Refractor telescopes
Same aperture, different focal length — which 70 mm refractor?
These two collect exactly the same amount of light. Everything that separates them follows from the focal length: 400 mm against 700 mm, which is f/5.7 against f/10 on the same 70 mm objective. The short one frames more sky and shows more false colour; the long one frames less and shows less. The eyepiece fitting decides the rest.
| Compared on | JUNIOR 70/400 | AC 70/700 AZ-2 |
|---|---|---|
| Aperture | 70 mm (from the title) | 70 mm (maker’s bullet) |
| Focal length | 400 mm (from the title) | 700 mm (maker’s bullet) |
| Focal ratio | f/5.7, the quotient of the two figures above | f/10, the quotient of the two figures above |
| Lens design | Refractor (lens objective) | Achromatic refractor |
| Glass type | Not published by the maker. | Not published by the maker. |
| Focuser | Not published by the maker. | Not published by the maker. |
| Eyepiece fitting | 24.5 mm (0.965 in), stated in the supplied-items list | Not published by the maker. |
| Supplied accessories | Backpack, 5x24 finder, star diagonal, 1.5x erecting lens, tripod, phone holder | Not published by the maker. |
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 arrives complete. A backpack, three eyepieces, a diagonal and an erecting lens make it usable on the evening it is unpacked, and f/5.7 gives the wide low-power view a beginner actually points at things with. The 24.5 mm eyepiece fitting is the catch: it is not the 1.25 in standard, so nothing else on this site threads into it.
The one that grows. f/10 on a 70 mm achromat is where a two-glass objective is at its best — the violet fringe that an f/5 achromat shows plainly on the Moon is barely there at f/10. It publishes less about what is in the box, and what it does publish is the optical design, which is the part that cannot be changed later.
For an extended object at the same magnification, yes: both objectives are 70 mm, so both collect the same light. What differs is the magnification each eyepiece gives, because that is set by the focal length.
Because 1.25 in is the standard almost every eyepiece, filter and diagonal on this site uses. A 24.5 mm fitting limits the instrument to what came with it, unless an adapter is found.
No. f/10 controls colour better on an achromat; f/5.7 gives a wider true field at the same eyepiece. They are different instruments for different jobs, which is why neither verdict here is a ranking.
70 mm aperture · 400 mm focal length
A 70/400 refractor sold as a first instrument for children from eight, supplied in a backpack with three eyepieces, a star diagonal and an erecting lens.
Best for: A child from about eight who will carry the telescope themselves
≈ £78 / €63
70 mm aperture · 700 mm focal length
An achromatic 70/700 refractor on an alt-azimuth mount with an aluminium tripod, sold as a first adult instrument.
Best for: An adult or older teenager starting with the Moon and planets
≈ £94 / €90
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.
