Tele Vue 13mm Ethos: 13 mm focal length, 100 degree apparent field, 2 in barrel. The magnification and true field it produces in real telescopes, worked from the specs.

Updated 05 September 2026.

Optical reference

Tele Vue 13mm Ethos: focal length, field and the magnification it gives

What the Tele Vue 13mm Ethos measures and the magnification and true field it produces once you drop it into a telescope of a given focal length.

Published specifications

Focal length
13 mm
Apparent field of view
100 degrees
Barrel size
2 in
Eye relief
15 mm
Optical design
Ethos

An eyepiece has no magnification of its own. The Tele Vue 13mm Ethos gives 50x in a 650 mm scope, 92x in a 1200 mm scope, 156x in a 2032 mm scope, because magnification is the telescope's focal length divided by this eyepiece's 13 mm focal length.

Its 100 degree apparent field becomes the true field of view once divided by that magnification: wider in a short-focal-length scope, narrower in a long one. The exit pupil equals this 13 mm focal length divided by the telescope's focal ratio, so at f/5 the exit pupil is 2.6 mm and at f/10 it is 1.3 mm.

The 2 in barrel decides which focusers it fits: a 2 in barrel needs a 2 in focuser or diagonal and will not drop into a 1.25 in one.

See it in a telescope

Tele Vue 13mm Ethos in the Sky-Watcher Classic 200P (8-inch Dobsonian)92x, exit pupil 2.2 mm
Tele Vue 13mm Ethos in the Celestron NexStar 8SE156x, exit pupil 1.3 mm
Celestron Omni 4mm Plossl4 mm, 50 deg

Questions and answers

What is the focal length of the Tele Vue 13mm Ethos?

13 mm

What is the apparent field of view of the Tele Vue 13mm Ethos?

100 degrees

What is the barrel size of the Tele Vue 13mm Ethos?

2 in

What is the eye relief of the Tele Vue 13mm Ethos?

15 mm