm will have a "larger" central sweet spot than the 32mm.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/05/2007 at 18:05
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Ah, maybe so... I haven't done any reading about that subject as I too am new to this field and my only past experience is with Cameras years ago.

I still have my Contax RTSII with Zeis lenses if anybody is in the market.... I never use it!..... but I do remember the larger lenses are easier and less expensive to get the same quality of perfection to the final picture as there is more room for error.
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