I received an SVBONY SV171 Zoom Eyepiece 8mm-24mm (SKU: W9131A). for testing. The zoom eyepiece is very convenient for quickly selecting the optimal magnification, or for cases when you need to quickly change the magnification without having to change eyepieces. The focal length in the zoom eyepiece is changed by rotating the body. Along with the focal length, the angular field of view of the eyepiece also changes.

Declared characteristics:

Focal Length	8-24 mm
Eye Relief	18-19.5 mm
Apparent Field of View	38°-56°
Barrel Size	1.25″
Lens Structure	7-Elements/4-Groups
Filter Threads Standard	1.25″(M28.5 x 0.6)
Coatings	Fully Multi-Coated
Blackened Lens Edges	Yes
Barrel Safety Recess	Yes
Eyecups	Twist-Up, metal

Measured characteristics:
Weight per box – 406.5 g
Weight with caps – 308.8 g
Length with caps – 100 mm
Length without caps – 96 mm
Diameter – 55 mm
Eyecup shift – 10 mm
Eyecup diameter – 53.7 mm
Ocular lens diameter – 29.1 mm
Skirt diameter – 31.7 mm
Skirt length – 24.9 mm
Skirt thread – М28
Eye relief from eyecup edge – nearly 14 mm.

Angular field of view:
24 mm – 35°
20 mm – 42°
16 mm – 46°
12 mm – 52°
8 mm – 55°

How the measurements were taken:
Field of view: Dioptra app for Android (measuring the angle of the phone when aiming at the top and bottom of the diaphragm, then summing the values without a minus sign).
Dimensions: metal vernier caliper with an accuracy of 0.1 mm
Weight: electronic scales (accuracy 0.5 g).

The eyepiece is delivered in a dense cardboard box. Inside the box there is an eyepiece, two protective covers and a cleaning cloth. Everything is neatly packed in bags and bubble-walled polyethylene. There is no damage, defects, dust inside the eyepiece or traces of use.

The eyepiece body is completely metal, except for the protective caps and ribbed rubber strip. The eyepiece is quite massive, weight with caps is 308.8 g. When shaking the eyepiece, a quiet knock is heard.

Eyepiece skirt with barrel Safety recess and thread for light filter. I tried to install the eyepiece in the Sky-Watcher BKP150750 telescope – the eyepiece was installed without friction or jamming.

Hard rubber eyecup 55 mm in diameter. The height of the eyecup is adjustable by rotation, the shitf is 10 mm. The base of the eyecup is metal.

All lenses of the eyepiece are coated – the reflections are light green, dark green and weak colorless. Ocular lens with a diameter of 29.1 mm, flat, recessed 2.5 mm relative to the metal. The edge of the eyecup protrudes another 1 mm from the metal. In my opinion, the twisted eyecup is too short for observation without glasses and does not allow you to accurately center the eyes. When the eyecup is screwed in, the entire field of view is visible with glasses.

The zooming is carried out tightly with a quiet rustle without jamming, the rotation angle is about 110 degrees. Perhaps in the future  the zoom mechanism will be more soft.

When zooming, the front lens moves towards the inside of the body. It is noticeable that there is no blackening on this lens. Glare from the inner walls of the eyepiece is also noticeable.

Sky test
Equipment: Sky-Watcher BKP150750 telescope (parabolic Newtonian 150 mm, f\5), Levenhuk ATZ alt-azimuth mount.
Test object: Sirius.
24 mm. The star is sharp on the axis. The edge of the diaphragm is slightly out of focus. When the star is displaced from the center of the edge, the image quickly degrades. Refocusing doesn’t help much.
20 mm. The image quality across the field has improved slightly, the field of view has increased.
16 mm. The image quality across the field has improved further, the field of view has increased.
12 mm. Equal image quality across the field.
8 mm. On the axis, the image is sharp. Slight curvature of the field, eliminated by refocusing. The chromatism of the increase is noticeable.

There is no parfocality – refocusing is necessary when the focal length is changed.
When testing on the Moon, I did not notice any bright glare or reflections. At 24 mm, a little more than two lunar disks fit into the field of view.

Summary
+good quality of optics and housing at a reasonable price
-short eyecup for observation without glasses
-no blackening on the front lens
-visible reflections from the inner walls of the eyepiece
-angular field of view is slightly less than stated
– the need for noticeable refocusing when changing the focal length

Generally, SVBONY SV171 Zoom Eyepiece 8mm-24mm – budget zoom eyepiece with a small field of view for observers undemanding to picture quality. The optics are fully coated, but I recommend that the manufacturer work on blackening the non-working surfaces. Correction of intrinsic aberrations at the long end of the zoom is far from ideal, so this eyepiece is best used in telescopes with “slow” telescopes (f\7-f\13). But at the same time, at “slow” telescope the exit pupil will be smaller, and parasitic light from light reflections inside the eyepiece can also enter the observer’s eye.

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