Aperture: The aperture of a spotting scope is the diameter of its objective
lens, in millimeters. A large aperture generally gives you brighter images than
a smaller aperture at any given power in low light and will show you (resolve) more
detail in all light conditions.
Exit pupil: The bright circle you see in the eyepiece
when you hold a spotting scope up to a light (not the Sun) is the exit pupil. The
larger the exit pupil on any given scope, the brighter the image. Since the exit
pupil diameter is equal to the aperture of the scope (in mm) divided by the magnification
of the eyepiece, you can see that brightness and magnification are directly linked.
The higher the power on a given scope, the smaller the exit pupil and the dimmer
the image.
A spotting scope is most effective when you match its exit
pupil to the light conditions. With a zoom eyepiece, this is easily accomplished
by changing powers on the eyepiece until the image looks best. Your eye will tell
you when the exit pupil is too small, for example, because the image will be dim
and the colors muted. Lowering the power brightens the image by enlarging the exit
pupil, although at the expense of image size. But giving up size for brightness
is usually a more-than-acceptable trade-off.
With fixed single magnification eyepieces, however, the
problem is a little harder, as you have to decide in advance what kind of lighting
conditions you're likely to encounter most often and pick your eyepiece(s) accordingly.
An exit pupil of about 4mm (15x on a 60mm scope) is best
for twilight observing, as your eye dilates to this size and larger as daylight
fades.
Eyepieces with 2mm to 3mm exit pupils (30x to 20x on a 60mm
scope) are best for average daytime use, as daylight causes the pupil of your eye
to contract to this size and smaller.
Exit pupils of 1.5mm and smaller (40x and up on a 60mm scope)
are generally usable only in very bright daylight.
Other considerations may be more important than exit pupil
when choosing eyepieces, however. If you bird at reasonably close-in distances,
for example, you may prefer the wider field of a 20x or 22x wide angle eyepiece
to help keep a whole flock in sight, rather than the bigger images of a narrower
field 30x - even if the larger exit pupil of the 20x wastes some light in bright
daylight.
No eyepiece will ever get so bright as to hurt your eyes,
so it's better to be more wary of too small an exit pupil rather than one too large.
A 60mm scope at 60x has a 1mm exit pupil, the smallest realistically usable in very
bright daylight - and its image will usually be too dim if skies are overcast.
It will almost certainly be unacceptably dim in early morning or twilight conditions.
The same 60mm scope at 20x would have a 3mm exit pupil, a closer match to your eye's
diameter in low light, and would be much more satisfactory - with smaller but sharper
images and more vivid colors.
Twilight factor: Exit pupils should not be your sole
judge of a spotting scope, however. A 60mm scope at 20x and a 90mm at 30x both have
3mm exit pupils - but the 90mm scope is brighter due to its better light gathering.
The twilight factor is a better way to judge how well a scope performs in low light
than the exit pupil, as it balances light-gathering and magnification (both
of which affect how much detail you can see.) You find the twilight factor by multiplying
the scope's magnification by the diameter of its lens, then finding the square root
of that product. The larger the twilight factor, the more suitable a scope is for
early morning and twilight use. A 60mm scope at 20x, for example, has a twilight
factor of 34.6, while a 90mm at 30x has a twilight factor of 52, which explains
the better low light performance of the latter despite their identical exit pupils.
If low-light birding is important to you, and you can't decide between two different
scopes, find the twilight factor of each at a similar magnification. The
scope with the higher twilight factor will show you the most detail in low light.
Magnification: How much larger an object appears
through a spotting scope than it does to your bare eye is the scope's magnification.
A spotting scope that magnifies twenty times (20x) makes things appear 20 times
closer than they actually are, so that a bird 100' away appears to be only 5' away
(100 ¸ 20 = 5). Keep in mind however, that a spotting scope will magnify everything,
including heat waves, tremors in your tripod, haze, air pollution, and so forth
- all of which limit potential maximum magnification.
A scope's magnification limit is about one power per mm
of scope aperture under excellent seeing conditions (bright daylight; still,
calm air; and little haze). A 60mm scope is therefore capable of 60x at the most,
which explains why 60mm scope manufacturers don't make eyepieces capable of higher
powers. Attempts to push the power beyond this point would only result in very dim
and increasingly unsatisfactory views. Low to medium powers usually provide more
satisfactory observing than high powers under average seeing conditions.
As a general rule, the highest consistently usable
power in average daylight is approximately equal to the aperture of the scope
in mm divided by 2 - 30x with a 60mm scope, 40x with a 77mm to 80mm scope, etc.
Field of view: This is the diameter of the circular
image visible in a spotting scope eyepiece at a distance of 1000 yards. A spotting
scope with a field of 52' at 1000 yards (only a 1° field, and not uncommon with
catadioptric scopes) will have a field 5' wide at a distance of 100 yards, and a
field only 6" across at 10 yards! You can easily see that spotting scopes are designed
primarily for observing at medium to long distances and that serious close-in birding
is best done with the much wider field of binoculars.
Focusers: There are two different spotting scope
focusing methods in general use - helical and knob focusing.
A helical focuser is a rubberized band or collar around
the barrel or eyepiece of the scope. Turning it changes the focus, although the
effort it takes may also move the scope off your target if the focusing mechanism
has become stiff due to cold temperatures or age. Usually less than a single 360°
turn of the collar is enough to move you from a close focus to infinity, making
helical focusing very useful for following fast-moving action.
Knob focusers use a small knob at the top or rear of the
scope that may require five turns or more to move from near focus to infinity -
making them at least five times more accurate than a helical focuser. While knob
focusing is more precise than helical, it is also slower to focus on moving subjects.
Because of this, knob focusing scopes are primarily suited to the photography and
observing of sedentary subjects - nesting birds, distant offshore rookeries, etc.
- where a very sharp focus is more important than focusing speed.
Near Focus: How close you can get to your target
and still see a sharp image in the scope is called the "near" or "close" focus.
Because a spotting scope's field of view is narrow, most scopes are not designed
to focus as close as binoculars, with 25' to 35' being the norm.
Eye relief: The minimum distance between a scope's
eyepiece and your eye that allows you to see the entire field of view is the eye
relief. While long eye relief is important for observing comfort, it is especially
important if you must wear eyeglasses to observe, as your glasses can keep you from
getting close enough to the eyepiece to see all of the already-narrow field. If
you are near or farsighted, or have only mild astigmatism, you can usually leave
your eyeglasses off when using a spotting scope, as the focuser has enough range
to accommodate these conditions. If severe astigmatism forces you to wear glasses
at all times, however, eye relief should be 12-15mm or longer to allow you to see
most of the scope's field of view.
Mountings:
All spotting scopes are equipped with a built-in 1/4"-20 thread adapter that allows
them to be mounted directly onto a standard camera tripod. The majority are light
enough to be usable with virtually any camera tripod at low powers - although skimping
on the stability of the tripod to save a few dollars can lead to frustratingly unsharp
images at high powers due to a trembling tripod.