henry link
Well-known member
Whew!...this review has been a long time coming. I received the first test specimen of the Zen-Ray ED2 scope way back in late December and a replacement for it in early January. I considered both of those to be too defective (or damaged) for a fair review. A unit from a new production run arrived in May. This one turned out to be good enough for me to feel confident that its optical quality is limited by the design rather than defects or shipping damage, so here, finally, are some test results. As usual, I’ll be concentrating entirely on the optics. I decided to treat the scope body and the eyepieces in separate parts, starting with the body.
Part One – Scope Body
From the outside the Zen scope closely resembles the Kowa 883, but that resemblance is only skin deep. Inside, the objective design, focusing system and erecting prisms are quite different from what‘s inside the Kowa. Instead of the Schmidt prism used in the Kowa the Zen uses a Porro, followed by semi-pentaprism to angle the light path. This erecting system has three separate prisms with more glass to air surfaces and more internal reflections than a Schmidt. It includes one reflection in the semi-pentaprism that has to be mirror coated. To my knowledge Zen is the first to use a dielectric coating for this surface.
Focusing is accomplished by moving one part of the Porro prism, so there is no moving focusing element as in the current alpha scopes. I think this design is a pretty good choice for a moderately priced scope. It avoids one potential optical defect by not having a roof prism split the light cone and in this case the dielectric coating keeps the light loss reasonable. This same system was used in many older high-end scopes, like the Swarovski AT-80 and Kowa 823. I haven’t seen every scope that uses it, but the two scopes I have tested (the Pentax 80ED and the Zen) both suffer some aperture loss as a result of the focusing prism moving out of its optimum position at close focus. I expect other scopes of this design have the same problem. The Zen begins to lose aperture at about 75-80’. At 60’ I measured an aperture of about 80mm, at 30’ it’s about 75mm and at closest focus of 16’ it’s down to about 70mm. As I recall the Pentax was worse, with the effective aperture already below 70mm at 30’. The first photo on the left below shows the difference between the 20x exit pupil size at infinity focus and at 16’. Part of the reduction in exit pupil is due to increased magnification at close focus and the rest is caused by the prism aperture blocking the edges of the objective. If at infinity focus the scope acts like a true 20x82, then at 16’ it’s effectively more like a 22x70.
The objective has 4 elements and a focal length of 464mm. That is, not so coincidentally as you will see later, about the same focal length as Swarovski scopes. There is a triplet up front consisting of a cemented doublet, closely air-spaced with a singlet and then down the tube close to the prisms there is another singlet in about the position a focusing lens would occupy. According to Zen there are two ED elements, but the type of ED glass is not specified. I don’t know of another scope using this particular configuration. The Pentax 80ED has a fixed singlet near the prism, but uses a doublet up front. All of the current alpha scopes use fixed triplets with moving focusing doublets.
I’ve included some star-test photos, which show a progression from intra-focus on the left through best focus at the center to extra-focus on the right. Unfortunately this is the best I could do, thanks to air turbulence. The diffraction patterns were much clearer through the eyepiece. There are a couple of items of interest that can at least be glimpsed in the photos. First, notice the flat spot on the outer edge centered at 4:00-5:00 in the intra-focal pattern. That appears to be a prism edge impinging on the light cone. It causes a diffraction spike that can be seen dimly at about 10:30 at best focus and in the close extra-focal pattern (right of center). That spike was more evident at the eyepiece than it appears in the photos. Second, notice the better defined intra-focal rings compared to the extra-focal rings. A perfect scope would show identical rings on both sides of focus. I believe the asymmetry in the Zen star-test comes mostly from sphero-chromatism, the form of spherical aberration that varies with wavelength. It causes the unfocused purple smear of color between the extra-focal rings that reduces their definition. While still not perfect, the symmetry of the extra and intra-focal rings is improved when viewed through a green filter that shows spherical aberration for a narrow band of color. Spherical aberrations are not too surprising in a fast scope like this. I also found it difficult to achieve a really good focused star. A perfect scope at high magnification shows a nice round airy disc surrounded by a clean first diffraction ring. Through the Zen a highly magnified star point always showed some flaring and asymmetry similar to what you see in the focused star image at the center. The impinging prism edge and the spherical aberrations were present in all three samples of the scope I tested, so I think it’s safe to conclude that they are characteristics of the design.
The spherical aberrations are probably doing most of the damage to the image quality at high magnification when the Zen is compared to a more “perfect” scope of the same aperture. I measured the resolution with the USAF 1951 resolution test pattern at 1.6 arc seconds. Not bad for a 82mm spotting scope, but not quite diffraction limited, which would be more like 1.42 arc seconds. At first glance the image at 60x looks good, but when directly compared to a scope with better corrections of about the same aperture (Takahashi SKY90 Fluorite APO reduced to 80mm) the image through the Zen appears somewhat degraded. This is why a reference scope of known quality is so important for comparisons. None of us has a built in sense of exactly how good a 60x image in an 82mm scope should look. For my eyes, the Zen at 60x loses about 1 element on the USAF chart (-12%) compared to the Tak and the image has the kind of gauzy appearance I associate with the presence of spherical aberrations or other defects. Chromatic aberration, on the other hand, appears well enough corrected for 60x. There is only a very little longitudinal CA visible. The color correction (like most ED spotting scopes) is not at the level of the best astronomical APO refractors, but it also doesn’t need to be that good for birding magnifications.
The USAF test patterns below are tight crops of photos made through the scopes using a Nikon D40 with a zoom lens set at 80mm. The left side is the Zen with its zoom eyepiece set at 60x compared to the Takahashi “80mm” on the right with a Baader zoom set to 60x. The Tak image is reversed because it uses a simple star diagonal for image erection. The photos don’t quite reveal the full resolution visible at the eyepiece through either scope, but the difference in image quality as it appeared to my eye comes across pretty well. What the photos don’t show is the difficulty one encounters trying to focus a scope with significant spherical aberration. No matter where the focus is set there is always some light from some part of the objective that remains out of focus. A scope with low aberrations, like the Tak, snaps into a definite sharp focus at 60x, but trying to find a completely sharp focus with the Zen at 60x involves a search back and forth that never ends. You think you’ve found the best focus, then you look again and it’s gone. The best 80-82mm alpha scope specimens would have images at 60x similar to the Tak and a larger aperture scope like the Kowa 883 would be even brighter and sharper than the stopped down Tak. The difference in image quality between the Zen and the Tak didn’t completely disappear even at the lowest magnification I used for comparisons (25x), but as magnification drops below about 35-40x the aberrations take less of a toll and the Zen image looks quite respectable, sharp enough to be largely indistinguishable from the Tak except under very close scrutiny.
The Zen’s color bias is slightly warm. The photo of a square within a square below shows a piece of white photo paper in sunlight. The square in the center is an image of the center of the same piece of paper photographed through the Zen scope. Just looking through the scope I had very little sense of this slight warm tint.
Part 2 – The Zoom Eyepiece
I discovered, when I compared them, that the Zen 20-60x zoom eyepiece is a direct copy of the Swarovski 20-60x zoom. The focal lengths, lens diameters, overall dimensions and everything else I could measure are the same within the margins of error of my kitchen table and backyard methods. The photos of the two eyepieces placed side by side (Zen on the left) show identical reflection patterns of the camera flash returning from the lens elements. This only happens if the optical formulas are identical. So, whatever has been said about the excellent Swarovski eyepiece also applies to the Zen, which is such a faithful copy that I doubt anyone could tell the difference between the two. Both are very sharp and bright at every magnification with excellent off–axis performance. As is often the case with spotting scopes, it’s the objective and prisms that limit the high magnification performance, not the zoom eyepiece. Charles at Zen-Ray does not recommend it, but as you can see in the photo that shows the bayonets, the mounts are so close that Swarovski eyepieces will fit the Zen scope (but not engage the locking pin) and probably vice versa.
I measured the eye relief and AFOV of the Zen eyepiece at the magnifications marked on the barrel. Eye relief was measured from the rim of the eyecup. It would be 6mm longer measured from the glass of the eye lens. AFOV measurements were done using the panning scale on a tripod head. These are the true angles subtended by the apparent field including any distortion, so they cannot be compared to specs derived mathematically from the real field.
20x, AFOV – 40 degrees, ER – 17mm
30x, AFOV - 48 degrees, ER – 12mm
40x, AFOV – 55 degrees, ER – 11mm
50x, AFOV – 62 degrees, ER – 13mm
60x, AFOV – 66 degrees, ER – 14mm
I should mention that I ran into a birder with a Vortex 85mm Razor scope this spring. I didn’t have the Zen for direct comparison, but I noticed that both the Vortex scope body and its zoom eyepiece looked very much like the Zen, including what looked like the very same “Swarovski” bayonet mount.
Part 3 – The 30XW eyepiece
This is another copy. This time the eyepiece being copied is from the Nikon SE series binoculars. The Zen version does have a slightly wider apparent field, but other than that I could detect no significant difference between the two when I mounted them on the same scope. You might expect this eyepiece design to deliver really good off-axis performance, but in fact it turns out to be more adequate than outstanding when used on a scope. There is some off-axis astigmatism, which oddly reaches a peak around 20 degrees from the center, then gradually diminishes toward the edge. The off-axis performance is OK, but not in the same class with the premium wide field astronomical eyepieces I have on hand for comparison (14mm Pentax XW, 19mm Televue Panoptic) and not quite as good as a 17mm Baader Hyperion. The Zen is quite small and light for a wide field eyepiece with long eye relief. The field is wider and eye relief longer than the zoom at 30x, but off-axis aberrations are worse, so the sweet spot is actually smaller. Too bad Zen didn’t copy the superb Swarovski 30XW, which is better in every way, but that would surely have resulted in a much more expensive eyepiece. The Swarovski 30XW will fit the Zen scope with excellent optical results, but it doesn’t engage the locking pin, so it may not be completely secure (once again, not recommended by Zen-Ray).
Eye relief measured 15mm from the eyecup, 21mm from the eye lens glass. AFOV was 59 degrees. That’s about 2 degrees more than the SE eyepiece measured the same way.
I notice that the Vortex Razor 30XW eyepiece shown on the Eagle Optics website looks very much like the Zen-Ray 30XW and has the same specs.
Part 4 – The Eyepiece Adapter
One more bit came with the Zen scope. It’s described as a 1.25” eyepiece adapter, but it won’t actually accept 1.25” eye barrels. The idea is to unscrew the eyepiece body from its barrel and then thread the eyepiece body onto the adapter. Unfortunately this will work only for the eyepieces that happen to have a male thread of the right size. Many of the most desirable astronomical eyepieces have field lenses in the barrel, so the barrel shouldn’t or can’t be removed. Charles is aware of the problem, so some modification of this adapter might turn up in the future.
I hope, by pointing out some faults, I haven’t left the impression that I have a negative opinion of the Zen scope. Provided you get a specimen as good as the cherry review sample here, the $1200 price of the Zen will buy you a superb zoom eyepiece, combined with scope optics that are good enough to stand comparison to average specimens of premium spotting scopes of about the same aperture (especially the Swarovski 80-HD since it uses essentially the same eyepiece). It’s true that the image quality, even of this cherry sample, is inferior to cherry samples of premium scopes, especially larger aperture scopes like the Kowa 883, but given the sample variation in expensive scopes you could wind up buying an alpha specimen with worse optics than this particular Zen. As ever, I’m afraid obtaining a good scope still comes down to finding a good individual unit no matter how much you’re willing to pay.
Finally, I’d like to thank Charles at Zen-Ray for patiently responding to my various quibbles and complaints over the last 5 months. He was always the perfect gentleman and a pleasure to deal with.
Henry Link
Part One – Scope Body
From the outside the Zen scope closely resembles the Kowa 883, but that resemblance is only skin deep. Inside, the objective design, focusing system and erecting prisms are quite different from what‘s inside the Kowa. Instead of the Schmidt prism used in the Kowa the Zen uses a Porro, followed by semi-pentaprism to angle the light path. This erecting system has three separate prisms with more glass to air surfaces and more internal reflections than a Schmidt. It includes one reflection in the semi-pentaprism that has to be mirror coated. To my knowledge Zen is the first to use a dielectric coating for this surface.
Focusing is accomplished by moving one part of the Porro prism, so there is no moving focusing element as in the current alpha scopes. I think this design is a pretty good choice for a moderately priced scope. It avoids one potential optical defect by not having a roof prism split the light cone and in this case the dielectric coating keeps the light loss reasonable. This same system was used in many older high-end scopes, like the Swarovski AT-80 and Kowa 823. I haven’t seen every scope that uses it, but the two scopes I have tested (the Pentax 80ED and the Zen) both suffer some aperture loss as a result of the focusing prism moving out of its optimum position at close focus. I expect other scopes of this design have the same problem. The Zen begins to lose aperture at about 75-80’. At 60’ I measured an aperture of about 80mm, at 30’ it’s about 75mm and at closest focus of 16’ it’s down to about 70mm. As I recall the Pentax was worse, with the effective aperture already below 70mm at 30’. The first photo on the left below shows the difference between the 20x exit pupil size at infinity focus and at 16’. Part of the reduction in exit pupil is due to increased magnification at close focus and the rest is caused by the prism aperture blocking the edges of the objective. If at infinity focus the scope acts like a true 20x82, then at 16’ it’s effectively more like a 22x70.
The objective has 4 elements and a focal length of 464mm. That is, not so coincidentally as you will see later, about the same focal length as Swarovski scopes. There is a triplet up front consisting of a cemented doublet, closely air-spaced with a singlet and then down the tube close to the prisms there is another singlet in about the position a focusing lens would occupy. According to Zen there are two ED elements, but the type of ED glass is not specified. I don’t know of another scope using this particular configuration. The Pentax 80ED has a fixed singlet near the prism, but uses a doublet up front. All of the current alpha scopes use fixed triplets with moving focusing doublets.
I’ve included some star-test photos, which show a progression from intra-focus on the left through best focus at the center to extra-focus on the right. Unfortunately this is the best I could do, thanks to air turbulence. The diffraction patterns were much clearer through the eyepiece. There are a couple of items of interest that can at least be glimpsed in the photos. First, notice the flat spot on the outer edge centered at 4:00-5:00 in the intra-focal pattern. That appears to be a prism edge impinging on the light cone. It causes a diffraction spike that can be seen dimly at about 10:30 at best focus and in the close extra-focal pattern (right of center). That spike was more evident at the eyepiece than it appears in the photos. Second, notice the better defined intra-focal rings compared to the extra-focal rings. A perfect scope would show identical rings on both sides of focus. I believe the asymmetry in the Zen star-test comes mostly from sphero-chromatism, the form of spherical aberration that varies with wavelength. It causes the unfocused purple smear of color between the extra-focal rings that reduces their definition. While still not perfect, the symmetry of the extra and intra-focal rings is improved when viewed through a green filter that shows spherical aberration for a narrow band of color. Spherical aberrations are not too surprising in a fast scope like this. I also found it difficult to achieve a really good focused star. A perfect scope at high magnification shows a nice round airy disc surrounded by a clean first diffraction ring. Through the Zen a highly magnified star point always showed some flaring and asymmetry similar to what you see in the focused star image at the center. The impinging prism edge and the spherical aberrations were present in all three samples of the scope I tested, so I think it’s safe to conclude that they are characteristics of the design.
The spherical aberrations are probably doing most of the damage to the image quality at high magnification when the Zen is compared to a more “perfect” scope of the same aperture. I measured the resolution with the USAF 1951 resolution test pattern at 1.6 arc seconds. Not bad for a 82mm spotting scope, but not quite diffraction limited, which would be more like 1.42 arc seconds. At first glance the image at 60x looks good, but when directly compared to a scope with better corrections of about the same aperture (Takahashi SKY90 Fluorite APO reduced to 80mm) the image through the Zen appears somewhat degraded. This is why a reference scope of known quality is so important for comparisons. None of us has a built in sense of exactly how good a 60x image in an 82mm scope should look. For my eyes, the Zen at 60x loses about 1 element on the USAF chart (-12%) compared to the Tak and the image has the kind of gauzy appearance I associate with the presence of spherical aberrations or other defects. Chromatic aberration, on the other hand, appears well enough corrected for 60x. There is only a very little longitudinal CA visible. The color correction (like most ED spotting scopes) is not at the level of the best astronomical APO refractors, but it also doesn’t need to be that good for birding magnifications.
The USAF test patterns below are tight crops of photos made through the scopes using a Nikon D40 with a zoom lens set at 80mm. The left side is the Zen with its zoom eyepiece set at 60x compared to the Takahashi “80mm” on the right with a Baader zoom set to 60x. The Tak image is reversed because it uses a simple star diagonal for image erection. The photos don’t quite reveal the full resolution visible at the eyepiece through either scope, but the difference in image quality as it appeared to my eye comes across pretty well. What the photos don’t show is the difficulty one encounters trying to focus a scope with significant spherical aberration. No matter where the focus is set there is always some light from some part of the objective that remains out of focus. A scope with low aberrations, like the Tak, snaps into a definite sharp focus at 60x, but trying to find a completely sharp focus with the Zen at 60x involves a search back and forth that never ends. You think you’ve found the best focus, then you look again and it’s gone. The best 80-82mm alpha scope specimens would have images at 60x similar to the Tak and a larger aperture scope like the Kowa 883 would be even brighter and sharper than the stopped down Tak. The difference in image quality between the Zen and the Tak didn’t completely disappear even at the lowest magnification I used for comparisons (25x), but as magnification drops below about 35-40x the aberrations take less of a toll and the Zen image looks quite respectable, sharp enough to be largely indistinguishable from the Tak except under very close scrutiny.
The Zen’s color bias is slightly warm. The photo of a square within a square below shows a piece of white photo paper in sunlight. The square in the center is an image of the center of the same piece of paper photographed through the Zen scope. Just looking through the scope I had very little sense of this slight warm tint.
Part 2 – The Zoom Eyepiece
I discovered, when I compared them, that the Zen 20-60x zoom eyepiece is a direct copy of the Swarovski 20-60x zoom. The focal lengths, lens diameters, overall dimensions and everything else I could measure are the same within the margins of error of my kitchen table and backyard methods. The photos of the two eyepieces placed side by side (Zen on the left) show identical reflection patterns of the camera flash returning from the lens elements. This only happens if the optical formulas are identical. So, whatever has been said about the excellent Swarovski eyepiece also applies to the Zen, which is such a faithful copy that I doubt anyone could tell the difference between the two. Both are very sharp and bright at every magnification with excellent off–axis performance. As is often the case with spotting scopes, it’s the objective and prisms that limit the high magnification performance, not the zoom eyepiece. Charles at Zen-Ray does not recommend it, but as you can see in the photo that shows the bayonets, the mounts are so close that Swarovski eyepieces will fit the Zen scope (but not engage the locking pin) and probably vice versa.
I measured the eye relief and AFOV of the Zen eyepiece at the magnifications marked on the barrel. Eye relief was measured from the rim of the eyecup. It would be 6mm longer measured from the glass of the eye lens. AFOV measurements were done using the panning scale on a tripod head. These are the true angles subtended by the apparent field including any distortion, so they cannot be compared to specs derived mathematically from the real field.
20x, AFOV – 40 degrees, ER – 17mm
30x, AFOV - 48 degrees, ER – 12mm
40x, AFOV – 55 degrees, ER – 11mm
50x, AFOV – 62 degrees, ER – 13mm
60x, AFOV – 66 degrees, ER – 14mm
I should mention that I ran into a birder with a Vortex 85mm Razor scope this spring. I didn’t have the Zen for direct comparison, but I noticed that both the Vortex scope body and its zoom eyepiece looked very much like the Zen, including what looked like the very same “Swarovski” bayonet mount.
Part 3 – The 30XW eyepiece
This is another copy. This time the eyepiece being copied is from the Nikon SE series binoculars. The Zen version does have a slightly wider apparent field, but other than that I could detect no significant difference between the two when I mounted them on the same scope. You might expect this eyepiece design to deliver really good off-axis performance, but in fact it turns out to be more adequate than outstanding when used on a scope. There is some off-axis astigmatism, which oddly reaches a peak around 20 degrees from the center, then gradually diminishes toward the edge. The off-axis performance is OK, but not in the same class with the premium wide field astronomical eyepieces I have on hand for comparison (14mm Pentax XW, 19mm Televue Panoptic) and not quite as good as a 17mm Baader Hyperion. The Zen is quite small and light for a wide field eyepiece with long eye relief. The field is wider and eye relief longer than the zoom at 30x, but off-axis aberrations are worse, so the sweet spot is actually smaller. Too bad Zen didn’t copy the superb Swarovski 30XW, which is better in every way, but that would surely have resulted in a much more expensive eyepiece. The Swarovski 30XW will fit the Zen scope with excellent optical results, but it doesn’t engage the locking pin, so it may not be completely secure (once again, not recommended by Zen-Ray).
Eye relief measured 15mm from the eyecup, 21mm from the eye lens glass. AFOV was 59 degrees. That’s about 2 degrees more than the SE eyepiece measured the same way.
I notice that the Vortex Razor 30XW eyepiece shown on the Eagle Optics website looks very much like the Zen-Ray 30XW and has the same specs.
Part 4 – The Eyepiece Adapter
One more bit came with the Zen scope. It’s described as a 1.25” eyepiece adapter, but it won’t actually accept 1.25” eye barrels. The idea is to unscrew the eyepiece body from its barrel and then thread the eyepiece body onto the adapter. Unfortunately this will work only for the eyepieces that happen to have a male thread of the right size. Many of the most desirable astronomical eyepieces have field lenses in the barrel, so the barrel shouldn’t or can’t be removed. Charles is aware of the problem, so some modification of this adapter might turn up in the future.
I hope, by pointing out some faults, I haven’t left the impression that I have a negative opinion of the Zen scope. Provided you get a specimen as good as the cherry review sample here, the $1200 price of the Zen will buy you a superb zoom eyepiece, combined with scope optics that are good enough to stand comparison to average specimens of premium spotting scopes of about the same aperture (especially the Swarovski 80-HD since it uses essentially the same eyepiece). It’s true that the image quality, even of this cherry sample, is inferior to cherry samples of premium scopes, especially larger aperture scopes like the Kowa 883, but given the sample variation in expensive scopes you could wind up buying an alpha specimen with worse optics than this particular Zen. As ever, I’m afraid obtaining a good scope still comes down to finding a good individual unit no matter how much you’re willing to pay.
Finally, I’d like to thank Charles at Zen-Ray for patiently responding to my various quibbles and complaints over the last 5 months. He was always the perfect gentleman and a pleasure to deal with.
Henry Link
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