Eyepieces determine your telescope's magnification. To calculate the
magnification of an eyepiece in your telescope, divide the focal length of the
telescope by the focal length of the eyepiece:
Magnification = telescope focal length / eyepiece focal length
THE FIELD STOP AND APPARENT FIELD OF VIEW
The field stop is the metal ring inside the eyepiece barrel that limits the
field size. It's projected by the eyepiece so that it appears as a circle out
in space when you look through the eyepiece. The angular diameter of this
circle is called the "apparent field" and is a fixed property for
each eyepiece design: Plossl eyepieces have a 50 degree apparent field of
view, Radian eyepieces have 60 degrees, Panoptics have 68 degrees, and Nagler
eyepieces have an apparent field of 82 degrees.
TRUE FIELD OF VIEW
Eyepieces also determine the true field you see in the sky. To calculate
the true field of view (in degrees) that you will see, divide the eyepiece
field stop diameter by the telescope's focal length and multiply the result by
True field of view (degrees) = (eyepiece field stop diameter /
telescope focal length) x 57.3
The following key spec summary for all Tele Vue eyepieces will help you
calculate the true field size you will achieve with your telescope in
combination with Tele Vue eyepieces. Please note that eyepieces are listed
according to their field stop diameter, from largest to smallest. Eye relief
is given for reference only.
LOW-TO-MEDIUM POWER VIEWING
For low-power viewing of large objects, or to use your telescope as a
low-power finder, you will want an eyepiece that delivers close to the maximum
possible true field of view (note that for 1.25" eyepieces, the maximum
field stop diameter is 27mm; for 2" eyepieces, it's 46mm). Then add
eyepieces covering uniform increments in smaller field stops. For example, if
your widest field eyepiece has a 40mm diameter field stop and you choose a
decreasing increment diameter factor of 2 (which results in a 4x decrease in
area size), you'll end up with eyepieces having field stop diameters of
approximately 40mm, 20mm and 10mm. To further fill in with incremental steps,
add eyepieces with approximate field stop diameters of 28mm and 14mm.
Of course, avoid duplicating focal lengths, when filling in. As an example,
there's no need for a 32mm Plossl (27mm f.s.) if you use a 31mm Nagler (42mm
In general, for each field stop size, choosing eyepieces with shorter focal
lengths and larger apparent fields of view will allow you to see more detail
and fainter stars. In addition, you'll have a smaller exit pupil to better
match your eyesight.
The exit pupil is the image of the objective that is formed by the
eyepiece. It's where you place your eye to see the full field of view. You can
calculate the diameter of the exit pupil by dividing the focal length of the
eyepiece by the telescope's focal ratio:
Exit pupil = eyepiece focal length / telescope f/#
For reflector telescopes, it's best to avoid exit pupils larger than 7mm or
smaller than 0.5mm. Refracting telescopes have no upper limits on exit pupil
If you wear glasses to correct astigmatism, try to choose eyepieces that
have at least 15mm to 20mm of eye relief.
IMAGE AMPLIFIERS (BARLOWS AND POWERMATES)
You can also choose a long focal length eyepiece with comfortable eye
relief and use Barlow lenses to increase power. Tele Vue makes Barlows and
Powermates (an improvement to the Barlow-type design) in magnifications
factors of 2x, 2.5x, 3x, 4x and 5x.
Once you've selected an eyepiece set based on field stop sizes, calculate
the magnifications produced with your telescope. For planetary or double star
observing, you'll want an eyepiece in at least the 150x range. For determining
maximum power, a good rule of thumb is to use no more than 50x per inch of
aperture for scopes with apertures up to 6". You can also consider
magnifications of 200x to 250x if the "50x per inch of aperture"
rule is maintained. Realistically, the atmosphere will limit your planetary
observing to a maximum magnification of about 300x, no matter how large your
telescope's aperture is.
Basically, you'll be choosing low and medium power eyepieces by field stop
increments, and high power eyepieces by magnification increments based on your
Be assured that by choosing Tele Vue eyepieces, you'll have the sharpest
and highest contrast images, not only at the center of the field, but at the
edge as well.
For more in-depth technical background on the optical factors that affect
choosing your magnifications, see the article
Your Telescope's Magnification".