Eyepieces
A Plössl, or an aspheric with more field?
Both of these publish more than most eyepieces on this site: weights, coatings, thread designations. They are different focal lengths and different designs, and the comparison is about what each design gives at the eyepiece.
| Compared on | Plössl 10 mm (4920210) | 4 mm aspheric |
|---|---|---|
| Focal length | 10 mm | 4 mm |
| Apparent field | The listing does not carry it | 62 degrees, as the maker states it |
| Eye relief | Fold-back rubber, described as suitable for spectacle wearers | Not published by the maker. |
| Barrel size | 1.25 in (31.7 mm) with a filter thread | 1.25 in, with an M28.5 × 0.6 filter thread named by the maker |
| Optical design | Plössl — four elements in two groups, as the maker states it | Aspheric |
| Coatings | Full multi-coating, as the maker states it | Fully coated, as the maker words it |
| Weight | 65 g, and 37 × 37 × 45 mm, as the maker publishes them | About 42 g, given by the maker in ounces |
| Supplied accessories | Rubber eyecup and a twist pack for storage | Two dust caps |
A tick means one listing published something the other did not, or published a better figure. It does not mean the product is better, and the column is not a score out of anything: a maker who says less loses lines here without losing a comparison. The two verdicts below are where the page is settled.
The one that states its construction and its coating. Four elements in two groups is a full description of a Plössl, and full multi-coating is the strongest of the four conventional labels. It also says the eyecup suits spectacle wearers, which is the practical form of an eye relief figure.
The one with the wider field and the named thread. Sixty-two degrees is a noticeably larger window than a Plössl gives, and naming the filter thread as M28.5 × 0.6 rather than "1.25 in" is the kind of precision that decides whether a filter will actually screw in. At 4 mm, eye relief matters more than at 10 mm — and it is not published.
Because short focal lengths traditionally put the eye very close to the glass. A 4 mm eyepiece with short relief is uncomfortable, and unusable with spectacles.
That at least one surface is not spherical, which lets a simpler design cover a wider field. It is a construction note rather than a quality grade.
Yes — it is the same thread written as a designation rather than as a barrel size. Publishing it is useful precisely because some listings on this site print a different pitch for what they call the same thread.
10 mm focal length · four elements in two groups · 65 g
A 10 mm Plössl with four elements in two groups, fully multi-coated, in a metal housing with a filter thread, published at 65 g.
Best for: A dependable mid-power eyepiece for a case that is being built properly
≈ €25
4 mm focal length · 62° apparent field · 42 g
A 4 mm aspheric eyepiece with a 62-degree apparent field, fully coated, in a 51 mm aluminium barrel with an M28.5 × 0.6 filter thread.
Best for: Lunar and planetary detail on a telescope with a long focal length
≈ £18 / €20
Any figure written into the body of this page was true on the day it was written. The amount shown next to each instrument is requested from the retailer as the page loads, and it is that amount, not the text, that stands.
