Re: [Phys-L] astronomy binocular buying advice

[John Denker <jsd@av8n.com>]

On 05/02/2017 04:53 PM, Larry Smith wrote:

> There seem to be contradictory opinions about size (the “giant” ones
> seem popular these days, but are they too heavy?).  Some guides
> recommend 7x50 or 9x63 as the best so we don’t have to buy tripods
> too.

I've always been happy with 7x50 wide angle.

The ratio of the two numbers is the size of the exit pupil:
 -- anything less than 7 means the image isn't as bright as it could be;
 -- anything more than 7 is wasted, because it's limited by the human iris.

Beware that the chart here has errors in the exit pupil column:
  http://www.space.com/27404-binoculars-buying-guide.html
  https://img.purch.com/w/640/aHR0cDovL3d3dy5zcGFjZS5jb20vaW1hZ2VzL2kvMDAwLzA0My8wMDgvaTAyL2Jpbm9jdWxhcnMtZm9yLWFzdHJvbm9teS0xNDEwMjBhLTAyLmpwZz8xNDEzOTI0MDQx

Old folks can't grab the full 7 mm, but teenage students can, which
is what matters.

Anything bigger than 7x50 will have weight and stabilization issues.
If you want more than 7x magnification, you probably want a lot more
than 7x, so get a telescope with a good mount.

If somebody wants to argue for something other than 7x50 I'm willing
to listen, but it would come as a surprise to me.

> Are image-stabilizing binoculars for astronomy worth the extra
> money?

Not now.  If the price comes down by a factor of ten we can revisit
the question.  Also if you're the lookout on a boat in a storm,
that's a different question.

General advice, not detailed or sophisticated:
  http://www.space.com/27404-binoculars-buying-guide.html
A bit more technical detail:
  https://www.astronomics.com/binocular-specifications_t.aspx



I haven't researched it in detail, but I reckon nobody would laugh
at you if you bought the Meade 7x50.  One could wish for more field
of view, but otherwise it's just fine, and affordable, with decent
eye relief and decent close focus (for non-astro work).

The Celestron 7x50 is comparable the above, with slightly less nifty
specs in a couple of categories:
  http://www.celestron.com/browse-shop/astronomy/astronomy-binoculars/cometron/cometron-7x50

The Orion 7x50 is more expensive and only marginally better in terms
of field of view.

The Nikon 7x50 is overpriced with a lousy field of view.  They brag
about being "sharp to the edge" but the edge is a lot closer in.

There are eleventy million off-brand products.  They might be just
fine, or not;  I have no idea.

Beware that the less-exalted makes & models economize on the coating.
This is hard to quantify, but it can make a noticeable difference
in brightness (and in extreme cases, color fidelity).  In a store
you would notice immediately, but when mail-ordering this is tricky.

I have nothing useful to say about "ruggedness".  My attitude is,
I'm not gonna drop 'em, so I'm never gonna find out whether they
are rugged or not.

I have no idea where to find a competent, systematic review of
the options.  (By way of contrast, note that cameras tend to
receive impressively systematic, detailed reviews.)

The last time I cared about this, I made a gnumeric spreadsheet,
with a figure-of-merit polynomial, and spent hours filling in
the numbers.  That was a long time ago, and your polynomial
coefficients will be different from mine.

-------

Note that in "most" cases eye relief doesn't matter much, but an
exception arises when the user has astigmatism.  The correct strategy
is to keep their glasses on, in which case generous eye relief is
helpful.  They may lose "some" field of view, but it won't be tragic.