Peter et al,
Firstly, my apologies if this rambles a bit and covers some items already
explained. I started composing it before some of the later postings. The
thread is active enough that it presents a moving target.
I am also an astronomer ("amateur", in the same sense that most members of
this list would be amateur birdwatchers, i.e. do not engage in it as a paid
profession) and have been watching this thread with interest. I would like
to add my bit - starting with the suggestion that most of the light enters
through the centre portion of the objective. I suspect your astronomer was
attempting to point out that the increase in light gathered is proportional
not to the diameter of the objective but to the area of the objective and
proportional to the square of the diameter.
Consider an objective 42 mm in diameter collecting 1 unit of light.
Increasing the diameter to 50 mm is like adding a 4 mm wide ring of extra
glass. The area of the objective is 50^2/42^2 = 1.42 times greater, now
collecting 1.42 units of light. of this, 1 unit or 70% is collected by the
inner 42 mm, and the remaining 30% is collected by the 4 mm wide "ring" of
The next bit in the optical train is the prism. There are various types
which I won't go in to except to say that one reason very cheap binoculars
are is that the manufacturers use a prism too small for the job, resulting
in vignetting. This is familiar to the photographers among us. Basically it
means the edge of the field of view receives less light than the centre.
The next bit in the equation is the eyepiece or ocular. It is essentially a
magnifying glass to magnify the image that is formed by the objective. There
are various designs used my manufacturers, all with there good and bad
points. The magnification is equal to the focal length of the objective
divided by the focal length of the eyepiece. All else being equal a higher
magnification means a dimmer image as you are trying to spread a smaller
amount of light (from a smaller part of the image formed by the objective)
over the same area (the eyepiece's apparent field of view.) This means
10X42s will give a dimmer view than 8X42s.
Much has been said about exit pupils. This is the image of the objective,
formed by the eyepiece, and it lies a short distance outside the binoculars,
behind the eyepiece. This distance is known as the "eye relief" and depends
primarily on the eyepiece focal length, but also on the eyepiece design.
Shorter focal length (higher magnification) means shorter eye relief, which
may be a problem if you wear glasses. The diameter of the exit pupil is
equal to the objective diameter divided by the magnification. Theoretically
all the light collected by the objective, (42 or 50 or whatever mm in
diameter) is "funnelled down" to the diameter of the exit pupil, then into
your eye making the image brighter than if seen by the naked eye alone,
which only utilises the light collected by the eye's pupil, normally never
more than 7 mm in diameter and usually considerably less. Because of other
factors this increased brightness is not readily noticeable in normal
daylight viewing, but becomes apparent under low light conditions. Turn your
binoculars to a starry sky to gain a good appreciation of this.
The final bit is the eye itself. After all of the above, the light still has
to get in to your eye through the pupil, which varies in diameter depending
on the amount of light entering the eye. It is generally accepted that the
maximum pupil diameter of a fully dark adapted eye is 7 mm. For this reason
many older binoculars were designed to achieve a 7 mm exit pupil to match,
hence the almost ubiquitous 7X50s. the important point to note here is DARK
ADAPTED. Think looking for the silhouette of a ship against the ocean in the
middle of the Pacific ocean with only the light of the stars. This is when
young healthy eyes can open up to 7 mm. In most practical birding
situations, even a rainforest understory on a cloudy day, there is way more
light than that, and we are deliberately trying to make it brighter by using
the best combination of aperture and magnification. I see no need to go for
an exit pupil of more than 5 mm. If you want 10X magnification then 10X50s
are OK. 8X40 should give similar brightness. the reduction in lens diameter
means the rest of the instrument, both body and internal prisms, can also
be smaller with a significant reduction in weight.
Close focus is one consideration that is very important for birders but
rarely presents a problem for other users such a sailors, sports watchers
and astronomers.Focusing depends on a mechanical system to vary the distance
between the objective and the eyepiece and has nothing to do with the
optical specification. Close focus is achieved by moving the eyepiece
further away from the objective.
I had a $200 pair of 10X50s which were great for astronomy but when I took
up birding I quickly realise their limitations. Heavy, smaller field of view
and unable to focus closer than about 12 metres. I now use a $250 pair of
William Optics 8X42 SEMI-APO with 2.5 metre close focus and weighing 675 gm.
The optical quality is very good, but obviously not in the class of
Swarovski or Leica, but I consider them excellent value for money.