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Eyepiece efficiency

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#1 Jeffmar

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Posted 29 November 2016 - 06:21 PM

I just bought a pair of inexpensive High Point brand 25mm eyepieces with a 52 degree apparent field of view to put on my binoviewers. The reason I go them was the advertised 22mm eye relief and the price. They claim that these eyepieces are fully multicoated and I can see a very obvious deep green tint. They also have only 4 lenses. I also have some higher end eyepieces that have up to twice as many lenses per eyepiece. How efficient are eyepieces now days. If each lens is 99% efficient then my cheap eyepieces only lose 4% of the available light. My Pentax eyepiece would loose 7% if it has 7 lenses and almost 8% if it has 8 lenses. That is just taking .99 to the n power where n is the number of lenses. If this is a topic that has been beaten to death could you guys let me know where and what the old topic is?

Thanks


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#2 jallbery

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Posted 29 November 2016 - 06:52 PM

Vision is logarithmic in its sensitivity to brightness.   You aren't going to to see the difference between 96% and 93% transmission.

 

I think the more important factor than the loss of transmission is that each additional element and surface can add scatter and stray reflections.   However, yhe manufacturers of those expensive multi-element eyepieces probably pay a lot more attention to controlling these problems.   That said, there is still an argument to be made for reducing elements, and there are people who appreciate high-quality simple eyepieces.  There is a reason why Televue still markets plossls (and it isn't simply price), while you can still buy quality orthos, and why Vernonscope still can sell Brandons.     Particularly for planetary (or other high-powered viewing), if you want to squeeze every last ounce of performance out of your scope, it's hard to beat a high-quality minimal glass eyepiece.

 

Unfortunately, short-focal-length four-element eyepiece designs typically come with very short eye-relief. 


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#3 MarkGregory

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Posted 29 November 2016 - 06:56 PM

Vision is logarithmic in its sensitivity to brightness.   You aren't going to to see the difference between 96% and 93% transmission.

 

I think the more important factor than the loss of transmission is that each additional element and surface can add scatter and stray reflections.   However, yhe manufacturers of those expensive multi-element eyepieces probably pay a lot more attention to controlling these problems.   That said, there is still an argument to be made for reducing elements, and there are people who appreciate high-quality simple eyepieces.  There is a reason why Televue still markets plossls (and it isn't simply price), while you can still buy quality orthos, and why Vernonscope still can sell Brandons.     Particularly for planetary (or other high-powered viewing), if you want to squeeze every last ounce of performance out of your scope, it's hard to beat a high-quality minimal glass eyepiece.

 

Unfortunately, short-focal-length four-element eyepiece designs typically come with very short eye-relief. 

Well said! Mark


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#4 The Ardent

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Posted 29 November 2016 - 07:00 PM

If you use a larger telescope you'll get the lost light back, and more.
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#5 Jeffmar

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Posted 29 November 2016 - 07:27 PM

I did know that sight and hearing are logarithmic, but I would still like to know the efficiency of lenses and coatings in eyepieces. I don't think I can ever get a big enough scope to be completely satisfied due to price, weight, and unreasonable expectations on my part. For now, at least, my 11" scope will just have to be enough. :grin:


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#6 Jon Isaacs

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Posted 29 November 2016 - 07:38 PM

Vision is logarithmic in its sensitivity to brightness.   You aren't going to to see the difference between 96% and 93% transmission.

 

 

Better eyepieces have better coatings so it is likely that even though the Plossl has fewer elements, the Pentax's have better transmission and less scatter...

 

Not all multicoatings are identical.  Better quality coatings are matched to the specific glass so the can be more efficient.

 

Jon


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#7 Jeffmar

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Posted 29 November 2016 - 08:55 PM

 

Vision is logarithmic in its sensitivity to brightness.   You aren't going to to see the difference between 96% and 93% transmission.

 

 

Better eyepieces have better coatings so it is likely that even though the Plossl has fewer elements, the Pentax's have better transmission and less scatter...

 

Not all multicoatings are identical.  Better quality coatings are matched to the specific glass so the can be more efficient.

 

Jon

 

Good point Jon. I am making a big assumption that the coating of a Pentax eyepiece and my inexpensive Plossls are equivalent. Better optics are also going to produce a better image even if we are looking at a few percent fewer photons. There is a noticeable increase in brightness between my newer Pentax eyepieces and my old Televue radians. I would guess that coatings have improved in the last ten or fifteen years.


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#8 Starman1

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Posted 30 November 2016 - 01:10 AM

I just bought a pair of inexpensive High Point brand 25mm eyepieces with a 52 degree apparent field of view to put on my binoviewers. The reason I go them was the advertised 22mm eye relief and the price. They claim that these eyepieces are fully multicoated and I can see a very obvious deep green tint. They also have only 4 lenses. I also have some higher end eyepieces that have up to twice as many lenses per eyepiece. How efficient are eyepieces now days. If each lens is 99% efficient then my cheap eyepieces only lose 4% of the available light. My Pentax eyepiece would loose 7% if it has 7 lenses and almost 8% if it has 8 lenses. That is just taking .99 to the n power where n is the number of lenses. If this is a topic that has been beaten to death could you guys let me know where and what the old topic is?

Thanks

Decent multicoatings pass 99.5% at 550nm, and you calculate loss at about 0.995 at each air-to-glass surface and about 1% per inch thickness of glass.

A good Plossl should have a transmission in the 97.5% range and an Ethos at roughly 93%.  Some eyepiece tests show those figures are about right.

Pentax XWs have from 6 to 8 lenses depending on focal length, but Pentax says about 96% transmission at 550nm.

Assuming equal spectra of transmission, an eyepiece would have to be in the 80s to be noticeably dimmer, and some older eyepieces ARE in the 80s in terms of % transmission.

Figure 96% transmission at an air-to-glass interface for an uncoated lens, about 98-98.5% for MgF2 coatings, and about 99.5% for multicoatings.

A Nikon engineer told me they have camera lens coatings on some glass types that have a 99.95% transmission, but these coatings aren't used on every lens material, nor in

affordable astronomical optics.

 

Some other things that influence how one eyepiece may appear darker than another in the same scope:

--focal length.  Even 1mm difference in focal length makes a noticeable difference in brightness

--spectrum of transmission.  A "bluer" eyepiece, at night, will appear brighter.

--types of glass used.  Lanthanum glass controls many aberrations, but it may appear darker than other glasses due to its spectrum of transmission.

--spot size.  A smaller spot size would make the faintest stars more visible by not blurring the stars.  A sub-factor here is that less astigmatism may make fainter stars appear, too.

--multi-coating efficiency.  The number of air-to-glass surfaces can be quite large--some 4 element Konigs have 6 air-to-glass surfaces, for example--and if the multi-coating is a little less efficient, transmission can suffer.

--lens polish.  Having a superb wavefront after passage leads to better focus, more light concentration, and brighter images.

--apparent field.  On average, a wider field will seem to be brighter, even if it doesn't measure so.  This is probably a psychological phenomenon.


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#9 paradise

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Posted 30 November 2016 - 08:02 AM

You must forget the idea that the number of lenses are detrimental to the quality of the transmission, the most important is the quality of the lenses and their treatment.


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#10 Starman1

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Posted 30 November 2016 - 09:52 AM

Apparently, that is true, since the eyepieces with the best wavefront error in the Ciel et Espace tests (admittedly, they only tested widefields)

had 8 and 9 elements.


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#11 MrJones

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Posted 30 November 2016 - 10:27 AM

The number of lenses and quality of coatings are the most important determinants of transmission.

 

Note that it's 4% loss *per interface* for uncoated lenses, improved to about 1% with single coating and as good as 0.2% with multicoatings. This adds up with multi-element eyepieces.

 

I'd think anyone can take an ortho (BGO for me) vs. same FL 82 deg. ES/Meade/Luminos for example and be able to see a transmission difference.

 

Anyway here's a nice article with some algebra.

 

http://www.canonrumo...-lens-coatings/


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#12 Starman1

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Posted 30 November 2016 - 02:01 PM

Ortho with best coatings: 98.0% transmission

6 element, 4 group UWA eyepiece: 96.0%

8 element, 5 group UWA eyepiece: 95.1%

All those with identical coatings.

 

Sorry, but if you see a difference it isn't due to the element count.  That difference is below the threshold of visibility.

 

BUT, the larger lens count eyepiece COULD have inferior coatings, a poorer wavefront accuracy, a difference in lateral field astigmatism, 

poorer lens polish, a less transparent glass type, or a severe roll-off in the blue end of the spectrum.  None of those is the case by necessity

as there are some multi-element eyepieces that do exceptionally well for high definition planetary and lunar viewing.

But it could account for a visible difference.

 

With equal coatings, though, you won't see a difference in brightness.


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#13 BLACKDRAGON

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Posted 30 November 2016 - 04:15 PM

I think that there's another factor that's not been mentioned during all the efficiency figures of glass etc and that is the limitations of our own eyes.   I doubt if the greater percentage of us have 20-20 vision and can appreciate the advanced  lenses and coatings to the full.  I know we all strive to get the most desired, efficient, top quality, light grabbing, aberration free, sharp image ep's that our wallets can afford, [although we sometimes ignore that. :lol: ], to give us that assurance and confidence that we are squeezing the most light and quality of image that we can so ensuring we are seeing the best images in viewing.   But, knowing that my own eyes have their own limitations I content myself with knowing that the ep's I have are the best I can afford, are probably more efficient at transmission than than my eyes are, and therefore I'm not able to appreciate the total high level qualities of my ep's..   So I peer through my scope completely ignoring the light transmission specifications, coatings quality, glass type, wave lengths error, etc, because I've got what I believe to be the best ep's for my use and then just wander through the Stars and Nebula with confident enjoyment. :shrug:   


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#14 Jeff Morgan

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Posted 30 November 2016 - 04:34 PM

You must forget the idea that the number of lenses are detrimental to the quality of the transmission, the most important is the quality of the lenses and their treatment.

 

When the 13 Ethos first came out my expectation (bias) was that it would not be as good (brightness/contrast) than my six element Nagler T4's (12 and 17).

 

But it was. A surprising result, but couldn't deny it. I sold the T4's, though I often question that decision.

 

Currently my "high count" eyepiece is the Leica ASPH (eight elements), and it is also surprisingly good. That was definitely one of the factors in thinning my planetary eyepiece collection. Only kept a 10mm Supermono.


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#15 MrJones

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Posted 30 November 2016 - 04:47 PM

"Sorry, but if you see a difference it isn't due to the element count."

 

Sorry, but wrong. The base determinants of lens/eyepiece transmission are number of elements and number of and quality of coatings. I have no doubt I could easily prove this statistically. Certainly other factors including defects can affect and even dominate transmission but these are the base ones. Take for example an uncoated 4 element lens vs. uncoated 16 element one assuming equal optical quality.

 

Anyway, this is still a fun article. Compare the Meade 6.7 UWA and Nagler 7mmT6. :blush:

 

http://www.amateuras.../tips/tips3.htm


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#16 Starman1

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Posted 30 November 2016 - 05:00 PM

"Sorry, but if you see a difference it isn't due to the element count."

 

Sorry, but wrong. The base determinants of lens/eyepiece transmission are number of elements and number of and quality of coatings. I have no doubt I could easily prove this statistically. Certainly other factors including defects can affect and even dominate transmission but these are the base ones. Take for example an uncoated 4 element lens vs. uncoated 16 element one assuming equal optical quality.

 

Anyway, this is still a fun article. Compare the Meade 6.7 UWA and Nagler 7mmT6. :blush:

 

http://www.amateuras.../tips/tips3.htm

I didn't say there wasn't a measurable difference, merely that the difference is not visible.

A 10% difference is 0.1 magnitude, less than the nightly fluctuations in the sky (I typically measure 0.2-0.4 magnitudes of difference in the sky

over the night), and we allow a 30% light loss at the edge of the field in a reflector because hardly anyone can see it.

The AAVSO says their best reporters can *just* see a difference of 0.1 magnitudes when the stars are side-by-side, but most of their contributors do a lot poorer than that.

If you see a visible difference in brightness between two eyepieces of exactly the same focal length (and that is almost never the case in anecdotal reports), IF it is due to a transmission

difference, then perhaps the eyepieces have radically different types of coatings (like one is uncoated, the other multi-coated).

But I stand by my statement: If you see a difference in brightness of the stars in the field, that difference isn't due to element count.

I would not argue that you cannot see any differences in brightness, either.

But I will clarify the statement by saying that if the multi-element eyepiece has low end or absent coatings, while the simpler eyepiece has state-of-the-art coatings, then

you MIGHT see a difference in brightness.

 

But if you perceive a difference in brightness, and the focal lengths between the compared eyepieces are EXACTLY the same (if they aren't, all bets are off), then there are 

perhaps many factors at work, and transmission is one of the lesser ones.

 

BTW, that Meade Series 4000 6.7UWA in that test had some uncoated interior surfaces, which explains its result.


Edited by Starman1, 30 November 2016 - 05:01 PM.

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#17 The Ardent

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Posted 30 November 2016 - 06:01 PM

you can achieve 99.9% transmission by removing all the lenses and just use an empty barrel. Your observing will be as superior as your statistics.


"Sorry, but if you see a difference it isn't due to the element count."

Sorry, but wrong. The base determinants of lens/eyepiece transmission are number of elements and number of and quality of coatings. I have no doubt I could easily prove this statistically. Certainly other factors including defects can affect and even dominate transmission but these are the base ones. Take for example an uncoated 4 element lens vs. uncoated 16 element one assuming equal optical quality.

Anyway, this is still a fun article. Compare the Meade 6.7 UWA and Nagler 7mmT6. :blush:

http://www.amateuras.../tips/tips3.htm


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#18 MarkGregory

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Posted 30 November 2016 - 06:24 PM

The number of lenses and quality of coatings are the most important determinants of transmission.

 

Note that it's 4% loss *per interface* for uncoated lenses, improved to about 1% with single coating and as good as 0.2% with multicoatings. This adds up with multi-element eyepieces.

 

I'd think anyone can take an ortho (BGO for me) vs. same FL 82 deg. ES/Meade/Luminos for example and be able to see a transmission difference.

 

Anyway here's a nice article with some algebra.

 

http://www.canonrumo...-lens-coatings/

Great article, thanks for sharing.  Mark


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#19 Jon Isaacs

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Posted 30 November 2016 - 07:34 PM

People do measure the transmission of eyepieces. A  I recall, the type 6 Naglers were about 96%, the Zeiss orthos were close to 98%. I'm on my phone, maybe someone can look it up...

 

Jon


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#20 Paul G

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Posted 30 November 2016 - 08:10 PM

People do measure the transmission of eyepieces. A  I recall, the type 6 Naglers were about 96%, the Zeiss orthos were close to 98%. I'm on my phone, maybe someone can look it up...

 

Jon

http://www.amateuras.../tips/tips3.htm

 

These measurements were made by a company that does that for a living, with equipment specifically made for that purpose, equipment that is regularly calibrated and accuracy regularly certified by the German government.


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#21 starcanoe

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Posted 30 November 2016 - 09:30 PM

Seems like one issue is transmission vs scatter. I'd happily loose a few percent more light IF the trade off was less scatter...particularly if lunar/planetary was my thing.

 

You get those high transmission numbers by using fancy multi layer AR coatings vs just a single (or a couple) layer AR coatings.

 

In practice do multilayer AR coating scatter the same or less than single AR coatings?



#22 Allan Wade

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Posted 01 December 2016 - 12:14 AM

Modern coatings are so good, I don't get too fussed about transmission levels between wide fields and minimum glass eyepiece's. But reportadly there is a difference to be seen in very large scopes. Where an eyepiece like a good Abbe Ortho comes into its own is scatter control. The difference compared to an Ethos or Delos is obvious. 

 

I often use my orthos when trying to see something faint near a bright target, like a faint moon around a planet, or a galaxy near a bright star. The faint object sometimes won't cut through the scatter in the wide field, but the clean view in the ortho usually gets the job done.


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#23 Shneor

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Posted 01 December 2016 - 02:26 AM

Apparently, that is true, since the eyepieces with the best wavefront error in the Ciel et Espace tests (admittedly, they only tested widefields)

had 8 and 9 elements.

In other words, better polish, making for smoother lens surfaces.


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#24 Starman1

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Posted 01 December 2016 - 02:46 AM

exactly.


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#25 marcus_z

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Posted 01 December 2016 - 05:19 AM

It is contrast what you can see and that's also what you need. You can not see brightness. I can remember forgetting an ND filter in my 2"->1.25" adapter when swapping from lunar to deep sky. It took me a long time to notice!
If you want contrast, then go to completely dark locations, otherwise straylight will deteriorate your contrast more than every lens count can do. Also note that large eye lenses, typically for wide angle EP, let more straylight in.
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