Free planning tools · Check equipment compatibility
Telescope eyepiece comparison tool
Compare magnification, exit pupil and approximate true field for an editable eyepiece shortlist.
No account needed. Results update here as you change the inputs. Measurements stay in this tab unless you choose to share a link; shared links contain the inputs, including any location coordinates.
Calculated from your inputs
Your plan
What to do next
Calculation details
Inputs used in this result
Compare a change before committing
Keep this plan, change the inputs and see the difference. The comparison stays in this tab.
Do the eyepieces in your shortlist offer useful different views?
An eyepiece shortlist is easier to judge when every option is calculated on the same telescope. Add the focal length and apparent field for each eyepiece to compare power, exit pupil and approximate true field together.
This telescope eyepiece calculator helps identify similar options or gaps in a set. It does not recommend the highest magnification by default, and it does not invent comfort or optical-quality measurements from a specification table.
How to use this comparison
- Set the telescope once, then add named eyepieces to the shortlist.
- Read each option’s magnification, exit pupil and estimated true field.
- Compare the practical differences and check eye relief, barrel size and your observing conditions separately.
The calculation
Magnification = effective telescope focal length ÷ eyepiece focal length. Exit pupil = aperture ÷ magnification. Approximate true field = apparent field ÷ magnification.
A worked example
A 1,200 mm, 200 mm aperture telescope with a 25 mm / 60° eyepiece gives 48×, a 4.17 mm exit pupil and about 1.25° true field.
Example inputs are demonstrations. Replace them with your own measurements, prices or documented requirements.
Assumptions and limits
True field is approximate; field-stop measurements give a better estimate when available. The results do not establish useful maximum magnification, eye relief, barrel fit, vignetting or observer comfort. Barlow factors can depend on spacing.
Questions about this calculation
Why is the true field labelled approximate?
Dividing apparent field by magnification is a useful estimate. A known field-stop diameter can give a better calculation, and distortion means the simple estimate is not exact for every eyepiece.
Can the calculator tell me the best planetary eyepiece?
It describes the geometry of your shortlist. It cannot establish useful power under tonight’s atmosphere, eye relief for you, mechanical fit or the optical performance of a particular model.
Put the result in context
Work through the next decision
Telescope camera field-of-view calculator
See the angular frame of your camera and compare it with simple target outlines or your own target dimensions.