On  7 Dec 01 at 19:01, Malcolm Stewart wrote:

> I have the Minolta Monocular converter, and I've tried using it with my 500
> f8 RF, 300 f4.5 MD and a few others.  Not a really worthwhile experience.
> Before you spend money, I suggest you try it out using adhesive tape or
> whatever to hold it roughly in place.  The converter has a focal length of
> 10mm and so you get an effective magnification of "lens focal length in mm"
> / 10mm.  i.e. 10-40x on your zoom.    What really kills it is the small exit
> pupil, making viewing seriously deficient.  Your zoom would offer something
> like 0.7 to 0.8mm  (approx.   filter diam./ 40)  at the long end, and I
> think you'll find it quite difficult to view in comfort.  Give it a try
> before committing yourself.


First, entrance/filter diameter on a 100-400/4.5-5.6 lens at 400mm
is 400/5.6 = 71mm.

Second, with magnification 40x (400mm lens divided by 10mm ocular), 
the exit pupil then becomes 71/40=1.8mm....at the wide end, 100mm, 
this would become 100/4.5 = 22mm entrance diameter, and an exit pupil of 

Third: we are talking about a functioning IS, which means that normal
rules of minimum exit pupil in relation to hand-held 'eye-ergonomy'
do not apply (6mm minimum, as in a 42x7 binocular)....otherwise
Fujinon wouldn't have made a mechanical image stabilized 40x16 with a
price tag of US$5k and more....nor would Zeiss have made a 60x20 IS
unit with a similar price tag.....once that exit pupil is rock
steady, you don't need that large diameter as much, at least not in
daylight (in daylight, 3mm is enough for a contracted eye pupil, at
night, enlarged, it becomes 6mm).


Willem-Jan Markerink

      The desire to understand 
is sometimes far less intelligent than
     the inability to understand

[note: 'a-one' & 'en-el'!]
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