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What's in an Eyepiece?

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#1 BarrySimon615

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Posted 05 March 2004 - 10:43 AM

I believe most of us also own telescopes so that means we all know a little something about eyepieces for telescopes. This carries over well in a discussion about eyepieces for binoculars as well.

In the "olden" days, pre-1980 or thereabouts; amateur astronomers had fewer choices in respect to eyepieces. If you had anything more than a less expensive small Japanese refractors with .965 eyepieces, your choices in 1.25" American standard were primarily -

Kellner - with a standard 40 to 45 degree field, a 3 element eyepiece which was and is a good all-around performer. Some ghosting, good edge correction and affordable. The Edmund RKE is a variation of the
Kellner.

Orthoscopic - a 4 element eyepiece with a 40 to 45 degree field, known for great image quality across the field. Usually found in short to moderate focal lengths and usually used for detailed viewing of the Moon and planets. More expensive.

Erfle - a 5 element type made popular in military optics where wide field was important. This type increased field size to about 70 degrees apparent. Edge of field correction is poor.

Symmetrical - a 4 element type containing 2 matched doublet pairs. It featured good corrections and long eye relief. With minor tweaking, primarily now two doublets of different focal lengths, this has become the plossl we know today.

Since the late 70's/early 80's, a variety of different eyepieces have made it to market - Koenigs, Panoptics, Naglers, etc., etc.

In binoculars, our eyepiece types in popular priced binoculars fit pretty much into the basic types that have been around for a long time. Your typical standard field binocular (40 degree to 50 degree apparent) is a tried and true Kellner type, and your wide field binocular (60 to 70 degrees apparent) is the old venerable Erfle type. This helps explain the typically less than "stellar" (pun
intended) performance at the edge of the field.

More expensive binoculars are generally more expensive for a number of reasons but one of the big reasons can be because they have better eyepiece pairs giving one better eye relief, and better corrections across the field including the edge, and a larger field.

In my recent testing of the 10x50 Nikon Action Extreme in comparison to other binoculars one simple fact was reillustrated. That is that it is easier to see right up to the field edge in a binocular with a standard apparent field and that the corrections are much better closer to the edge. The end result being that in practical use the
standard field binocular may have just as large a "practically seen" field as what you can see with a wide field model. Another variable in the equation is that the further out angularly your own vision goes, the less acute it is and the more likely that aberrations will be seen when looking thru optics. Sometimes we blame the binoculars
for a poor image near the field edge, yes the eyepieces are generally not as good near the field periphery, but so are our own eyes near their periphery. So a combination of factors results in poorer performance away from field center.

With the very modest standard angle 42 degree apparent Carton Adlerblick 7x50 with a 6 degree true field, I could clearly and sharply see Orion from the belt star most distant from the great nebula M-42 to the last bright star in the scabbard. This was no less a field than what the 10x50 Extremes gave me with their 65 degree apparent field and 6.5 degree true field as that binocular is not sharp up to as close to the field edge like the Adlerblick. Of course the image scale was larger at 10x power, but the edge of field was cleaner and sharper in the 7x50 pair. Additionally there was less noticeable image shake at 7x than 10x, and because there was a natural decrease in the size of my exit pupil under suburban skies, the background did not seem brighter via the 7 mm exit pupil Adlerblicks than it did in the 5 mm exit pupil Nikon Extremes. (My eyes exit pupil determined the sky darkness, not the binoculars.)

This begs the question - Which do I like better? That is a question I still can not answer fully. I like them both. The standard angle Adlerblicks with crisp, sharp views always continue to impress. (The Fujinon FMT-SX 7x50 with a wider 52.5 degree apparent/7.5 degree true field impress me even more.) However the larger image scale and larger apparent field of wide field model 10x50's with all their
faults - eyepieces with poorer edge correction and a lesser ability for steady views as the power magnifies shake, impress me too.

My advise would be for any prospective buyer to not overlook a standard angle model as eyepieces here typically will give you better edge correction and your own eyes will let you actually see more of the field. If the opportunity presents itself, check out both standard angle and wide angle binoculars side by side and let your eyes decide what eyepieces they like best.

Barry Simon


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Posted 05 March 2004 - 11:35 AM

This is a very interesting and informative post Barry! I think another thing that contributes to the lack of performance in some wide angle binoculars are poorly designed eyeguards. I got rid of the rubber eyeguards that came with my Oberwerk 15x70mm binoculars(they didn't allow me to get as close as I wanted and also slightly irritated my skin) and replaced them with the ones found on Orion's Sirius plossels. The view seems much clearer now and it's easier to see the field stop.

I'm planning on getting a high end pair of binoculars in a few months. I hadn't even considered using anything with an AFoV less than 60 degrees, but you've convinced me to compare the views in wide and conventional angled binoculars very carefully before I buy.

#3 KennyJ

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Posted 05 March 2004 - 12:37 PM

Very interesting post Barry,

There are exceptions of course, of which my Zeiss 7 x 42s are a classic ( pun intended ! ) example , but you have to pay a higher premium for better in both worlds.

After many years using a 10 x 50 with a 5 degree TFOV it was like a "different world" looking at it through the Zeiss

Unfortunately that particular 10 x 50 was such a mediocre model anyway that I could see no benefit from the narrower field whatsoever in terms of edge performance.

More recently when anyone has asked me for advice on a "good astronomy" binocular I have deliberately kept wide -field models out of the short -list.

But I think for example the Nikon Superior E with it's 60 degree AFOV and around 4mm exit -pupil is a very nice compromise all round.

Regards --Kenny.

#4 BarrySimon615

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Posted 05 March 2004 - 05:42 PM

No question about the Nikon Superior E series. Good edge performance with a relatively wide 60 degree apparent field. (Now if they could only offer that series in 8x42 and 10x50, I think they would sell a lot more units. A standard bracket socket and a focuser move to between the two body hinges would also be nice.)

The Fujinon FMT-SX 16x70 with very good edge performance and a 64 degree apparent field is also an example of a binocular that has a somewhat more sophisticated eyepiece design allowing for that great performance.

Barry Simon

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Posted 05 March 2004 - 06:21 PM

No question about the Nikon Superior E series. Good edge performance with a relatively wide 60 degree apparent field. (Now if they could only offer that series in 8x42 and 10x50, I think they would sell a lot more units. A standard bracket socket and a focuser move to between the two body hinges would also be nice.)

The Fujinon FMT-SX 16x70 with very good edge performance and a 64 degree apparent field is also an example of a binocular that has a somewhat more sophisticated eyepiece design allowing for that great performance.

Barry Simon


A very interesting and informative post on eyepieces. One thing I have noticed is that my microscope uses what I think are 2 element Huygenian eyepieces. The problem is that the top element focusses on one surface of the bottom element, and hence dirt and crud on the bottom element is clearly visible.

I do wonder why I can buy an 8x32 binocular with superb eyepieces for ~£440, and yet one 32x eyepiece for my scope costs ~£150. Maybe the scope eyepiece needs larger diameter lenses, or is harder to make due to the different focal length? The binocular eyepiece will have a FL of about 32x6/8 = 24 mm assuming an F6 objective. The scope eyepiece will have a FL of about 440/32 ~ 13mm.

I do hope they introduce an 8x42 SE though I suspect it would be too bulky for me to hold. I hope they don't move the focus wheel forwards. For me the whole beauty of the SE shape is the position of the focus wheel that allows me to grip it between forefinger and thumb, and hence obtain a very precise degree of control, even when it gets stiff in cold weather!

Perhaps the major fault for me is the rubber eye cups. Modern twist up ones would be so much nicer for me, though not for Mr J.

#6 lighttrap

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Posted 06 March 2004 - 08:41 AM

Barry Simon wrote: "(Now if they could only offer that series in 8x42 and 10x50, I think they would sell a lot more units...)


Nikon's optical engineers apparently believe there is no point to doing this, other than to appeal to a buying market that insists on certain sizes, rather than performance standards. They reportedly believe that their 8x32 SE and 10x42 SE perform to the limits of their respective magnifications, and would not bennefit from having larger objectives. In other words, what they've done is to give us the performance of 8x42s and 10x50s in smaller packages.

I was initially, VERY skeptical of this approach. However, in real world resolution testing with the 8x32 SE and 12x50SE, I find that they do in fact outperform a variety of 8x42s and 12x60s. To date, I've not found any 12x binoculars that can outperform the 12x50SEs regardless of objective size. The only thing that keeps me from saying that about the 8x32SEs is that most of their really serious competition ups the magnification to 8.5x such as the excellent Swarovski 8.5x42 ELs. That particular comparison amuses me because it essentially takes a binocular costing 2.5x as much, and using a slightly higher magnification and objective size to really equal the Nikon 8x32SEs for brightness and color correction. Not to mention that the ELs weigh more than the SEs. I certainly couldn't see any notable improvement when going from the 8x32SEs to the B&L Elite 8x42s, or Leica BN 8x42s. Leica BNs have some weird distortion artifacts, and some chromatic abberations that are totally lacking in the smaller, lighter, cheaper SEs. In fact, the Pentax DCF WP 8x42s were actually MUCH WORSE both in resolution, contrast and edge sharpness and brightness than the smaller Nikon 8x32SE porros. Granted, these comparisons are comparing a really, really great porro prism binocular to some popular roof prism binos. But, it was enough to make a believer out of this skeptic.

The thing about the SEs is that they are incredibly bright for their sizes. I've read critics that claim that the SEs actually beat the theoretical limits of resolution, but can't find any instance of Nikon actually claiming that feat. I've not tried the 10x42s, but am finally, totally convinced that the 8x32s and 12x50s would not bennefit from larger, heavier objectives.

Zeiss takes a similar approach to the 15x size, by putting forth a 15x60 instead of the traditional 15x70.

I don't know how Nikon gets a completely clear to the edge 5* FOV out of their 12x50SEs, but whatever they're doing works, very, very well for me.

Mike Swaim



#7 BarrySimon615

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Posted 06 March 2004 - 10:20 AM

Mike,

Yes, I totally agree that Nikon is able to wring amazing performance out of their whole Superior E series with the 10x42 equaling or exceeding performance you will find with 10x50 binoculars and the 12x50 doing the same head to head against 12x60.

That being the case, what is wrong with the logic of expecting great results too with 8x42 Superior E's and 10x50 Superior E's? A 5 mm exit pupil is still a 5 mm exit pupil (5.25 mm in the case of 8x42). The 60 degree apparent field that the series shares would mean a true field going from 6 degrees in the 10x42 to 7.5 degrees in 8x42 if the 60 degree apparent field could be maintained. I think many people would find that very appealing. In addition the reduction in magnification for both objective sizes would result in shake being less noticeable. I think many have not jumped on the 12x50's simply because you are getting to a magnification which makes hand holdability impractical for many binocular users.

If I saw an ad for Nikon today offering either an 8x42 Superior E or a 10x50 Superior E, I would be on the phone placing an order within 10 minutes. My only dilemma would be deciding which one I would want more.

Barry Simon

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Posted 06 March 2004 - 11:36 AM

I suspect that what Mike Swaim is saying is that during daylight hours an 8x42 SE would offer no noticeable advantage over the 8x32 in terms of image quality. Where it certainly would gain is in low light, for Owl watching for example, but maybe this is a niche market?

In the UK the SEs have suffered from IMO an ill-informed birding press who are incapable of objectively testing optics.

I see quite a few people carrying 8x50 and 10x50 Leica bins for birding during daylight. By the above arguments these should offer no advantage over an 8x42. I have never tried such bohemoths. Perhaps they do offer an advantage e.g. more contrast, or maybe these are people who buy just one pair of bins, and buy them big for end of day viewing?

#9 sftonkin

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Posted 06 March 2004 - 06:22 PM

Barry Simon wrote:
>I believe most of us also own telescopes so that means we all know a
>little something about eyepieces for telescopes. This carries over
>well in a discussion about eyepieces for binoculars as well.

Apparently the big Miyauchis have a "reversed Koerner" eyepiece. I've never heard of this, it
doesn't seem to be in any of my optics books, and I can't find anything about it on the web
(except that it's used in Miyauchis!). Anyone here know what the lens configuration is?


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