Why aren't EP's rated for transmission like camera lenses?

[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' :D 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

[Don Pensack]

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/InstrumentTest/201306__6_oculaires_10mm.pdf
http://www.cieletespace.fr/files/InstrumentTest/201102_test_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.

[Stonius]

Don, you legend! Thanks for such a comprehensive and thorough reply. Makes total sense! :-)

I know manufacturer's specs are rarely accurate, but there are independent grass-roots comparator sites for camera lenses, such as this one (http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx).

I wonder if there's anything like that for EP's? Or are the lab tests for camera lenses more affordable/widely available than for eyepieces? I guess one difficulty is that any eyepiece is going to be working from the light off an objective lens/mirror which would either have to be optically perfect to begin with, or standardised in it's imperfection across multiple repeatable tests in different locations.

Hmm, interesting to consider, anyway :-)

-Markus

[barx1963]

Markus
The thing to remember is that a camera lense is a complete system, an eyepiece is only part of a system, it only works when attached to a telescope. Also by definition, a camera lense is only used for imaging, while an eyepiece is (normally) only for visual use. Camera sensor accumulate signal while eyes don't so "speed" is more important in a camera lense than in an eyepiece.
Speed is more important in terms of the telescope being used. There is sometimes confusion about the relevance of the speed of a scope to a visual observer. For an imager, speed is a relevant factor as a faster scope will give you a brighter image. For a visual observer, it is less important, although a "faster" can give benefits in terms of portability, not having to climb a ladder etc. As a visual observer I am more interested in the raw numbers, aperture and focal length as that gives me an idea how bright(determined by the aperture) and large it is (determined by the focal length) for a particular eyepiece. Speed in terms of f ratio is a handy shorthand for me if combined with the size of the scope. For example if I see 20" f5, I know my 17mm eyepiece will give me 150x and that's what I want to know.
With zooms, I have never used one but there are people who do and love them. As with camera lenses it is very hard (impossible? maybe or at least very expensive!) to make a zoom that has no aberrations across its entire range. They exist for the same reason that zoom lenses exist, the convenience outweighs any issues.
Malcolm

[Don Pensack]

The equipment to test things like wavefront accuracy at different f/ratios is expensive, i.e. not common.
And the low profit margins don't really allow for such equipment to be easily affordable.
So lab tests for eyepieces are quite rare.
Many of the manufacturers have such data, but are not going to release it.
Imagine: Lab T tests 13 samples of an eyepiece and they vary from 1/133 wave error on the wavefront to 1/65 wave. Next thing you know, customers are requesting the 1/133 version, not the 1/65 version and the manufacturer is either forced to measure every eyepiece and sell the poorer ones for a discount (incurring huge cost increases thereby) or to ignore the test lab tests and lose sales to another brand that had one sample at 1/135 wave error.
Even with camera lenses, most readers of reviews skip the data (which they don't understand) and go straight to the conclusions. And if a lab says brand X is better than Brand Y, it can kill the sales of Brand Y.
And look at the data in the two comparative test reports I linked. If you understand the data, note that the worst eyepiece tested among all of them was better than the best mirror.

The data we really need to know is:
--field curvature from center to edge, in diopters
--separation of sagittal and tangential focal planes (i.e. astigmatism)
--spot size on axis, half way out, and at the edge
--measured apparent field versus claims
--measured eye relief versus claims
--spectrum of transmission and percentages.
--lowest f/ratio maintaining design correction at the edge (i.e. Critical f/ratio).

I don't expect to see that data in my lifetime. So far, only Pentax has posted some of that data for their eyepieces, and no one else.

So, read the forums to get an impression of the eyepiece you might want to buy. Right now, that's about as scientific as it gets.
Would that we'd have more tests like the Ciel et Espace ones, eh?

[N1]

I do find the "speed" of the telescope helpful in determining the image brightness for a given eyepiece. The same f ratio will give the same exit pupil size for different instruments of the same design (and obstruction in percent) regardless of aperture.

[Wavytone]

What I'd like to see is

1) the field curvature of the eyepiece (+/- and radius), this is arguably the #1 problem with mismatched eyepieces and telescope;
2) the longitudinal spherical aberration on-axis (the cause of the "kidney bean" effect), annoying in some eyepieces (Televue come to mind);
3) distortion (%), also annoying in some eyepieces, notably RKE's;
4) 3rd order coma and astigmatism values, for example the Konig eyepieces are real shockers, full of seagulls at the edges;
5) lateral chromatic error at some typical off-axis angles, say 25, 30, 40 degrees.
6) Actual field of view across the field stop in mm (frequently less than manufacturers would have you believe).

All easily measured on an optical bench.

Transparency is not an issue thanks to modern multi-coatings.