This may seem obvious to many, but I'll explain my reasons for posting later.
A telescope or binocular can be regarded as a "light-funnel" in which light is collected in a large diameter objective and channelled into a small exit pupil.
The magnification is the ratio of objective diameter to exit pupil diameter, so a 10x50 binocular would have a 5 mm exit pupil.
Used terrestrially this same 10x binocular provides a 10x angular magnification of an object in height and width and a solid angle gain of one hundredfold.
If the binocular were well designed and manufactured and tripod mounted it would potentially be able to relay one hundred times as much information of that object as the naked eye. We have an enormous light gain but no gain in brightness because the light is spread across that hundredfold solid angle and the brightness of the image would in fact suffer a little due to transmission losses.
Now binoculars and telescopes can reveal stars that are not perceptible with the naked eye. I'm not a hobby astronomer but recommend having a look at the night sky with binoculars even in an urban environment. Stars in our galaxy can be tens or even thousands of light years distant, so are essentially dimensionless light sources. Our 10x50 binocular with its 1963 sq. mm objectives collects parallel light rays from a star and delivers them to the oculars, where they emerge again parallel from 19,6 sq. mm exit pupils. We consequently have the potential of increasing the brightness 100x but with losses due to absorption (transmission losses) and discrepancies in eye pupil and exit pupil size.
I have held a long written discussion with another birdforum member of undoubted competence in some areas, who saw a contradiction in the reduced brightness of terrestrial objects through a binocular and the increased brightness of stars. I think this can be explained though by the fact that the binocular cannot bring the point light source of the star to a point focus but rather to a finite sized Airy disc. Viewed through the oculars this is a diffused image even if it appears as a point source to our eyes.
The topic is open to discussion.
John
A telescope or binocular can be regarded as a "light-funnel" in which light is collected in a large diameter objective and channelled into a small exit pupil.
The magnification is the ratio of objective diameter to exit pupil diameter, so a 10x50 binocular would have a 5 mm exit pupil.
Used terrestrially this same 10x binocular provides a 10x angular magnification of an object in height and width and a solid angle gain of one hundredfold.
If the binocular were well designed and manufactured and tripod mounted it would potentially be able to relay one hundred times as much information of that object as the naked eye. We have an enormous light gain but no gain in brightness because the light is spread across that hundredfold solid angle and the brightness of the image would in fact suffer a little due to transmission losses.
Now binoculars and telescopes can reveal stars that are not perceptible with the naked eye. I'm not a hobby astronomer but recommend having a look at the night sky with binoculars even in an urban environment. Stars in our galaxy can be tens or even thousands of light years distant, so are essentially dimensionless light sources. Our 10x50 binocular with its 1963 sq. mm objectives collects parallel light rays from a star and delivers them to the oculars, where they emerge again parallel from 19,6 sq. mm exit pupils. We consequently have the potential of increasing the brightness 100x but with losses due to absorption (transmission losses) and discrepancies in eye pupil and exit pupil size.
I have held a long written discussion with another birdforum member of undoubted competence in some areas, who saw a contradiction in the reduced brightness of terrestrial objects through a binocular and the increased brightness of stars. I think this can be explained though by the fact that the binocular cannot bring the point light source of the star to a point focus but rather to a finite sized Airy disc. Viewed through the oculars this is a diffused image even if it appears as a point source to our eyes.
The topic is open to discussion.
John