Binoculars and light amplification

[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.