AlanFrench
Well-known member
solentbirder said:I think I might throw some light on this (if you excuse the pun). I've been playing around with optics for as long as I can remember. This experience leads me to the following conclusion (which I expect will be strongly refuted !).
A larger objective lens has higher resolving power, regardless of the size of the your eye pupil. The 'field stop' argument only applies to the brightness of the observed image. Why is this so ? When you look through a pair of binoculars or a telescope you are viewing the image formed at the focal plane of the objective. The job of the eyepiece is simply to magnify this image.
There is nothing else going on. Imagine you inspect a very finely printed postage stamp with a magnifying glass. Imagine the stamp is the image at the focal plane of your binocular/telescope (in this analogy a larger objective 'prints' a more finely detailed stamp). Does the detail of the printing change under different light conditions because your exit pupil alters ? No.
Images formed at the focal plane of larger objectives always contain more detail than those from smaller objectives. The 'field stop' argument mistakenly treats the binocular as a single compound lens and misses the fact that you are actually viewing the image at the focal plane of the objective. At any magnification, a larger instrument will show more resolution than a smaller one (Assumptions: 1. The optical quality is equivalent. 2. The the lenses in your eyes are of good quality out to the edges. For a lot of people this is not true which is why some report a sharper image with binoculars having small exit pupils. In this case the beam of light is passing through the best central area of their eye lenses).
Let the slings and arrows begin !
You are forgetting what the exit pupil of an optical system is. It is an image of the light coming through the objective, and there is an exact correspondance between the aperture and the exit pupil. The center of the exit pupil contains the light coming through the center of the objective. The outer part of the exit pupil contains light coming from the outer part of the objective (and light coming through each area contains light from the entire field of view).
You can easily prove this to yourself. Hold binoculars so you can see the exit pupil. Now place a something - perhaps the tip of a pencil - just in front of the objective. You can see that the placement of the pencil tip in front of the objective exactly matches the placement in the exit pupil.
The exit pupil is aperture, and stopping down the exit pupil is exactly the same as stopping down the aperture, and has exactly the same effect on light grasp and resolution.
I have been through this discussion several times in the past, and have gotten quite a bit of disagreement and even abuse in these discussions. During these heated discussions I talked with several folks who make a living designing and making optical systems, and their response was uniformly in agreement - a stop at the exit pupil has the same effect as a stop at the aperture.
Clear skies, Alan