henry link
Well-known member
I borrowed a Zeiss Gavia 85 for a couple of days last week with the idea of writing a review. Two days turned out to be too little time to get around to checking out everything, so here are the results of the tests I had time to do.
The Gavia is one of several recently introduced iterations of a Kamakura design that has been around for at least six years. I have one of the earlier versions (the discontinued Brunton ICON), so I was particularly interested to see how the Gavia might differ from the ICON. My original review of the ICON can be found here:
http://www.birdforum.net/showthread.php?t=297258
One positive change in the Gavia is a new zoom eyepiece with improved performance. I don’t know whether this eyepiece is a unique Zeiss design or is also available on the current Kamakura siblings of the Gavia, like the Vortex Razor or Maven spotting scopes. Looking at reflections returning from the lens surfaces of the old and new eyepieces shows very little in common. No visible mechanical parts look interchangeable. Bayonet mounts are not compatible.
The basic specs of the new and old eyepieces are not very different. Eye relief and AFOV are about the same for both over the zoom range, but it was quickly obvious that the new one has lower field curvature and off-axis astigmatism and a worthwhile reduction in lateral color. Distortion has changed from mustache (which I initially mistook for barrel) in the old eyepiece to fairly strong barrel at 30x in the new one that gradually weakens to approximately zero rectilinear distortion at 60x. The barrel distortion is necessarily accompanied by very strong angular magnification distortion at 30x. AMD is weaker but still strong at 60x. The distortion pattern over the zoom range resembles the distortion of the Kowa 25-60x zoom.
Moving on to the scope body, I found only one significant difference in the basic optics between the ICON and the Gavia and that, I’m sorry to say, was inferior color correction in the Gavia.
The star test photos below show a focused artificial star in the center, flanked by defocused diffraction patterns, inside-focus on the left and outside-focus on the right. The Gavia is the top row. At best focus the Gavia isn’t nearly as color free as the ICON. The outside-focus diffraction pattern shows a very vivid red fringe surrounding it in contrast to a milder green fringe surrounding the inside-focus pattern. At high magnification the red fringe on a high contrast focused object is quite obvious. The red fringe becomes wider and more vivid on objects that are slightly closer to the viewer than a perfectly focused object. The misty green fringe is less obvious, but still visible on objects that are slightly farther away than best focus.
I think the most likely explanation for the Gavia’s poorer color correction is that the ICON probably used a Fluorite equivalent ED glass (like Ohara FPL53) with an Abbe value around 95. That gave it truly apochromatic correction and establishes that this Kamakura objective design can be essentially color free with the right glass. For the Gavia Zeiss presumably specified one of the ED glass types with an Abbe value around 80-85 (like Schott FK51). Those have a lower potential for color correction, but they may be seen as good enough for a scope where very high magnification isn’t likely to be used. It would be very interesting to see the level of color correction for the Vortex and Maven siblings of the Gavia.
One other unpleasant surprise was an undersized internal stop in the Gavia that reduces the scope’s effective aperture to about 80mm. I almost didn’t bother checking this, because I just assumed Zeiss would get it right.
I used two methods to determine the effective aperture and repeated the measurements several times. One test is the “flashlight” method, proposed by Glenn LeDrew at “Cloudy Nights”. It involves shining a collimated flashlight beam through the eyepiece end of the scope and measuring the diameter of the light cylinder that emerges from the objective lens. The other method is to examine a transparent ruler stretched across the front of the objective lens by looking through the eyepiece with a magnifying loupe. Both methods were in good agreement and showed effective apertures of 80mm for both the ICON and the Gavia.
I’ve used both methods many times and have always found high end optics to have clear apertures as specified or very close. Inexpensive binoculars, on the other hand, are notorious for undersized prisms or other internal stops that reduce the axial clear aperture. Also scopes that use moving prisms for focusing typically loose effective aperture as the prism moves out of its optimum position at close focus. The Gavia has quite effective internal baffling, usually a good thing, but I think it’s likely that at least one of those baffles is a bit too small.
Just like the ICON, the Gavia suffers from serious spherical aberration at all distances, but SA strongly increases in both scopes at close focus. The image quality of the Gavia at the closest focus is very poor, though a bit better than the truly terrible ICON. That might be attributable to the Gavia’s slightly higher focal ratio. Both scopes are effectively 80mm, but the ICON has a focal length of around 450mm (f/5.6) and the Gavia around 495mm (f/6.2).
Using my indoor set-up I measured the Gavia’s resolution on a USAF 1951 glass slide as 2.17” at 13’ and 1.9” at 30’, poor results for an 80mm scope. I couldn’t conduct a reliable resolution test using my outdoor set-up because the air was too turbulent during the time I had the scope. My estimate is that this particular Gavia’s resolution at long distance is about the same or slightly better than my ICON, which would make it about 1.6”- 1.65”. That might rise to the level of mediocre resolution by spotting scope standards, but it's pretty far from 1.36”, which is what a diffraction limited scope with a clear aperture of 85mm should do.
One other similarity between the Gavia and ICON is Kamakura’s excellent variable speed focuser. I didn’t have time to determine if the focusing speeds are identical, but they seemed so in normal use. As I said in the original ICON review this feature is something that would benefit every scope.
In summary I have to say that I found the Gavia to be a somewhat disappointing mixture of a very nice eyepiece mounted on a not so nice scope body. A street price of $2000 places it $400 above two of its closest competitors, the 85mm Vortex Razor (which could be exactly the same scope) and the 82mm Nikon Monarch HG (which looks like it could be the old 82mm Fieldscope resurrected with a new wide angle zoom). Then there is the Kowa 883, a much better scope for about $600 more. I think if I were shopping for a $2000 telescope I would be inclined to just skip the current $1600-$2000 offerings and save up for the Kowa, which is looking like the biggest bargain in scopes right now.
Henry Link
The Gavia is one of several recently introduced iterations of a Kamakura design that has been around for at least six years. I have one of the earlier versions (the discontinued Brunton ICON), so I was particularly interested to see how the Gavia might differ from the ICON. My original review of the ICON can be found here:
http://www.birdforum.net/showthread.php?t=297258
One positive change in the Gavia is a new zoom eyepiece with improved performance. I don’t know whether this eyepiece is a unique Zeiss design or is also available on the current Kamakura siblings of the Gavia, like the Vortex Razor or Maven spotting scopes. Looking at reflections returning from the lens surfaces of the old and new eyepieces shows very little in common. No visible mechanical parts look interchangeable. Bayonet mounts are not compatible.
The basic specs of the new and old eyepieces are not very different. Eye relief and AFOV are about the same for both over the zoom range, but it was quickly obvious that the new one has lower field curvature and off-axis astigmatism and a worthwhile reduction in lateral color. Distortion has changed from mustache (which I initially mistook for barrel) in the old eyepiece to fairly strong barrel at 30x in the new one that gradually weakens to approximately zero rectilinear distortion at 60x. The barrel distortion is necessarily accompanied by very strong angular magnification distortion at 30x. AMD is weaker but still strong at 60x. The distortion pattern over the zoom range resembles the distortion of the Kowa 25-60x zoom.
Moving on to the scope body, I found only one significant difference in the basic optics between the ICON and the Gavia and that, I’m sorry to say, was inferior color correction in the Gavia.
The star test photos below show a focused artificial star in the center, flanked by defocused diffraction patterns, inside-focus on the left and outside-focus on the right. The Gavia is the top row. At best focus the Gavia isn’t nearly as color free as the ICON. The outside-focus diffraction pattern shows a very vivid red fringe surrounding it in contrast to a milder green fringe surrounding the inside-focus pattern. At high magnification the red fringe on a high contrast focused object is quite obvious. The red fringe becomes wider and more vivid on objects that are slightly closer to the viewer than a perfectly focused object. The misty green fringe is less obvious, but still visible on objects that are slightly farther away than best focus.
I think the most likely explanation for the Gavia’s poorer color correction is that the ICON probably used a Fluorite equivalent ED glass (like Ohara FPL53) with an Abbe value around 95. That gave it truly apochromatic correction and establishes that this Kamakura objective design can be essentially color free with the right glass. For the Gavia Zeiss presumably specified one of the ED glass types with an Abbe value around 80-85 (like Schott FK51). Those have a lower potential for color correction, but they may be seen as good enough for a scope where very high magnification isn’t likely to be used. It would be very interesting to see the level of color correction for the Vortex and Maven siblings of the Gavia.
One other unpleasant surprise was an undersized internal stop in the Gavia that reduces the scope’s effective aperture to about 80mm. I almost didn’t bother checking this, because I just assumed Zeiss would get it right.
I used two methods to determine the effective aperture and repeated the measurements several times. One test is the “flashlight” method, proposed by Glenn LeDrew at “Cloudy Nights”. It involves shining a collimated flashlight beam through the eyepiece end of the scope and measuring the diameter of the light cylinder that emerges from the objective lens. The other method is to examine a transparent ruler stretched across the front of the objective lens by looking through the eyepiece with a magnifying loupe. Both methods were in good agreement and showed effective apertures of 80mm for both the ICON and the Gavia.
I’ve used both methods many times and have always found high end optics to have clear apertures as specified or very close. Inexpensive binoculars, on the other hand, are notorious for undersized prisms or other internal stops that reduce the axial clear aperture. Also scopes that use moving prisms for focusing typically loose effective aperture as the prism moves out of its optimum position at close focus. The Gavia has quite effective internal baffling, usually a good thing, but I think it’s likely that at least one of those baffles is a bit too small.
Just like the ICON, the Gavia suffers from serious spherical aberration at all distances, but SA strongly increases in both scopes at close focus. The image quality of the Gavia at the closest focus is very poor, though a bit better than the truly terrible ICON. That might be attributable to the Gavia’s slightly higher focal ratio. Both scopes are effectively 80mm, but the ICON has a focal length of around 450mm (f/5.6) and the Gavia around 495mm (f/6.2).
Using my indoor set-up I measured the Gavia’s resolution on a USAF 1951 glass slide as 2.17” at 13’ and 1.9” at 30’, poor results for an 80mm scope. I couldn’t conduct a reliable resolution test using my outdoor set-up because the air was too turbulent during the time I had the scope. My estimate is that this particular Gavia’s resolution at long distance is about the same or slightly better than my ICON, which would make it about 1.6”- 1.65”. That might rise to the level of mediocre resolution by spotting scope standards, but it's pretty far from 1.36”, which is what a diffraction limited scope with a clear aperture of 85mm should do.
One other similarity between the Gavia and ICON is Kamakura’s excellent variable speed focuser. I didn’t have time to determine if the focusing speeds are identical, but they seemed so in normal use. As I said in the original ICON review this feature is something that would benefit every scope.
In summary I have to say that I found the Gavia to be a somewhat disappointing mixture of a very nice eyepiece mounted on a not so nice scope body. A street price of $2000 places it $400 above two of its closest competitors, the 85mm Vortex Razor (which could be exactly the same scope) and the 82mm Nikon Monarch HG (which looks like it could be the old 82mm Fieldscope resurrected with a new wide angle zoom). Then there is the Kowa 883, a much better scope for about $600 more. I think if I were shopping for a $2000 telescope I would be inclined to just skip the current $1600-$2000 offerings and save up for the Kowa, which is looking like the biggest bargain in scopes right now.
Henry Link
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