1. Eye Relief And Eyepiece Aperture

(This formula seems only to give approximate results.)

- eye-lens aperture

- pupil size (used in design) (usually 7mm)

- eye relief

- apparent field of view

2. Apparent and True Field of View

- magnification which equals the ratio of objective & eyepiece focal lengths.


- true field of view.


- apparent field of view.

3. Apparent field of view, field stop, and eyepiece focal length.

- entrance aperture of field stop

- eyepiece focal length

- apparent field of view

This only works for "normal" eyepieces, like Kellners, Plossls, and Panoptics. It does not work for eyepieces that have a built-in Barlow, like Vixen Lanthinum, Nagler, and Meade Ultrawide.

Note that "s" is what one measures using the star drift test. So it is not possible by that test alone to measure both the eyepiece focal length and the apparent field of view: either you need another thest (like the size of a focused image of the sun, made using only the eyepiece) or you must assume the manufacturer's value of alpha or "l" is correct (it never is exact).

4. Maximum true field of view in a focusing tube.

- focusing tube diameter

- effective focal length (objective focal lenth times any barlow factor or divided by any telereducer factor).

- maximum possible true field of view.

This only gives the upper limit on the true field of view -- what you would get from longest focal length/widest field eyepiece usable in that scope. Usually the eyepiece will impose the limit on the field of view. This also does not consider whether vignetting from baffles or from a secondary mirror will prevent the full field of view from being completely illuminated.

The picture is for a refractor, but the formula also works for Netonians and Catadioptrics.

5. Maximum field of view with a dew shield or telescope tube.

- aperture (diameter) of the dew shield (or telescope tube wall).


- diameter of the objective.


- tube length from outside of the dew shield.


- maximum possible field of view fully illuminated.

This formula is only approximate for catadioptrics, where the corrector lens spreads out the light path. It's exact for Newtonians and Refractors.

Note: Formulas 2, 4, and 5 give three different answers for the (maximum) true field of view. What you get is the smallest number -- usually from the eyepiece, hence formula 2.

Notice that this is the same formula as 3 -- the telescope's pupil is the objective diameter, and its outermost aperture is like the eyepiece eye-lens diameter.