SVBONY SV191 變焦目鏡
cloudynight
https://www.cloudynights.com/topic/813089-svbony-sv191-72-to-216mm/SVBONY 7-21 (SV135) vs 7.2-21.6 (SV191)
https://www.cloudynights.com/topic/903759-svbony-7-21-sv135-vs-72-216-sv191/SV135:
Very decent eyepiece! The image quality across the field is head and shoulders above that of Plösslov, in the center it is tolerable. The angular field of view is small and decreases at lower magnifications. There aren't enough clicks to fix focal lengths. The eye relief is quite comfortable, even when observing with glasses. It is parfocal to itself when changing magnifications and to other eyepieces with zero (the most common) parfocal parameter. Probably suitable for binocular observations. Conditionally recommend!
SV191:
A very mediocre eyepiece in terms of image quality. It’s not “ultra-wide-angle” at all, and the eye relief isn’t amazing. It seems there is not a single declared characteristic in it that would be confirmed by direct measurements (including the mass of 167 g versus the real 224 g). I do not recommend.
So Earnest is clearly not impressed with the larger, more expensive SV191! Of course with these inexpensive eyepieces there is always the possibility one gets a poor copy.
My Use Case
I wanted a compact, inexpensive zoom primarily for use with an Astro-Tech AT70ED as a daytime terrestrial spotting scope. I have plenty of nicer fixed eyepieces that I would typically use for astronomical observing so I do not intend to use this zoom for astronomy. (And I already have the TeleVue 3-6mm Nagler as a planetary zoom).
I am willing to accept some compromises in performance for my relatively infrequent secondary use as a spotting scope. So I don't expect either eyepiece to be spectacular. I am mostly curious as to whether the SV191 is worth the price and size premium over the SV135 for my use case.
The AT70ED is a 420mm F/6 scope, so probably a bit taxing on an eyepiece, and a scope that will have fairly pronounced field curvature as well due to its short focal length. Earnest tests at F/5 if I understand correctly, so his tests might be a bit more stringent than mine. I presume if I slapped these on my SW 100ED, which is F/9, they'd look better - but I have no intention of using these on that scope so I didn't try.
Build/Mechanical Notes
The overall build quality is similar. The SV191 is larger, heavier, and has a twist up eye cup. The zoom on the SV135 was smooth. The zoom on the SV191 was too tight in my opinion. Also it was awkward to try to grip the bottom of the SV191 to prevent the eyepiece from rotating in the holder when zooming because the lower portion is quite thin and smooth. Besides the tight zoom on the SV191, there is really little to complain about for zooms in this price range.
Test Setup
My main question was how appreciable were the differences in TFOV and AFOV between these two, how usable was any extra FOV, and were there any significant image quality differences between the two. I had no need for absolute measurements, Ernest has already provided those, so I only cared about direct relative comparisons. Largely these comparisons were made at the shortest and longest focal lengths.
I setup the AT70ED about 10m from a test target which was simply concentric circles printed on A3+ glossy photo paper that also included a radial series of asterisks to better see any truly obvious astigmatism. At this distance the SV191 TFOV covered about 15.1 inches and thus mostly fit on the target and being substantially less than the width of the target could be easily measured. The target was well lit with an 60W equivalent LED bulb so my eye pupil was not remotely dark adapted.
First I measured the TFOV in the sense of the width of the target the FOV covered at each labeled focal length. Obviously there is uncertainty in the labeling, but the extremes were what I cared about most. I also noted at what radius the image quality degraded significantly, but I did allow for accommodation by my eye from the substantial field curvature. Those with less accommodation would have worse results at this short focal length while those using longer focal length refractors would have an easier time.
Results
Largely my results of TFOV matched what Earnest already reported. The SV191 does in fact have a bit more TFOV and AFOV at every setting. The difference is not Earth shattering at all and would likely be irrelevant for anyone with eye glasses. I had a slightly larger TFOV at the widest setting for the SV191 compared to the SV135 than Earnest reported. I suspect Earnest's measurements are more reliable than mine.
As for image quality, well there wasn't much difference really. I would say I could find no case in which the SV191 image quality was better than the SV135. Earnest reported quite poor edge performance, especially at the wide end of the SV191. My results were similar. In fact if I were to consider the "sharp" TFOV in both eyepieces then I'd actually give a small nod to the SV135 at the longest focal length, which despite the shorter focal length and smaller AFOV has a slightly larger sharp TFOV than the SV191. So while the SV191 does in fact have a wider TFOV at its widest, the extra FOV compared to the SV135 is rather compromised. At the short focal end of things, not surprisingly the SV191 had a wider TFOV and in this case the sharp portion was a bit more than the SV135 but of course the SV135 was also magnifying a bit more. In this refractor both eyepieces were showing a bit of color at their shortest focal setting on this fairly high contrast target.
Conclusion
Again, this is a very simplistic review of a specific use case on a specific telescope. That said, I really saw nothing in the results to make me choose the SV191 over the SV135. In fact, were they the same price I'd probably still pick the SV135 as it is smaller and lighter.
I'll be keeping the SV135 and sending the SV191 back.
It would be interesting to compare on a longer focal length and slower F-number telescope to see if the SV191 might better realize its increased AFOV as sharp. Sadly I am too busy and lazy to do that comparison myself
. I did answer my specific questions for my specific use cases and figured I could take a moment to share if it is of any use to folks in the future.
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