Binoculars and light amplification

[cube137]

Because the single lens acts as a simple magnifying glass. Without the eyepiece the objective lens can't be used to magnify far-away objects. Any telescope or binoculars needs both the objective and the eyepiece to function correctly.

Yes. When I put an eyepiece between the objective lens the ground, the brightness at center is gone.. but it still is not same brightness as the sunshine. A telescope with eyepiece is supposed to make the brightness of target object and the magnified view same brightness. For those who think that a telescope focus all light to a point.. it's not entirely accurate.. the objective lens alone can act like magnifying glass.. But when you put eyepiece.. it redistributes the light to the magnified image. Anyway I still can't understand why the sunshine on the ground is not same brightness as the objective lens plus eyepiece.. you can't reason the telescope focus the light to smaller area (because the eyepiece redistributes) the light. I'm thinking hard and reviewing the thread over and over davenn..

I wish I had time to find some good pictures and to go into detail on this subject, but I'm doing homework and studying for a physics exam tomorrow.

physics exam? I thought physicsforums Mentors are former retired Nobel Laureates.

 

[russ_watters]

A telescope with eyepiece is supposed to make the brightness of target object and the magnified view same brightness.

That isn't true. As I said before, the two issues are not that closely related. This should be obvious given that you can use different eyepieces in the same telescope, which will produce different magnifications and brightnesses.

See the telescopes in my avatar? I can use different eyepieces on them to obtain the same magnification:
3000/40 = 75x
600/8 = 75x (actually, the smallest I have is a 9mm, but close enough...I also have Barlows and focal reducers)

But when viewing the moon through the big telescope, I need to use a filter to block most of the light, otherwise it hurts my eyes. Why? Because it pulls in a lot more light.

you can't reason the telescope focus the light to smaller area (because the eyepiece redistributes) the light.

Which is bigger: the lens or the spot on the ground the lens focuses the light to?

 

[Drakkith]

A telescope with eyepiece is supposed to make the brightness of target object and the magnified view same brightness.

Nope. Only a single magnification for each objective will do this. Changing out the eyepiece in a telescope for one of a different power will alter the brightness of the image.

Anyway I still can't understand why the sunshine on the ground is not same brightness as the objective lens plus eyepiece.. you can't reason the telescope focus the light to smaller area (because the eyepiece redistributes) the light.

See the following image:

Light, in this case from the center of the telescope's field of view, is focused down to a spot by the objective. Since there is nothing at the focal plane, the light continues on, diverging some before the eyepiece catches the diverging light and turns the rays parallel again. These parallel rays then enter your eye. Now, if you take away the eyepiece and place the objective such that the focal plane lies on the ground, the incoming light is focused down to a spot on the ground. So all that light that would be spread out is instead concentrated at a single spot.

physics exam? I thought physicsforums Mentors are former retired Nobel Laureates.

Someone must have forgotten to give me my check then.

 

[cube137]

That isn't true. As I said before, the two issues are not that closely related. This should be obvious given that you can use different eyepieces in the same telescope, which will produce different magnifications and brightnesses.

About 10 years ago. I bought a lot of astronomical equipments.. but didn't find a dark sky to use them so just put it in closet. I also bought a book Telescope Optics.. so I'm very familiar with basic stuff like Airy discs, exit pupils, etc. I just couldn't find the book now and kinda reviewing because of my new binocular hobby. By the way.. the refractor I bought is called APM 4" Apo.. I bought a special collector item with strehl ratio of 0.989 (equal to maybe 1/10 wavefront error). Do you think it's really good. The following is the spec.

See the telescopes in my avatar? I can use different eyepieces on them to obtain the same magnification:
3000/40 = 75x
600/8 = 75x (actually, the smallest I have is a 9mm, but close enough...I also have Barlows and focal reducers)

But when viewing the moon through the big telescope, I need to use a filter to block most of the light, otherwise it hurts my eyes. Why? Because it pulls in a lot more light.

Which is bigger: the lens or the spot on the ground the lens focuses the light to?

I know exil pupil is the determining factor of brightness. So viewing the moon through your big telescope. If the exit pupil is 7mm.. and you watch it with a smaller aperture and same exit pupil but lower magnification, the brightness is the same right??

I'm applying all it to terrestrial daytime view of my new binocular (I know exit pupil becomes 2.5mm). Looking at the sunshine at ground and fully intact binocular with sunshine shining thru it.. the spot on the ground even with eyepiece is still brighter than the sunshine... I guess the reasoning is like the brightness of the sun when viewed is not the same brightness of the sunshine on the ground.. isn't it.. the sun being brighter than the sunshine on ground.. using an intact binocular with eyepiece, you are magnifying the image of the sun.. and this is brighter than sunshine on ground.. what is the physics explanation of it.. that the sunshine on ground is less brighter than the sun when viewed directly.. by the way.. I know looking at sun naked eye or binocular/telescope can cause instant blindness so it's just theoretical.

 

[Drakkith]

I'm applying all it to terrestrial daytime view of my new binocular (I know exit pupil becomes 2.5mm).

A pair of 50x7 binoculars has an exit pupil of 7.14 mm, not 2.5 mm.

 

[cube137]

A pair of 50x7 binoculars has an exit pupil of 7.14 mm, not 2.5 mm.

Our eyes at daytime has pupil size of 2.5mm.. therefore the effective size of the 7x50 binocular becomes 2.5x7 = 17.5mm.. meaning you are only using 17.5mm of the 50mm objective.. it's like the resolution of 17.5mm objective lens.. you know.. stopping down aperture can lower resolution. etc

 

[Drakkith]

But you're not talking about the exit pupil of the binoculars, you're talking about the entrance pupil of the eye. A very important difference.

 

[cube137]

But you're not talking about the exit pupil of the binoculars, you're talking about the entrance pupil of the eye. A very important difference.

I know the distinction.. but since our pupil doesn't open more than 3mm at daytime.. I automatically assume the 7x50m 7.1mm exit pupil won't be utilized but only less than 3mm. This is why the worlds' best daytime binocular is size 8x30W. http://www.greatestbinoculars.com/a...vski/swarohabicht8x30w/swarohabicht8x30w.html I bought it for $1000. Now with all the data I learned in this thread. I know I have to avoid reflections from sunshine and at least know that viewing trees and leaves won't be dangerious even with more blue light because they won't be concentrated at the macula.. I was concerned before the light intensity would be increased at macula.. but reading that the brightness of the view is the same as the naked eye and only magnification increased (this is the reason for this thread).. then I'm relieved to know it is not harmful. Thanks to all who responded. I guess I'll move on now from theoretically optical physics to just enjoy the views.

 

[sophiecentaur]

There is no mystery here. "Bigger" and "brighter" are two separate things. Binoculars often make things both bigger and brighter. Bigger because of the magnification, brighter because of the size of the lens.

We have all told him as much but he can't / won't take it on board. I have given up.

 

[cube137]

We have all told him as much but he can't / won't take it on board. I have given up.

I already understood it. I'm now moving on from comprehending optical principles to just enjoy the view. Thanks a lot.

 

[litup]

Lens size has essentially nothing to do with magnification. You can get very high magnification with tiny lenses. Look at the size of the lens on a microscope.

Magnification is determined by the ratio of the image size to the size of the original. That will turn out to depend on the focal lengths of the lenses in use and on their exact arrangement. It will not depend upon their size.

If your resolution is diffraction-limited then using a larger lens (i.e. larger aperture) can improve matters. If your resolution is intensity-limited then using a larger lens (i.e. larger aperture) can improve matters. If your only problem is that the image is too small or too far away to make out clearly then magnification is a remedy. Magnifying an image reduces its intensity. This can put you into an intensity-limited situation. Hence the motivation for bigger lenses.

Larger lenses collect more light so you can focus more energy in a smaller spot so you can get more magnification over a wider area, if you have a 7X50 you get that 7 power that fits through the pupil fully. If you have a 70 X 500 you get 70 X magnification in the same pupil area. If you tried to get 70 X out of a 50 mm objective lens you can do that but the image size will be proportionally smaller so you have a narrow field of view since the light would only be going to a small percentage of the retina area.

You could get 700 X if you use a 5000 mm lens, 5 meters, about the size of the Mt. Palomar scope. So the 2 meter rough size lens of Hubble would only give about 300 X if it was used as a telescope for human viewing and you wanted to fill the pupil area. Of course if your sensor has millions of pixels, you can get a lot more magnification in a small area. Anyone know the effective magnification of Hubble? I found a couple of sites that say between 4800 and 8000X effective magnification but they also say that is the least important aspect of telescopes, the main thing being resolution and light collecting area.

 

[Drakkith]

If you tried to get 70 X out of a 50 mm objective lens you can do that but the image size will be proportionally smaller so you have a narrow field of view since the light would only be going to a small percentage of the retina area.

What do you mean here? Why would the light be going to only a small section of the retina?

 

[Anand Sivaram]

I think people are getting confused about Exit Pupil https://en.wikipedia.org/wiki/Exit_pupil
It is the ratio between diameter of the binoculars to the magnification
So, 7x50 has an exit pupil of (50/7) = 7.15mm

Magnification is not related to the diameter of the main lens/mirror. It is the ratio between focal length of primary/eyepiece.
But, if you magnify too much with a primary with lower diameter, then the image brightness would be reduced much.
The rule of thumb is that for every inch of diameter, keep the magnification upto 50x, means if you have an 8 inch telescope, go upto 400x magnification.

 

[sophiecentaur]

I think people are getting confused

Yes. And that's because the posts have not been concise enough. There is, as you say, a strong association between objective size and useful magnification but it's not a causal relationship - which is that magnification depends on Focal length of the objective.

 

[Drakkith]

There is, as you say, a strong association between objective size and useful magnification but it's not a causal relationship - which is that magnification depends on Focal length of the objective.

And the eyepiece?

 

[sophiecentaur]

And the eyepiece?

Naturally but is the magnification due to the diameter of either lens? (Is my point)

 

[Drakkith]

Naturally but is the magnification due to the diameter of either lens? (Is my point)

Ah, I see what you're getting at. No, the magnification is not due to the diameter of the lens, but to focal length.

 

[sophiecentaur]

Ah, I see what you're getting at. No, the magnification is not due to the diameter of the lens, but to focal length.

But the slightly red herring (pinkies. actually) about pupil size and permissible magnification kept raising it head as if diameter was directly related to magnification. (Usual PF style discussion. ). I do wonder what the casual reader makes of some of these threads.