Do I want flat-field binoculars?

[Robert Silvers]

I need to decide between two Fujinon binoculars. They
are identical in quality, but one has more eye relief and is
of flat-field design. The flat-field ones cost more, $500 instead
of $400. I understand what this means, but aren't most camera
lenses curved field? (except for copy-stand cameras?) Is this
something I should care about?

I do not wear glasses, so I do not want to pay for the
extra eye relief.

Thanks,
--Rob.

[William Pope]

In article <1993Jan21....@nynexst.com> rsil...@cirrus.nynexst.com (Robert Silvers) writes:

> I need to decide between two Fujinon binoculars. They
> are identical in quality, but one has more eye relief and is
> of flat-field design. The flat-field ones cost more, $500 instead
> of $400. I understand what this means, but aren't most camera
> lenses curved field? (except for copy-stand cameras?) Is this
> something I should care about?

Try them. I prefer the non-flat field and I do wear glasses. Go to the
store take them (and a salesman) outside and try them. If the store won't
let you do this go somewhere else.

=bill
--
============================================================================
William Z. Pope Phone: +1 (617) 621-8889
Open Software Foundation Fax: +1 (617) 621-0584
11 Cambridge Center Email: zp...@osf.org
Cambridge, MA 02142 OSF DME Integration Project

[Keith Benton]

In article 1jnqh1...@crcnis1.unl.edu, vpor...@unl.edu (victor porguen) writes:
>rsil...@cirrus.nynexst.com (Robert Silvers) writes:
>
>
>> I need to decide between two Fujinon binoculars. They
>>are identical in quality, but one has more eye relief and is
>>of flat-field design. The flat-field ones cost more, $500 instead
>>of $400. I understand what this means, but aren't most camera
>>lenses curved field? (except for copy-stand cameras?) Is this
>>something I should care about?
>

>> I do not wear glasses, so I do not want to pay for the
>>extra eye relief.
>

>I must be not up to date on binoculars, because I cannot understand
>how a pair of binocular can be of "flat field" design. Binoculars are
>supposed to be able to focus on everything between say, distance "x"
>and infinity. This is definitely not a "flat" field. It's about as
>non-flat as one can imagine.
>
>Cameras and microscope objectives, which have notoriously curved
>fields unless corrected, are something else. Here, you might want to
>have a very sharp, flat "field", either because your object is flat
>(microscopes) or because your sensor is flat (cameras).
>
>But what do you do with "flat field" binoculars...? Look at a wall?
>
>As far as the eye relief, it can be extremely important, especially if
>you wear eyeglasses, and even if you do not - it's a lot more
>comfortable to use high-eye-relief systems. But if you can live
>without it, I would buy the least expensive pair.
>

I thought flat-field had to do with the field of view and not
depth of field. This means that the lenses are corrected so that
the whole field of view is in focus and not just the center of the
field of view with distortion out to the edges.

---
---------------------------------------------------------
| Keith Benton: k...@s1.gov |
| ME!?!? Speak for anyone here!?!? HAHAHAHAHAHAHAHAHAHA |
| Standard Disclaimers apply |
---------------------------------------------------------

[Daniel P. B. Smith]

You must try them. You MUST try them. You must TRY them.

Binoculars, like many other products (perhaps all) can't be accurately
and succinctly characterized by a marketeer's catalog description. It's
an easy trap to fall into. I find myself falling into it myself all
the time. Don't buy any binoculars you haven't tried, and try HARD to
make good comparisons between the models you're really interested in.
Buying a pair of binoculars you haven't tried would be a FAR worse
mistake than picking the wrong "features."

Here's my GUESS as to what "flat-field" means. It probably means that
if something is in focus in the center of the field, and you swing the
binoculars a bit so it's at the edge of the field, it has less of a
tendency to go out of focus. (At binocular distances, over binocular
fields of view, I don't believe there's any meaningful difference between
a truly flat surface at 100 meters and a spherical surface of 100 meter
radius centered on the viewer!).

In practice, desirable features don't just add, they always trade off.
For example, the "flat field" design might have more lenses in it, which
would explain the extra cost. But more lenses would mean more light loss
and flare. It's also possible that the flat field design would somehow
appear less sharp or crisp in the CENTER, when compared side-by-side with
the non-flat.

When we went shopping for binoculars, a LONG time ago, we were absolutely
astonished at the lack of correlation between what we saw and price, type,
maker, or product line. We were ready to buy some $250 ultracompact
binoculars, even though we didn't want to pay for compactness, because
they just plain had a better image than the $100 models we were looking at...
then we found a $100 non-compact model that was just fine. The funny
thing was that the $100 model, which happened to be a 9X magnification,
was terrific, but similarly priced, similarly "featured" 7X models from
the same manufacturer were not so hot.

I wear glasses but I don't have much astigmatism. I don't like high
eye relief models because I find that you have to get your eyes in just
the RIGHT place behind the eyepiece; and they're STILL not good enough to
clear my eyeglass lenses; and I like to have eyecups screening my eyes
from the side. So I take off my glasses when I use binoculars and one
of the things I look for is convenient diopter adjustments that fit MY
eyes. My eyes are pretty far apart, and some binoculars don't feel as if
their interpupillary wants to get set quite that far. The point is that
I could NEVER find that out from a description!

--
Daniel P. B. Smith
dpbs...@world.std.com

[victor porguen]

Superb advice for buying binoculars from Daniel P. B. Smith in the
previous article. I wish I'd written that. Couldn't have put it
better.

VP

[Francis Muir]

victor porguen writes:

On a related matter, bird watchers emphasize the importance of color
correction, and this may be important to boaters, too. color problems
are inportant in twilight when the perception of color is at its lowest.

I have one quite poor eye ("the Nelson touch") and I've been looking
around for telescopes. Surprisingly hard to find. At the moment I'm
thinking of getting an instrument maker to cut my binocs into two.
Twice the number, half the weight. Any thoughts? Oh yes, I am also using
a Celestion C65, a 6.5cm telescope. lightweight plastic thing.

Cap'n Fido on
F E L I X

[Richard A. Schumacher]

"Flatness of field" in binocs has nothing to do with distance to
the subject. (Would that it did! Astronomical use would be much
easier...) It has to do with whether
focus is maintained across the field of view. And do not
confuse "flatness of field" with "depth of field", as did a previous
poster.

By all means, try and compare before buying,

[Don Tuite]

Try the monocular at REI. Minolta, I think.

Don Tuite

[Richard A. Schumacher]

ARRGGHH! Once again: flat-field != fixed focus != depth of field

Flat-field: "Flatness of field" refers to whether all objects in view
at the same distance are all in focus at the same time, or not. The
"field of view" is what you see when you look through the binoculars
(or telescope, or whatever). If an object 500 feet away from you in
the center of the field of view (that is, you're pointing the binoc
straight at it) is in focus, and if another object 500 feet away but
off to one side (so that it's at the edge of the field of view) are
both in focus at the same time, then the binocular has a "flat field".
If all objects at the same distance are not all in focus at the same
time, then the field is not flat. Everything taken as equal, you'd
rather have a flat field, but you can probably live without one.
As it turns out, it's very expensive to make a binocular with
a perfectly flat field. Some are much better than others. You just
have to try them to see whether you can live with a particular one
or not. A flat field is fairly important for astronomy, but often
not so important for sporting.

Fixed focus: A binocular with no focus adjustment. Usually cheap.
They assume that you have normal vision, so if you don't, you
have to keep your glasses on while using the binoc. Their focus
is fixed at some long distance on out to infinity, so if you
want to look at closer objects you're out of luck. I wouldn't
want to live with these limitations, but people do buy these
things so there must be some use for them. If you want to
save money, try one of these and see whether it works for you.

Depth of field: this is a measure of the range of distances that
will be in focus at the same time. If the depth of field of a
certain binocular is one inch, then you'd have to change the focus
every time you looked at a new object that was more than half
an inch closer or farther away than the object now in focus.
(It would be better to express depth of field as a fraction
of the focus distance, but I used an explicit distance to make
the example simpler.) If you had a magic binocular with an infinite
depth of field, then everything would always be in focus, so you'd
never have to fool with a focus knob (and, as a bonus, it would have
a perfectly flat field). There is no such animal; pinhole cameras
approach this, actually, but they they trade off brightness for
sharpness and magnification, and so they make lousy binoculars.
If you use a binocular only for astronomy, you need very little
depth of field: the moon, planets and stars are all essentially
at the same distance as far as focussing is concerned (namely,
infinity). If you use it for birding, you'd probably want quite
a lot of depth of field, to reduce the amount of fiddling with
the focus knob as you sweep from the hummingbird at your back
yard feeder to that buzzard riding a thermal over in the next county.

There are other considerations like coma or chromatic aberration,
but these are usually only important for astronomical binoculars,
and aren't usually problems for sporting use.

If you want lots of info on what to look for when choosing a
binocular, see the article in the Nov '92 issue of "Astronomy",
or send away for the reprint from "Sky and Telescope".