henry link
Well-known member
The image provided by the best current binoculars looks so good that it’s easy to fall into the assumption that binoculars are now so close to optical perfection that further improvement wouldn’t accomplish anything as it would be invisible to the eye. Every now and then I disabuse myself of this notion by comparing the best binoculars I have around to an almost perfect low power telescope. I combine a Takahashi FC-50 Fluorite APO refractor with a 48mm eyepiece to make a nearly aberration free 8x50. Compared to this telescope the image in the very best binoculars has always looked quite obviously mushy and dull. It’s really a surprise to see just how far binocular image quality is from what is actually possible from an aberration free low magnification telescope. The main reason for this is simple. Binoculars, even the best ones, are two optically low grade little refractors strapped together. These telescopes need to be very small and light so we can hand hold two of them. That means they have to have very short focal length objectives and that leads to focal ratios so low (around f/4 or less) that the optics inevitably have very high aberrations. The steep f/4 light cone also causes the eyepieces to show worse off axis aberrations than the same eyepiece would have if combined with a slower objective. The trick for the designers is to try to keep the aberrations just low enough so that the low power images from these things will look unobjectionable to the viewer, in spite of all those nasty aberrations lurking just at the threshold of visibility.
I’m always on the lookout for a binocular design that might take those aberrations below the threshold of visibility and approach the image quality of the 8x50 Takahashi. For a while I’ve been intrigued by the potential of the Zeiss 8x56 FL as a binocular that could have unusually low aberrations under daylight conditions. I know this binocular is of very little interest to birders. It’s quite monstrously large and heavy (1210g) compared to 42mm binoculars, some of which birders already consider to be too heavy. I have no interest at all in the 56mm aperture, but the relatively long focal length attracted my attention. The overall length of the 8x56 is about 36mm longer than the 7x42 FL, but actually only 10mm longer than the old Zeiss 7x42 B/GAT. If Zeiss had been willing to make the 42mm FL’s as long as the old 7x42 they would be almost the same length as the 8x56 FL, so you might imagine that optically there is a svelte long focal length 8x42 embedded in the portly 8x56. Recently the binocular buying stars aligned themselves favoraby. An offer to buy my Swarovski 8.5x42 EL coincided with the opportunity for me to buy a demo 8x56FL at a very attractive price. I went for it and I’ve been measuring, testing, comparing it to other binoculars (mainly to the 8x42FL) and using it in the field for a couple of weeks now. I don’t think too many people are interested in this binocular so what follows is not a complete review, but an effort to explain why its image quality is so unusually good and what that says about the compromises in binocular design.
The pair I bought turned out to have one excellent barrel and one barrel with a defective roof prism. A star test of the bad barrel (with the magnification boosted to 64x) shows a spike from the roof prism edge running through the middle of the diffraction pattern on one side of focus and two(!) Airy discs connected by a thin bridge of light on the other side. This causes significantly worse measured resolution and an obviously softer image quality in that barrel compared to the other one at 64x. If this had been a spotting scope I would certainly have returned it. Fortunately I had a nearly perfect barrel to use as a reference and after a lot of scrutiny under every kind of lighting condition at 8x, I concluded that the defect was doing no visible harm to the 8x image. Since my intended use is birding in daylight I was most interested in the binocular’s performance at stopped down apertures in the range of 24mm to 42mm. I measured the resolution and performed star tests with aperture stopdowns to duplicate the limits that the eye’s stopped down entrance pupil would cause in daylight. At each aperture (56mm, 42mm, 32mm and 24mm) I compared the good barrel of the 8x56 to the better barrel of my 8x42FL.
At 56mm: Star test is similar to the 8x42. Perhaps the 8x56 has a liltle more chromatic and spherical aberration. I suspect the 56mm objective is faster than the 42mm, maybe f/3.2-3,4 vs f/3.5-3.7. There is plenty of CA in both, but in fact much less than a conventional binocular. There is also lots of SA, maybe 1.5 waves undercorrection in the 8x42, 1.5-2 waves in the 8x56. Very bad for a telescope, but typical for binoculars. Resolution is about 2.5 arcsec for the 8x56 using the USAF 1951 test pattern. Very good for a binocular, but actually no better than when it’s stopped down to 50mm. The 64x image looks a little cleaner and sharper when the objective is stopped down to 50mm because the aberrations are lower.
Stopped down to 42mm: Things get interesting. Now the 8x56 is clearly superior to the 8x42. Measured resolution is excellent for both, about 2.9 arcsec, but the 64x image looks better in the 8x56. It’s cleaner, with higher contrast and visibly less chromatic aberration. The star test shows improved correction for spherical aberration in the 8x56 to probably better than 1 wave. The improvements are explained by the change in focal ratio. The stopped dowm 8x56 is now operating at around f/4.5 instead of below f/3.5
Stopped down to 32mm: Both stopped down binoculars have about 3.9 arcsec resolution and both show improvements in the 64x image quality. The 8x56 is better. Its 64x image now looks quite respectably clean and contrasty with very little chromatic aberration. Spherical aberration in the 8x56 improves to perhaps 1/2 wave. Now its optics are operating at about f/5.8. The stopped down 8x42 is operating at about f/4.8.
Stopped down to 24mm: Now the 8x56 becomes quite a good f/7.5 telescope, almost a true APO with about 1/4 wave SA..
The point of all this is to show that the 8x56 really has no better (perhaps slightly worse) optics than the 8x42 when they are compared at full aperture, but when the 8x56 is stopped down to 42mm and below it shows significantly lower aberrations than the 8x42 (at the same aperture) simply because the 8x56 focal length is longer. If the 8x42 had the same focal length it would certainly perform just as well.
Now, does any of this matter when you simply look through the binoculars at 8x? To my delight the answer is yes. In daylight he 8x56 FL produces the sharpest, cleanest and most transparent image I’ve yet seen in a binocular. It’s very obvious comparing it to other binoculars tripod mounted, but even hand holding I’m always aware that the image is unusually fine by binocular standards. I wouldn’t have expected any binocular to make the 8x42FL, Nikon 7x50 Prostar and 8x32SE look mushy and dull in sunlight, but the 8x56 FL does it. Besides the reduced longitudinal CA and SA seen in star testing there is also a reduction in lateral color that is quite obvious in daylight. Lateral color is probably almost always what people are seeing when they complain about “color fringing” in binoculars. There is also a modest but welcome increase in the size of the “sweet spot” compared to the 8x42FL. Less lateral color and a bigger sweet spot are two more benefits that come from the higher objective focal ratio, because the less steep light cone allows the eyepiece to perform better off-axis. But, alas, edge of the field astigmatism is still this binocular’s weakest performance characteristic, just like the 8x42FL. The 7mm exit pupil also has a benefit in daylight. There is virtually complete freedom from “flare”. When bright reflections from the edge of the objective reach the eye they are out at the edge of a 7mm circle of light, so the flare tends to fall invisibly on the iris rather than entering the eye.
After experiencing the outstanding daylight image quality of this binocular for the last two weeks there is simply no turning back for me. Even if I look like a 5 year old struggling with his daddy’s big binoculars, the 8x56FL is what I will be using for birding until something better comes along. In spite of the optical quality I doubt that I will ever run into another birder using a pair. I can’t help but think that Zeiss missed an opportunity for making the 42mm FL’s just as optically superior in daylight, if only they had been willing to allow the 42’s to be about 1” longer and a little heavier. I understand that birders want their binoculars short and light, and that drives design decisions, but it also takes a toll on the optical quality.
I’m always on the lookout for a binocular design that might take those aberrations below the threshold of visibility and approach the image quality of the 8x50 Takahashi. For a while I’ve been intrigued by the potential of the Zeiss 8x56 FL as a binocular that could have unusually low aberrations under daylight conditions. I know this binocular is of very little interest to birders. It’s quite monstrously large and heavy (1210g) compared to 42mm binoculars, some of which birders already consider to be too heavy. I have no interest at all in the 56mm aperture, but the relatively long focal length attracted my attention. The overall length of the 8x56 is about 36mm longer than the 7x42 FL, but actually only 10mm longer than the old Zeiss 7x42 B/GAT. If Zeiss had been willing to make the 42mm FL’s as long as the old 7x42 they would be almost the same length as the 8x56 FL, so you might imagine that optically there is a svelte long focal length 8x42 embedded in the portly 8x56. Recently the binocular buying stars aligned themselves favoraby. An offer to buy my Swarovski 8.5x42 EL coincided with the opportunity for me to buy a demo 8x56FL at a very attractive price. I went for it and I’ve been measuring, testing, comparing it to other binoculars (mainly to the 8x42FL) and using it in the field for a couple of weeks now. I don’t think too many people are interested in this binocular so what follows is not a complete review, but an effort to explain why its image quality is so unusually good and what that says about the compromises in binocular design.
The pair I bought turned out to have one excellent barrel and one barrel with a defective roof prism. A star test of the bad barrel (with the magnification boosted to 64x) shows a spike from the roof prism edge running through the middle of the diffraction pattern on one side of focus and two(!) Airy discs connected by a thin bridge of light on the other side. This causes significantly worse measured resolution and an obviously softer image quality in that barrel compared to the other one at 64x. If this had been a spotting scope I would certainly have returned it. Fortunately I had a nearly perfect barrel to use as a reference and after a lot of scrutiny under every kind of lighting condition at 8x, I concluded that the defect was doing no visible harm to the 8x image. Since my intended use is birding in daylight I was most interested in the binocular’s performance at stopped down apertures in the range of 24mm to 42mm. I measured the resolution and performed star tests with aperture stopdowns to duplicate the limits that the eye’s stopped down entrance pupil would cause in daylight. At each aperture (56mm, 42mm, 32mm and 24mm) I compared the good barrel of the 8x56 to the better barrel of my 8x42FL.
At 56mm: Star test is similar to the 8x42. Perhaps the 8x56 has a liltle more chromatic and spherical aberration. I suspect the 56mm objective is faster than the 42mm, maybe f/3.2-3,4 vs f/3.5-3.7. There is plenty of CA in both, but in fact much less than a conventional binocular. There is also lots of SA, maybe 1.5 waves undercorrection in the 8x42, 1.5-2 waves in the 8x56. Very bad for a telescope, but typical for binoculars. Resolution is about 2.5 arcsec for the 8x56 using the USAF 1951 test pattern. Very good for a binocular, but actually no better than when it’s stopped down to 50mm. The 64x image looks a little cleaner and sharper when the objective is stopped down to 50mm because the aberrations are lower.
Stopped down to 42mm: Things get interesting. Now the 8x56 is clearly superior to the 8x42. Measured resolution is excellent for both, about 2.9 arcsec, but the 64x image looks better in the 8x56. It’s cleaner, with higher contrast and visibly less chromatic aberration. The star test shows improved correction for spherical aberration in the 8x56 to probably better than 1 wave. The improvements are explained by the change in focal ratio. The stopped dowm 8x56 is now operating at around f/4.5 instead of below f/3.5
Stopped down to 32mm: Both stopped down binoculars have about 3.9 arcsec resolution and both show improvements in the 64x image quality. The 8x56 is better. Its 64x image now looks quite respectably clean and contrasty with very little chromatic aberration. Spherical aberration in the 8x56 improves to perhaps 1/2 wave. Now its optics are operating at about f/5.8. The stopped down 8x42 is operating at about f/4.8.
Stopped down to 24mm: Now the 8x56 becomes quite a good f/7.5 telescope, almost a true APO with about 1/4 wave SA..
The point of all this is to show that the 8x56 really has no better (perhaps slightly worse) optics than the 8x42 when they are compared at full aperture, but when the 8x56 is stopped down to 42mm and below it shows significantly lower aberrations than the 8x42 (at the same aperture) simply because the 8x56 focal length is longer. If the 8x42 had the same focal length it would certainly perform just as well.
Now, does any of this matter when you simply look through the binoculars at 8x? To my delight the answer is yes. In daylight he 8x56 FL produces the sharpest, cleanest and most transparent image I’ve yet seen in a binocular. It’s very obvious comparing it to other binoculars tripod mounted, but even hand holding I’m always aware that the image is unusually fine by binocular standards. I wouldn’t have expected any binocular to make the 8x42FL, Nikon 7x50 Prostar and 8x32SE look mushy and dull in sunlight, but the 8x56 FL does it. Besides the reduced longitudinal CA and SA seen in star testing there is also a reduction in lateral color that is quite obvious in daylight. Lateral color is probably almost always what people are seeing when they complain about “color fringing” in binoculars. There is also a modest but welcome increase in the size of the “sweet spot” compared to the 8x42FL. Less lateral color and a bigger sweet spot are two more benefits that come from the higher objective focal ratio, because the less steep light cone allows the eyepiece to perform better off-axis. But, alas, edge of the field astigmatism is still this binocular’s weakest performance characteristic, just like the 8x42FL. The 7mm exit pupil also has a benefit in daylight. There is virtually complete freedom from “flare”. When bright reflections from the edge of the objective reach the eye they are out at the edge of a 7mm circle of light, so the flare tends to fall invisibly on the iris rather than entering the eye.
After experiencing the outstanding daylight image quality of this binocular for the last two weeks there is simply no turning back for me. Even if I look like a 5 year old struggling with his daddy’s big binoculars, the 8x56FL is what I will be using for birding until something better comes along. In spite of the optical quality I doubt that I will ever run into another birder using a pair. I can’t help but think that Zeiss missed an opportunity for making the 42mm FL’s just as optically superior in daylight, if only they had been willing to allow the 42’s to be about 1” longer and a little heavier. I understand that birders want their binoculars short and light, and that drives design decisions, but it also takes a toll on the optical quality.
