by Chris Peterson » Fri Jul 03, 2009 1:38 pm
pmp613 wrote:When you say "Aperture" - there are two apertures - the objective lens and the exit pupil measurement. Is there any kind of ratio or other calculation that can be derived from this, including the magnification as well, to help guide someone who wants to make a purchase decision?
In normal usage, "aperture" always refers to the entrance or objective size.
The exit pupil is simply the entrance aperture divided by the magnification. There is limited value in increasing the aperture of a telescope beyond the point where the exit pupil is larger than the pupil of the observer's eye. For astronomical viewing, that value is usually taken as 7mm (but does vary somewhat between different people).
So your basic 7X50 binoculars, probably the most popular for astronomy, have an exit pupil of 7.1mm- just right. There would be no point to making 7X binoculars with a larger aperture, because the additional light would be wasted. If you wanted some big, 100mm aperture binoculars, you would want at least 100mm/7mm = 14X or higher. Of course, such binoculars couldn't practically be used handheld- they would be too heavy, and too hard to hold steady.
How much magnification people require depends on their observing goals. You could specify an "ideal" binocular by first determining the desired magnification, and then figuring out what aperture, used at that magnification, gives a 7mm exit pupil. You could calculate magnification by considering that the unaided eye has a resolution of about one arcminute (the Moon is 30 arcminutes across). So at 7X, you could see features, like craters, that are 1/7 of an arcminute. But most people don't look at low power optics this way. When it comes to binoculars, it is common to simply consider what magnification you can use and still keep the image steady. That's around 7X for most people, although certainly a few can go higher and still keep things steady.
[quote="pmp613"]When you say "Aperture" - there are two apertures - the objective lens and the exit pupil measurement. Is there any kind of ratio or other calculation that can be derived from this, including the magnification as well, to help guide someone who wants to make a purchase decision?[/quote]
In normal usage, "aperture" always refers to the entrance or objective size.
The exit pupil is simply the entrance aperture divided by the magnification. There is limited value in increasing the aperture of a telescope beyond the point where the exit pupil is larger than the pupil of the observer's eye. For astronomical viewing, that value is usually taken as 7mm (but does vary somewhat between different people).
So your basic 7X50 binoculars, probably the most popular for astronomy, have an exit pupil of 7.1mm- just right. There would be no point to making 7X binoculars with a larger aperture, because the additional light would be wasted. If you wanted some big, 100mm aperture binoculars, you would want at least 100mm/7mm = 14X or higher. Of course, such binoculars couldn't practically be used handheld- they would be too heavy, and too hard to hold steady.
How much magnification people require depends on their observing goals. You could specify an "ideal" binocular by first determining the desired magnification, and then figuring out what aperture, used at that magnification, gives a 7mm exit pupil. You could calculate magnification by considering that the unaided eye has a resolution of about one arcminute (the Moon is 30 arcminutes across). So at 7X, you could see features, like craters, that are 1/7 of an arcminute. But most people don't look at low power optics this way. When it comes to binoculars, it is common to simply consider what magnification you can use and still keep the image steady. That's around 7X for most people, although certainly a few can go higher and still keep things steady.