Quote:
Originally Posted by Stonius
Seems an obvious question to me. By way of analogy, some camera lenses are faster than others, and in an application where a faster eyepiece would mean coaxing dimmer objects into visibility you would have thought it was an important spec.
Similarly, with camera lenses the best quality in terms of resolution and contrast comes from primes, not zooms. So I'm curious as to why zoom eyepieces are a thing?
Not 'casting Nasturtiums'  at people who like them, just wondering why these things that are true for camera lenses don't seem to apply to EP'S?
And in any case, for things like the speed of an EP, or contrast, resolution, even the shape of the focal plane as pointed out by Alexander - these things are surely objectively measurable in the same way that camera lenses can be tested?
And lastly, why are EP'S with more elements considered better? If each air/glass boundary represents possible light loss isn't that a liability? And given that each lens is built to a tolerance, shouldn't more lenses lead to more variability in the quality of that design?
Not wanting to ruffle feathers here, just curious as to why things that apply to camera lenses dont apply to EP's and why this stuff doesn't form part of the spec sheets.
Markus |
Eyepieces are not rated by transmission because the eye can barely detect, in a lab, a difference in brightness of 10% (0.1 magnitude) and most eyepieces easily fall in that 10% range.
Zoom eyepieces ARE, by and large, poorer in resolution etc. than fixed power eyepieces, but they're convenient. Many people are willing to trade optimum performance for convenience.
Eyepieces can be measured for the things you mention (except, perhaps, contrast, which is a synergy of many characteristics operating together) but there are no test labs with the expensive equipment to test them and the manufacturers won't release the data for fear of losing sales. The camera market is many many times larger than the astronomy market. Astronomy equipment is made in small lots (like 300 pcs), and there isn't enough margin in the product for extensive testing.
More elements aren't necessarily better, though it does take more elements to control aberrations at the edge of the field in a wide angle eyepiece used at fast focal ratios. In a long f/ratio scope, with a narrow field eyepiece, some low element count eyepieces can function quite well. Make the field wider or the scope shorter, though, and you'll be adding elements to control aberrations.
With high-transmission multi-coatings, adding a couple more air-to-glass surfaces won't affect the brightness of the image. Some of the best eyepieces out there have very high transmission with lots of air-to-glass surfaces internally.
Here are a couple eyepiece tests with numbers from the lab:
http://www.cieletespace.fr/files/Ins...aires_10mm.pdf
http://www.cieletespace.fr/files/Ins..._oculaires.pdf
Note that the eyepieces with the lowest wavefront aberrations were eyepieces with a high lens count. The point is that lens count isn't critical. Optical quality (including design) is.