Intrigued by the prism roof-line "beam" phenomena of my Kowa 6.5x32s, just for fun () I took a number of my binoculars out several nights ago to look at some bright objects: Jupiter, Saturn, Mars, Vega, full Moon, and MW star clouds in Cygnus (admittedly not very bright in my skies). I viewed with my new eyeglasses which significantly reduced, but did not eliminate, spikiness of these objects naked eye.
This is a non-scientific comparison to say the least, the only constants in the "testing" being me and the sky conditions. But I have been purchasing binoculars for different purposes over the past two years and was curious to see if any other of my units exhibited the prism roof "beams" of the little Kowas or other aberrations on bright objects. In decreasing order of magnification with approximate retail price and exit pupil noted:
- Vortex Diamondback 15x56 ($350, EP 3.7 mm): bit of bloat and small amount of CA on Jupiter, smaller bloat on Saturn, very small spikes on Vega and Mars, slight CA on limb of moon, MW clouds easily visible. OK on planets, very good on the Moon. No roof "beams."
- Vortex Diamondback 12x50 ($250, EP 4.2 mm): bloat and small spikes on Jupiter, small bloat on Saturn, small spikes on Vega and Mars, moderate CA on limb of moon, MW clouds faintly visible. OK on planets and Moon. No roof "beams."
- Nikon Monarch HG 10x42 ($1,000 EP 4.2 mm): no bloat, spikes, or CA on any targets, good definition of MW star clouds. Aesthetically, gave the best overall views and perceived sharpness. No roof "beams."
- Nikon Monarch 5 8x56 ($520, EP 7.0 mm): small spikes on Jupiter, Saturn, Vega and Mars, just a trace of CA on the moon, good definition of MW star clouds. Very good overall on bright objects, spikiness perhaps due to large EP and my eyes. No roof "beams."
- Oberwerk 8x32 HD II ($200, EP 4.0 mm): minor bloat and small spikes on Jupiter, small spikes on Saturn and Mars, but curiously no spikiness on Vega, slight CA on the Moon, MW star clouds faintly visible. Good views on stars, not as good on brighter objects, perceived sharpness on moon worst of this particular set, but not bad overall. No roof "beams."
- Kowa BDII32-6.5XD ($350, EP 4.9 mm): small spikes on Jupiter, Saturn and Vega, very faint but detectable roof "beams" on Mars, prominent roof "beams" on the Moon but very good perceived sharpness, very good definition of MW star clouds with full moon at roughly the same elevation as Cygnus and about 90 degrees in azimuth away. No CA to speak of.
Well, I don't know if there's many cosmic conclusions to be drawn from these bright-object observations, except maybe that optically you get what you pay for (i.e., with the two Nikons) and that the Kowas generate these prism roof beams when an object reaches a certain brightness threshold. Despite the "beams" the Kowas do provide satisfying views and the perceived sharpness is very good, day or night. Also, the nearly complete absence of CA is a real plus of these binoculars. I have no idea whether this is due to the objective design (a triplet seems unlikely at this price point, but I'd love to know) or if it might be due to the lower magnification. Either way, this is icing on the cake for a lightweight, well-built, very wide FOV binocular.