The zoom eyepiece is a variable focal length eyepiece that allows you to quickly change the magnification by simply rotating the special ring on the body. In fact, “zoom” can replace several eyepieces, and at the same time, you can easily find the optimal magnification for various objects. Zoom eyepieces are also very convenient for mass telescope observations (“sidewalk observations”), where changing the eyepiece takes a lot of time. A few years ago, I was able to observe the planets in a unique short-focus zoom eyepiece Nagler 3-6 mm – I really liked it for the convenience of choosing the best magnification, as well as its small size. About a year and a half ago, Svbony sent me the SVBONY SV171 Zoom Eyepiece 8mm-24mm for review, and after testing, I suggested that Svbony consider developing a planetary zoom eyepiece.

And now, a year later, Svbony told me the good news – the new zoom eyepiece Svbony zoom 3-8 mm SV215 is ready, and they need my help in testing 🙂 Great!

I want to note that this eyepiece is an experimental model – shortly after the release of the review, Svbony finalized the design of the eyepiece, adding intermediate focal lengths (7 and 5 mm), as well as slightly reducing the size of the eyepiece. So in the review – the first version of the eyepiece (I’ll call it v1).

Appearance, equipment
The eyepiece is supplied in a small cardboard box measuring 112 x 84 x 63 mm and weighing 270 grams. Inside the box is an eyepiece with protective caps, surrounded by a bag and protective foam.

Casing, mechanics
The eyepiece is small, but at the same time weighty – the case is metal, only covers are made of rubber, an elastic band on the zoom ring and an eyecup. Metal skirt with a groove. There are no markings on the eyepiece – only the focal length and a round mark. In conventional zoom eyepieces (8-24 mm and similar), the dimensions of the eyepiece do not change when the focal length is changed. Here, when zooming, the block of the eye lens with the eyecup is shifted towards the observer by 27 mm. At focal lengths 8, 6, 4 and 3, click-lock occurs. Little effort is required to rotate. I twisted the zoom about 20-30 times – everything works flawlessly. The “trunk” of the eyepiece has a little backlash, as does the zoom ring.

In personal correspondence with the manufacturer, I found out the declared characteristics.
Focal length: 3-8mm
Exit pupil: 10mm
Field of view: 56 degrees
Size: 1.25 inches
Lens structure: 6 elements / 4 groups
Coating: SMC
Eyecup Type: Rubber
Thread specification: M28.5*0.6

Measured characteristics
Weight with caps: 190.3 grams
Weight without caps: 179 grams
Weight of caps: 11.3
Length with eyecup unscrewed (without caps): 72 mm
Eyepiece extends 27 mm when zoomed
Eyepiece length at 3 mm position: 99 mm
Skirt diameter: 31.76 mm
Skirt length: 39.7 mm
Inner diameter skirt thread: 27.8 mm
Body diameter: 40 mm
Zoom ring rotation angle: approx. 170 degrees
Eyecup height unfolded: 6.4 mm
Eyecup height folded: 1.3 mm
Eye cup thread: 37.9 mm
Eye lens diameter: 13 mm
The eye lens is recessed by 0.45 mm relative to the metal
Front aperture: 8.75 mm, moves 9.51 mm deeper when zooming at 8 mm focal length
Field lens lens recessed by approx. 25 mm
Eye relief from the metal at a focal length of 8 mm: 10 mm
Eye relief from the metal at a focal length of 6 mm: 8 mm
Eye relief from the metal at a focal length of 4 mm: 7.7 mm
Eye relief from metal at 3 mm focal length: 6.8 mm
Field of view at 8 mm: 55 degrees
Field of view at 6 mm: 56 degrees
Field of view at 4 mm: 57 degrees
Field of view at 3 mm: 58 degrees
The focal length was not measured, however, when compared with the 3mm Levenhuk LER eyepiece, the angular size of the image turned out to be identical to that shown by the Svbony eyepiece.

The eye lens is convex, with a diameter of 13 mm. Glare from blue, green, yellow lenses, there is also a dull colorless glare. The eye relief is small, about 11 mm at 8 mm focus, and when you zoom in it becomes even smaller. The eyecup is soft rubber, you can roll it up or remove it altogether. Due to the small eye relief, it is better to look through the eyepiece without glasses.

There is a 27.8mm internal thread on the front of the eyepiece. Next, a small aperture with a diameter of 8.75 mm is installed, which, when zoomed, moves deeper by 9.51 mm. Next comes a field lens with a flat outer surface and blue coating – perhaps something like scattering lens cell. Blackening of internal surfaces is present.

When checking for light on a white object, it turned out that the eyepiece introduces a slight yellowness into the image.

Testing
To test the eyepiece, I used a Levenhuk Ra R66 ED Doublet Black refractor (aperture 66 mm, focal length 400 mm, f\6), a diagonal mirror, as well as three more eyepieces – a Celestron X-Cell XL 7 mm, a Celestron 6 mm plossl and also Levenhuk LER 3 mm. The tested eyepiece has strong contenders – the 7 and 3 mm have excellent image quality across the field, while the 6 mm has high contrast.

The first thing I noticed when using the eyepiece was a noticeable non-parfocality compared to other eyepieces, the telescope focuser had to be screwed in 15 mm deep. Perhaps this is because of the long skirt, which simply did not fit all into the diagonal mirror.

A test on Altair, Saturn and Jupiter showed that closer to the edge of the image, the eyepiece has a slight curvature of the field. At a focus of 8 mm, the image quality across the field is the best, closer to 3 mm, inclusive, the eyepiece works worse across the field. However, the image quality on the axis and in almost 70-80% of the field of view did not cause any complaints. Yes, the X-Cell LX 7mm worked better in the field as expected, as did the LER 3mm, but the picture is very similar on the axis and in the central region of the field. In the field, the 6 mm plossl turned out to be worse, comparable in sharpness and contrast on the axis. I did not notice any spurious glare or re-reflections, but I still need to check on a larger aperture. I checked it – with a Schmidt-Cassegrain 127 mm (Celestron Omni XLT 127) on Jupiter, I found glare at all focal lengths – most likely from the cornea. Moreover, there are several glare – one glare walks in 2-3 diameters from Jupiter, in brightness it is about several times dimmer than Jupiter’s satellites. The other flare is dim cone-shaped, approximately between the center and the edge of the field of view. When the eye moves away from the lens, the glare disappears. Flares are sometimes visible, sometimes not, most often they appear when the planet is directly in the center of the field of view. At a small aperture (66 mm) I didn’t notice any glare, but at 127 I immediately saw it. I’ll have to check on the Moon.

About refocusing. I observe without glasses, my vision is about -1.75 (myopia).
When switching from 8 to 6 mm, refocusing is not required.
When switching from 6 to 4 mm (large shift of the eyepiece body), a slight refocusing is needed (with a focus of 400 mm – about 0.5 mm shift of the eyepiece from the lens).
When switching from 4 to 3 mm, refocusing is not needed.

When observing with glasses, the need for focusing is less noticeable, especially if you first focus on 3 mm, but it is almost impossible to observe with glasses (only 8 mm, and then at the limit). So, perhaps, people with normal vision will not need refocusing.

I also observed the Sun through the Levenhuk Ra R66 ED Doublet Black telescope, Herschel Lacerta 1.25 wedge, ND 3.0 filter, polarizing filter and Svbony zoom 3-8 mm SV215 eyepiece. The eyepiece showed excellent versatility in observations – with such a focal length of the telescope (400 mm), the magnification range was from 50 to 133x. I admired the sunspots with great pleasure.

When testing on ground objects, I noticed pincushion distortion, and the smaller the focal length of the eyepiece, the stronger the distortion. The edge of the aperture during observations is sharp, with a greenish (8 and 6 mm) or bluish (4 and 3 mm) tint. I took pictures through a closed window only to assess distortion.

Summary
+ Sharp and contrasting image on the axis and most of the field of view
+ Large range of focal lengths
+ Compactness
+ Good build quality
+ Affordable price
– Small eye relief, impossible to observe with glasses
– There is a noticeable distortion, especially at short focus
– Slight color distortion
-When changing the focal length, a slight refocusing is necessary
– Glare (presumably from the cornea of the eye) at all focal lengths.

Chinese engineers came up with a very interesting eyepiece with good image quality, which is great for observing the Moon, planets, the Sun, double stars. Due to the wide range of focal lengths, the eyepiece can be productively used on various telescopes, but it will be most interesting for owners of telescopes with a relative aperture from 1:5 to 1:10. Also, the eyepiece can be successfully used for ground-based observations, including through spotting scopes with a 1.25″ output. I really liked the eyepiece and I will be happy to use it when observing the Moon, planets and the Sun. In the near future I will try to get a Nagler eyepiece for a test 3-6mm and compare it with Svbony.

Many thanks to Svbony for the provided eyepiece.
The eyepiece Svbony zoom 3-8 mm SV215 is available for sale at link.

Have questions? Ask in the comments, I will be happy to answer.

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