Binoculars
and Telescopes: Technical matters – a more detailed guide.
Introduction
In most of the information provided by
Optical Hardware, you’ll find us advising that modest magnifications are best
and that ‘exit pupil’ is very important in assessing how bright the image will
appear to be. For most practical terms when choosing a binocular this is true
however, as is often the case with scientific things, it isn’t the whole story.
The laws and rules of optics can become quite mathematical and complicated, We’ll
attempt here to outline some of the more detailed considerations when choosing
an optical instrument without getting too technical.
First the basics.
All binoculars, telescopes and monoculars (
a monocular is really just a small telescope ) are designed to produce magnification.
You will usually see a binocular or
spotting scope described as an 8x30 or 10x50 or 20x80 etc. The first number
refers to the magnification, so a 20x80 for example will make the image appear
20x larger than the object viewed unaided.
Higher magnification isn’t always better,
because high magnifications usually mean a less stable image, lower fields of
view and the image can appear duller. ( Field of view is how much of the object
you can actually see without moving the binocular or telescope and is discussed
in more detail later )
The second number refers to the size of the
objective ( front )lens, so on a 20x80 the objective lens(es) are 80mm wide.
Bigger lenses can gather more light, so produce brighter images in low light,
the downside is that bigger lenses means
the binocular or scope will be bigger and heavier to carry.
Some instruments have “zoom” magnification,
for example 20-60x70. This means that the instrument has variable magnification
from 20x to 60x and the objective(s) are 70mm
The above applies to binoculars and
spotting scopes. A spotting scope is a term used for most telescopes designed
for normal observation. Telescopes designed for astronomy are often described
slightly differently. You will usually find
these types of instrument described by the “focal length” of the telescopes
main tube and the size of the objective lens or primary mirror. (many
astronomical telescopes use mirrors instead of lenses, more about that later )
So an astronomical telescope may be listed as a 1000mm / 114mm, meaning that
the telescope has a focal length of 1000mm and an objective lens ( or mirror )
114mm across. You often see telescopes measured in inches and it’s quite common
to see 4 ½ inches, 6 inches, 8 inches in lists of telescopes, it may seem a bit
old fashioned but there is a historic tradition. 114mm is about 4 ½ inches.
The magnification of a telescope is found
by dividing the focal length of the telescope main tube by that of the selected
eyepiece. For example, a 1000mm scope
tube used with a 10mm eyepiece gives 1000/10 = 100x magnification. Used with a
5mm eyepiece, magnification would be 1000/5 = 200x
Most astronomical telescopes conform to
standard eyepiece fittings which means there are a vast number of different
eyepieces available giving a wide range of magnification options and these may
also differ by design to give a wider field of view or longer eye relief ( more
about eye relief later )
An astronomical telescope is intended for
use looking at the night sky, Moon, planets, stars etc , whereas a spotting
scope is for down to earth things, birdwatching, ships, long range observation
etc. An astronomical scope can be used for normal observation and a spotting
scope can be used to look at things in the night sky, but each is designed to
perform best in the use it was intended. If you are only buying one telescope, In
practice it’s easier to use a spotting scope for astronomical observation than
it is to use an astronomical scope for normal observation. Here’s some of the
considerations :-
Astronomical telescopes usually give
inverted images, this doesn’t matter when you are viewing something in the
night sky, but can be disconcerting when looking at a ship which appears
upside-down. Eyepiece accessories can be obtained to erect the image though
this can make the telescope a little clumsy
to use. A spotting scope is designed to
give erect, right way round images.
Astronomical telescopes are not usually
designed with any degree of water resistance, after all you wouldn’t be using
an astronomical telescope in the rain because the cloud means you wouldn’t see
anything, but you might well be birdwatching in heavy rain.
One of the main differences is ability to
interchange eyepieces. Most astronomical telescopes conform to standard
eyepiece fittings which means there are a vast number of different eyepieces
available. Spotting scopes tend to be supplied with modest magnifications and
if they have interchangeable eyepieces the range available is more limited than
for an astronomical telescope. This is done for good reason for most
observation purpose 20x – 30x is more than sufficient, and magnifications much
more than 60x are not really practical. ( see the important notes later about
magnification and brightness )
Another big difference between an
astronomical scope and a spotter is the type of tripod mount usually supplied
with them. Spotting scopes usually have a standard tripod screw fitting
allowing a normal ( camera type ) tripod to be used. The head movement here is
side to side and up and down which is relatively quick and easy to use. When
viewing the night sky however the movement of the earth has to be taken into
account as this causes objects in the night sky to appear to move in an arc.
Astronomical telescopes are usually designed for “equatorial” tripod mounts
which is system of geared movement allowing the telescope to track the apparent
movement of a body in the night sky. An equatorial mount can be used for normal
observation too but it is less convenient and can take longer to find your
subject.
Mirrors, lenses and prisms
Optical instruments use lenses and/or
mirrors to produce magnification. Binoculars and spotting scopes tend to use
lenses (and usually prisms too).
Astronomical telescopes sometimes use lenses, called a REFRACTING TELESCOPE, or use mirrors
(with lenses), called a REFLECTING TELESCOPE.
Refracting ( lens) telescopes often give
better contrast of image ( difference in tone between the light and dark parts
of the image ) which can make the image easier to view with a refractor than a
reflector. However telescopes with
large lenses can be difficult and more expensive to manufacture than with large
mirrors, and large lenses may change shape with time. The debate between astronomy enthusiasts of
whether refractors and reflectors are best will no doubt continue. For most
practical and affordable purposes we would suggest that astronomical refractors
(lens) are good up to about 150mm to 200mm (6 to 8 inches) bigger than this a
reflector may be better, and there are many quality compact reflectors made
with 100mm – 150mm mirrors, particularly good if size is important.
Most binoculars and spotting scopes use
prisms along with lenses to ‘fold’ the path of the light and make the
instrument more compact. The common type being the porro prism design, named
after its inventor, this uses two triangular faced prisms to provide an erect
image. Another prism type is the roof (
or Dach ) prism, usually used to make more compact instrument designs.
Field of view (FOV)
This is how much of the object you can actually see. For a binocular or
spotting scope It is usually measured in degrees or as an indication of width
at a given distance. A common expression would be 100m/1000m. This means that
viewing over a distance of 1000m ( 1km) you could see two objects 100m apart
within the field of view of the binocular or telescope. 100m/1000m could be expressed as an angle,
this is about 6 degrees. It may be written in yards, for example
100yards/1000yard which is just the same field of view as 100m/1000m.
Confusingly some binoculars may show this in feet and yards, for example
300ft/1000yds, which is again the same
as saying 100yds/1000yds, or 100m/1000m. These are just notation traditions.
Astronomical telescopes usually indicate field of view by degrees or sometimes
arcminutes ( another convention of angle measuring ) though this is usually
described for the selected eyepiece and the observed field of view has to be
calculated, more about that soon
A
wide field of view is good because it makes viewing more comfortable and you
don’t have to move the instrument so much to see objects across a wider
area. Field of view is mainly related to
magnification, bigger magnification usually means less angle of view. The
design of the instrument can also have an effect on angle of view, hence there
will be some variation from instrument to instrument at any given
magnification.
Typically an 8x binocular would have an
angle of view of between 5 degrees to 8 degrees. Anything less than 5 degrees
for 8x would be considered a very narrow
field, anything above 8 degrees is exceedingly wide. About 6.5 degrees would be
average. For a 12x binocular, the field of view would likely vary between 4
degrees and 6 degrees, with around 5 being the average.
For astronomical telescopes, the field of
view is usually given for the eyepiece, and you have to calculate the true
field of view for the combination of telescope and eyepiece. This is found
quickly by dividing the stated field of view by magnification. For example you may select a 10mm eyepiece
with a stated field of view of 50 degrees and use this with a 1000mm tube. The
magnification is 1000/10 = 100x, the real field of view is 50/100 = 0.5
degrees.
Eye relief
This is the distance from the binocular or
telescope eyepiece where you can comfortably view the full image. Typically
many binoculars and scopes have eye relief of about 10mm-12mm and this is fine
for viewers with normal eyesight. Spectacle users can usually roll back the rubber
eyecups (some instruments now use twist eyecups) to get a little closer to the
eyepiece lens but often this isn’t close enough to see a full clear image. Eye
relief of 15mm-20mm would be better, and these can also be used with normal
viewing too so long as the eyecups are a little longer, these then roll, or
twist back to allow use with spectacles. The only downside of using a long eye
relief instrument with normal vision is that setting the eyecup to the correct
position may take a moment or two longer. Instruments with eye relief of about 15mm or
more are often called long eye relief in their description or ‘LER’. Eye relief
above about 25mm may be too long for normal use, the only exception may be a
target scope for a rifle where you would want the scope some distance from your
eye otherwise the recoil, when the gun is fired, may hurt ! Rifle scopes often
have eye relief above 30mm.
More on eyepieces for Astronomical
telescopes
Eyepieces for astronomical telescopes mostly conform to standard fittings. The main
fitting is 31.7mm ( 1.25” ) most good quality telescopes use this and just
about any eyepiece will fit and work with any scope. Other fittings in use are 0.98 inches, though
this only tends to be used with very low price telescopes, and 2 inch which is
used with very expensive instruments such as our Ostara F7 series. The use of a
2” fitting allows for wider eyepieces making wide angle viewing easier to
achieve. As there are less 2 inch eyepieces available, this type of telescope
can usually be easily adapted to work with 1.25 inch type eyepieces.
Brightness, Exit pupils, twilight factors,
relative brightness ... and more
How bright the image appears to be is an
important consideration when selecting a binocular or telescope, a bright image
makes viewing more comfortable and enjoyable..
At Optical Hardware we tend to talk about
exit pupils when referring to the “brightness” of a binocular, we believe this
is the most useful comparison for most practical purposes but it is not the
whole story. If you’ve read information from various other sources, especially
on the internet, you’ll probably have noticed there is a lot of confusing and
sometimes contradicting information given. It is a complicated topic and open
to various interpretation so I’ll try here to cut through this and give a
simple, usable, overview.
Exit pupil is the size of the bright area (circle)
seen on the eyepiece of a binocular or telescope, ( it is not the size of the
eyepiece lens itself ) - the bigger, the better ( within limits )
The size of the exit pupil INCREASES as the objective lens size increases.
The size of the exit pupil DECREASES as the magnification increases
Exit pupil size is calculated by dividing the objective size by magnification,
so a 10x50 binocular will have a (50/10=) 5mm exit pupil. An 8x40 will also have a (40/8=) 5mm exit
pupil.
A 7x50 has a (50/7=)7mm ( approx) and a 20x50 ( 50/20=) 2.5mm.
The human eye also has a pupil which opens
and closes to control the amount of light entering the eye. The biggest this
can typically open to in low light is about 7mm, closing down when it is
bright.
In low light the human eye will be fully open at 7mm, so a binocular which is
able to produce an exit pupil of 7mm will show a brighter image, so a 7x50 or
8x56 etc perform incredibly well in low light.
There would be little point in having an exit pupil bigger than 7mm because the
eye would not be able to use the extra light, so an 8x80 for example giving an
exit pupil of (80/8=) 10mm is not a particularly efficient design. There is a
school of thought which says that producing very large
( bigger than 7mm ) exit pupils can be an advantage because the eye is always
comfortably looking into the bright exit pupil, even if the binocular is
shaking. In practice however it makes little difference. 7mm is about the best
the human eye can reach in low light, when it is brighter the eye closes down,
in bright daylight this will perhaps be around 3mm, so for example a 10x30,
exit pupil (30/10=) 3mm, will appear equally bright. The need for a 7mm exit
pupil is only important in low light. Furthermore 7mm is a ‘perfect’ eye which
many of us haven’t got, and as we get older the ability of the eye to open
reduces further, typically to around 5mm at best.
What this means in practice is that a 10x50
or 8x40 binocular ( exit pupil 5mm ) is great for most purposes. Use 7x50 or 8x56 if low light is really
important. If your requirement is normal daylight observation then most
binoculars of modest magnification will work very well.
Relative Brightness is another term used by
some. This is the calculated by squaring the exit pupil. So a 10x50, exit pupil
5mm, has a relative brightness of 25. A 10x30 (3mm) has a relative brightness
of 9.
Twilight factor is a common term for
comparing binoculars in low light conditions but is often misused. Twilight
factor is found by multiplying the size of the objective lens in mm by the
magnification and then taking the square root.
So a 10x50 has a twilight factor of (approx square root of 500=) 22. An
8x40 would have a twilight factor of (approx square root of 320=) 18, and a
20x80 has a TF of 40. This implies that
increasing either magnification, or increasing objective lens size, or both,
will make the binocular perform better in low light. This is true, to a point, as both increased
magnification and increased objective size can improve effectiveness of
observation in low light conditions especially when considering large objective
binoculars and astronomical telescopes. There is also an argument that in
falling light, when the eye may not be fully dark adapted, increased
magnification can improve viewing. But
effectiveness of observation and a comfortable bright image are not the same.
Twilight factor implies that a 10x50 and a 50x10 would be equally good in low
light, I can be reasonably sure that you would not find this to be the case ! (
and no one, as far as I know, makes a 50x10 )
Always, in practice, modest magnification
and a reasonably large objective size (within the size and weight limits that
you are happy to carry) is the best choice for general purpose binoculars and
scopes.
The situation is slightly different when
considering large objective scopes and observation binoculars and astronomical
telescopes used for looking at very dim subjects where both increasing
magnification and objective lens, or mirror, size is useful. Bigger objectives
mean better resolution and light gathering, increasing magnification shows
detail however this only works within practical limits. For an astronomical
telescope when viewing dim deep sky objects, a rough guide would be a maximum
magnification of about twice the size of the objective, so for an 80mm scope,
maximum usable magnification would be about 160x, a 200mm scope would be 400x.
You do not require the really high magnifications for all objects, it depends
on what you are looking at. Astronomy publications give suggestions for best
magnification but actually there is no substitute for the enjoyment of finding
out yourself by trail and error. Remember,
the magnification of an astronomical scope is determined by your choice of
eyepiece (see earlier discussion) so you
would select an appropriate eyepiece to give the required magnification. Many
scopes are supplied with a number of eyepieces and additional ones are
available at modest cost.
Even
with scopes of 200mm or larger, which would imply that you could work with
magnifications of over 400x, the maximum practical magnification, no matter how
big the scope, is around 300x-400x. Beyond this atmospheric distortions becomes
a more important factor and if the scope isn’t on a stable tracking mount,
maximum magnification is further limited by vibration.
Quality of optics
All of the above discussions about
magnification and light gathering and all other considerations assume that you
are comparing like for like. Exit pupil, relative brightness, twilight factor
etc, tell you that all 8x42’s are the same, in
fact the optical quality of the instrument, the materials used and careful
construction and alignment can make a huge difference to image resolution and
brightness. When selecting a binocular or telescope there is no substitute for
trying it, and if possible, in the conditions and environment where you plan to
use it.
Clear skies and good seeing.
Another major consideration which, especially
for astronomy, can be very important is the clearness of sky. Dust and dirt pollution reduces the ability
to view clearly, ambient light is also a significant. For best viewing you need
a clear night away from town and city lights and even then there are more
problems to deal with. What appears to be a perfectly clear sky can have air
disturbances caused by wind, temperature differentials and other factors.
There’s no easy answer, getting on high ground reduces the amount of atmosphere
you have to look through and can help but for the most part astronomy requires
a degree of patience, but with that comes much fun and enjoyment.
Digiscoping
This is the science ( or art ? ) of
coupling a camera, usually digital these days but most of the considerations
also apply to film SLRS’s, to a telescope to achieve high magnification
photography. Essentially there are two ways to do this. You can attach a
digital or film SLR camera directly to a scope without the scopes eyepiece in
place, this usually requires a camera adaptor on the scope and a T2 mount to
match the camera. This is called the Prime Focus method. It has the advantages
of (theoretically) better quality because it minimises the number of glass
elements the light has to travel through and gives short shutter capture speeds
however it is only usable with scopes with removable eyepieces and SLR cameras.
The other method is “AFOCAL” – this is where the telescope with eyepiece
attached is used with a camera with lens attached and is mounted close to the telescope eyepiece.
This method is really only suitable for digital compact cameras where there is
a viewing screen. It’s better if the camera has a filter thread around the lens
as this allows for easier and more accurate coupling and alignment but it’s not
essential as most telescopes have a variety of camera adaptors and brackets
available. There are many considerations in achieving good photographs, we
cover many of these in the digiscoping section of the Optical Hardware website
and there are many other good resources.
This
information is written with my best efforts to balance technical accuracy and
detailed explanation with a need for simplicity of description and
understanding for the practical user. I appreciate that sometimes these
explanations may overlook fine points and so not give a fully exhaustive
explanation. It is often the case with scientific and technical issues that a
meticulous coverage could bring an unwieldy amount of unnecessary information. Getting
the right balance is no easy task. If
you require further clarification of any points, or have any further questions,
please use the contact us section of the website and I will try to help. I’m
often quite busy so please be patient in waiting for a reply. Many questions
arising from this may already be covered in our FAQ sections, so please take a
look there first. I welcome comments and suggestions from like minded readers
on how our technical guides could be improved .
P.J Sokell B.Sc. Product Manager, Optical Hardware Ltd.
