> There is also an optimum for night vision.

 NP> By optimum do you mean, in light gathering capacity?

  Yes. A large lens is best for that, but it also depends on the 
magnification.

 > 8x40 or 10X50 gives a 5mm exit diameter and that is about the
 > minimum for general use. 7X50 is better.

 NP> It must be a bad day. I am lacking in comprehension. :-(

  The light captured by the 50mm lens emerges smaller and brighter. 
That's how it magnifies. 10x50 emerges as 5mm in diameter (or so). 
This light goes into your iris and focuses on your retina. Anything 
larger than 5mm wastes too much light... most of it doesn't enter 
your eye. Anything smaller than 5mm is too difficult to align with 
your iris... you get blank spots when you move it.

  It's quite critical. 7x50 binocs are much easier to use than 10x50,
and the image is almost as bright.

Regards,
Bob
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