Hi Peter



 >   There is also an optimum for night vision.



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



 > 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. :-(





Crash course in elementary telescope optics:





____________________

               ^   l\

               |   l \

                   l  \

               |   l   \     FLe

                  Ol    \ ->|   |<-

               |  bl     \       ________

                  jl      \    /l

               |  elL      \  /elp

               D  cle       \/ yli

               |  tln       /\ ele

                  ils      /  \ lc

               |  vl      /    \le_______

                  el     /

               |   l    /

                   l   /

               |   l  /

               |   l /

___________________l/



                   ||

                      FLo



The above shows an objective lens, diameter D, focussing a (nearly)

parallel beam of light from a distant object to a point a distance

FLo, the focal length of the objective, away. An eyepiece lens of

focal length FLe picks up the cone of light that expands away from

the focus, and turns it into a parallel beam again (I'm departing

from reality quite a bit here, but I always did have trouble with

virtual images, and this explanation will do ok), for your eye to

see. The focal ratio, f, of the objective, is defined as



  f = FLo / D



The parameters D, f or FLo, and FLe are usually known for a

telescope. For example, my Tasco clone has D=115mm, FLo=900mm (so

f=7.9), FLe = 6 and 20mm for the two eyepieces. The scope I wanna

buy has D=300mm, and f=4.8 . The smaller f ratio is necessary, if

it was 7.9 the telescope tube would be about 2.2 metres long.



To find FLo (and thus f) for a binocular lens you would have to

remove it, and measure how far you have to hold it from an ant to

get maximum frying power from the sun.



Magnification is easy: M = FLo / FLe. Given M and FLo for a

binocular, FLe can be derived from this.



Exit pupil, the diameter of the image as it emerges from the

eyepiece, is D / M. In general, a larger exit pupil means a

brighter image, but exit pupils over 7mm are wasted, because your

eye opens only that far in the dark. Older people (>40) have

smaller max pupils, 5 or 6 mm.



This doesn't matter for daytime use, but at night, every photon

helps. You still see the entire image when the exit pupil is bigger

than your pupil, just as you can see the whole image by day when

your pupils are much smaller. It's just that some of the light

doesn't get into your eye, and an image of identical brightness

could be got by using a smaller objective lens.



Going on these figures, 7x50 binocs would waste some light on your

eyes; 10x50 would give higher magnification with the same image

brightness. 10x50 is just a bit harder to hold steady, but I think

they'd be a better choice if you were buying.



Clear as mud?



Cheers



--- PPoint 1.88