Oculaires

How to Choose Eyepiece Focal Lengths

Magnification is your telescope's focal length divided by the eyepiece's. Exit pupil — eyepiece focal length divided by focal ratio — determines how bright and comfortable the view is, and it is the number that actually sets the useful range.

• Low power, 5 to 7 mm exit pupil: Widest true field for large nebulae, star fields and finding targets. Above 7 mm your eye's pupil clips the beam and you waste aperture.
• Medium power, 2 to 3 mm exit pupil: The workhorse range for most deep-sky observing, balancing image scale against background darkness.
• High power, 0.7 to 1 mm exit pupil: Planetary, lunar and double star work. Canadian seeing usually caps useful magnification well below the theoretical limit.
• Apparent field of view: 50 degree designs are compact and inexpensive; 82 and 100 degree designs give an immersive view and much longer tracking time in an undriven Dobsonian.
• Eye relief: Eyeglass wearers need roughly 15 mm or more. Short focal length simple designs often have very little, which makes them uncomfortable regardless of optical quality.
• Barrel size: 2" barrels are needed for long focal length wide-field eyepieces; 1.25" covers everything from medium to high power.

Why Choose David Astro for Eyepieces

An eyepiece recommendation is only useful if it accounts for the telescope it will be used in and the sky it will be used under.

• Set planning, not single sales: We space focal lengths so each step is a meaningful jump, typically by a factor of roughly 1.4 to 1.5, instead of selling you two eyepieces that give nearly the same view.
• Focal ratio honesty: Below f/5, simple designs show edge aberrations that no coating fixes. We say so, and we also say when a premium eyepiece will not visibly improve a slow telescope.
• Exit pupil calculated for you: We work out the useful range for your specific aperture and focal length rather than quoting general rules.
• Authorized dealer: Genuine eyepieces with manufacturer warranty, in stock here and shipped without customs delays for Canadian buyers.
• Domestic shipping: Ships from Canada with no duties or brokerage fees for Canadian orders.

Eyepieces in Canada — Frequently Asked Questions

How many eyepieces do I actually need?

Three or four covers almost everything: a low power for finding and wide fields, one or two medium powers for deep sky, and one high power for planets. A good Barlow can substitute for one of them.

Are 100 degree eyepieces worth the price?

In an undriven Dobsonian, very much so — objects stay in the field far longer and the view is immersive. On a tracking mount at medium power the advantage is real but smaller, and the weight can upset balance.

What is the highest magnification I can use?

Optically, roughly twice the aperture in millimetres. Practically, Canadian seeing limits most nights to 150 to 250x regardless of aperture, and pushing past that gives a larger, softer image rather than more detail.

Do I need 2 inch eyepieces?

Only for long focal length wide-field views. If your telescope's maximum true field is already limited by a 1.25 inch barrel and you want more, a 2 inch focuser and eyepiece is the way to get it.

Building an eyepiece set?

Tell us your telescope and observing site and we will map out a complete set. Authorized dealer, Canadian stock.