I came across this url:
http://www.wwnorton.com/college/astronomy/astro21/sandt/powerbinocs.html. It's pretty clear that Bishop is talking about exactly the same thing. Note in the graph he uses exit pupil as a primary parameter. Taking the square root of a product is simply a way to scale large numbers. He just uses the product and calls it the "visibility factor," which is what it is.
QED
It all ties together.
Thanks,
Ed
Very interesting link.
But I don't see it having much relevance to terrestrial bin use. Terrestrial bins are not being used to observe extended, low contrast targets in the dark using a large entrace pupil with a diffuse external light source reducing effective contrast (sky glow). Perhaps it comes closest to being applicable at twilight.
In fact in general it only applies when the eye's entrance pupil is larger than the exit pupil of the bin (under the given circumstances). And when you are dealing with diffuse skyglow that reduces the contrast of the image. In fact the article states that
For characterizing bin performance in the night sky it's right on the money - I saw similar effects when describing starfields and Jovian moon observations with the budget bins.
The problem with the argument that it might apply to birding bins is birding bins have other (conflicting) needs.
Larger magnifications have:
- narrower FOV so that worse for following closer birds;
- larger objectives, for a given exit pupil, you need which increases the weight;
- worse aberrations (especially for less expensive bins);
- a smaller exit pupil makes the bin more of a pain to use;
- more shake and so maybe more tiring for extended use;
But a higher magnification increases the amount of detil seen (up to about x10 or perhaps x12 with handheld bins and even higher with mounted bins).
Again the article points out most of these these.
Picking ideal bin parameters is not possible without reference to use and habitat. And worse still it depends on the observer (edge sharpness preference anyone?).
And yes, I've noticed the weight dilemma that Tero has pointed out too. Lighter to carry but a little heavier for good handling. The neoprene straps help a lot e.g. the 30oz Celestron Ultima DX comes with a non-stretchy around-the-neck only strap which has limited my use of it despite it being a good bin. I can't imagine anyone carrying 30oz around the neck for hours.
