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Olivier Biot
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Comparison of 24mm-26mm Eyepieces
      #5076852 - 02/17/12 11:36 AM

Comparison of 24mm-26mm Eyepieces

By William Paolini.

Edited by jrcrilly (02/17/12 11:40 AM)


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dyslexic nam
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Olivier Biot]
      #5076853 - 02/17/12 11:38 AM

Yay

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jrbarnett
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Loc: Petaluma, CA
Re: Comparison of 24mm-26mm Eyepieces new [Re: dyslexic nam]
      #5076928 - 02/17/12 12:16 PM

An absolute gold mine of interesting data.

A few initial observations:

1. Not all "orthos" are created equal.

2. Televue knows edge correction.

3. On a budget? Think UO Abbe Ortho or Sterling Plossl.

4. And, most importantly, Brandon FTW!

Regards,

Jim


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Doug Culbertson
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Reged: 01/06/05

Re: Comparison of 24mm-26mm Eyepieces new [Re: jrbarnett]
      #5076930 - 02/17/12 12:18 PM

Wow, what an amazingly well done comparison. I must admit that there were some surprises, along with some expected results. A thoroughly enjoyable read!

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WarrenM
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Re: Comparison of 24mm-26mm Eyepieces new [Re: dyslexic nam]
      #5076958 - 02/17/12 12:32 PM

Wow! Now thats what I call a well done comprehensive review. Well done William!

Warren


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Sean Cunneen
Let Me Think
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Re: Comparison of 24mm-26mm Eyepieces new [Re: WarrenM]
      #5076966 - 02/17/12 12:37 PM

Fantastic! A round of applause!

now I am off to Google Sterling...
Sean


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BDS316
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Sean Cunneen]
      #5076984 - 02/17/12 12:48 PM

One take-home message is that you don't have to spend a lot of money to get a great eyepiece. This point was made with STERLING clarity.

Would have been interesting to have seen how well a generic Asian plossl, such as a Synta or GSO would have done...


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Jason B
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Sean Cunneen]
      #5076987 - 02/17/12 12:49 PM

Very well done. The wealth of information is staggering.

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coutleef
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Jason B]
      #5077001 - 02/17/12 12:56 PM

Well done Bill. We were waiting for that review for a while and it was definitely worth waiting for.

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Scott99
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Re: Comparison of 24mm-26mm Eyepieces new [Re: coutleef]
      #5077143 - 02/17/12 02:02 PM

great job Bill! That's a lot of work and it's much appreciated.

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Starman1
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Scott99]
      #5077246 - 02/17/12 03:11 PM

This review is exhaustive and much attempt is made to make comparisons valid.

There are a few quibbles I have with his test procedures however:

1) Comparing a 68 degree eyepiece's edge correction with a 50 degree eyepiece's edge correction would seem to be valid, given that the widefield user will be looking for definition at or near the edge of the field just like the narrower field user. However, if we assume that the edge correction of a 50 degree eyepiece is less than perfect at the edge, while a 68 degree eyepiece is perfect at the same 50 degree point in the field, but deteriorates more at the edge, then which eyepiece actually has the better correction? To give a valid comparison, I would want to see the correction obtained at 20, 25, 30, and 34 degrees off axis for each. It could be the widefield eyepiece yields a better-corrected 50 degree field than the 50 degree eyepiece. If so, the edge correction would have to be "field size compensated" to yield a relative ranking.

2) Bill seems to have some confusion about rectilinear distortion and angular magnification distortion and which aberration causes what in the image. Perhaps a good way to think about these two distortions is to picture map projections. If you reduce a globe to a flat representation, then there will be geometric distortions in the map, worst at the edges. We see this all the time in standard flat maps of the world. This is similar to what we would see as rectilinear (geometric) distortion.
If the globe itself is presented and rotated, a country will come around the edge of the globe, come toward you, and move away from you, changing size as it goes, but it will not be geometrically distorted. yet, it will appear to change in size.
Case one represents rectilinear distortion, while case two represents (it's an inexact analogy, I admit)) angular magnification distortion.

Would we want the planet to appear smaller at the edge of the field than the center? Probably not, though this wouldn't seem to be an issue in a driven scope with the planet's image held in the center. So, in the undriven scope, angular magnification distortion would not appear to be a desirable trait, especially in star fields or for viewing double stars.

But, the wider the field, the more the correction of one aberration leads to more of the other, so an eyepiece corrected for AMD will, necessarily, have RD. Since our eyes tend to have a little barrel distortion (positive RD)(it's a brain thing) when viewing the edge of a field, some amount of negative RD (pincushion) seems to result in an undistorted image to our eyes.
If there is too much RD, the object will appear to stretch and change shape as it nears the edge, similar to that Mercator map projection we talked about.

So what's ideal?
Well, in an undriven scope, a low level of AMD for sure. And for the least distortion visible to our eyes in images, a certain amount of RD. Not surprisingly, many good eyepieces are designed this way.

But what about the person who pans across the field? Well, if there is AMD in any significant amount it would appear as if the image is rolling across the surface of a globe. I have a kaleidoscope that does this--the edge appears farther away than the center because the object changes in size as it crosses the field and our brains tell us we are watching a globe rotating.

With a lot of RD and no AMD? Well, we'd see the object change its shape as it moves across the field, but the field could still appear flat. And if the RD amount is chosen to leave only a tiny bit of AMD in the field, we might see very little distortion in the field.

But, extend the field even more, to 82 degrees, or 90 or 100 or 110 degrees. The curves of solution for AMD and RD diverge even more. Having that *just right* about of RD in the field will result in noticeable AMD. So, a designer could solve for AMD and let RD be a larger amount. Given that most of these eyepieces are deep-sky eyepieces, that is probably the right choice. Used as terrestrial eyepieces, the edge of the field would appear quite distorted because we know how tree trunks and building edges are supposed to appear.
We might even tolerate that, though, if the eyepiece was free from astigmatism and the edge was sharp.

But it does indicate that the demands of designing the perfect astronomical eyepiece and perfect terrestrial eyepiece can be quite different.

I refer you to the discussions of RD and AMD which exist on the web, especially as they relate to binoculars and terrestrial usage.

3) Bill also missed the opportunity to discuss astigmatism and field curvature in the eyepieces. Field curvature's visibility might be related to the scope in which the eyepiece is used, but astigmatism is not. Since most of these eyepieces have narrow fields, where astigmatism should be tightly controlled, this could have been an illuminating comparison. I suppose some of that could be garnered from the comparison with Paracorr used, since, if coma and field curvature are more or less corrected, what's left is astigmatism [excluding chromatic aberrations].

All-in-all, a good step toward trying to be fair in comparisons. It was illuminating to see some highly-touted eyepieces come in down the list, and I applaud Bill's lack of that annoying drone about "tinted" eyepieces.


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mercedes_sl1970
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Starman1]
      #5077493 - 02/17/12 06:20 PM

An impressive and thoughtful review. Thank you!

Andrew


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KidOrion
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Re: Comparison of 24mm-26mm Eyepieces new [Re: mercedes_sl1970]
      #5077528 - 02/17/12 06:50 PM

I'm going to go give my $99 24mm SWA a hug.

Great work, Bill!

Edited by KidOrion (02/17/12 06:50 PM)


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Arizona-Ken
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Re: Comparison of 24mm-26mm Eyepieces new [Re: KidOrion]
      #5077613 - 02/17/12 07:57 PM

An outstanding review.

An excellent attempt at describing the subtle differences among very good eyepieces and enough detail to allow both beginners to understand the differences and also to allow one to understand why some eyepieces are a bit better for different kinds of objects than others.

Here at CN we get good people to provide great insights on equipment but this is an order of magnitude above most efforts.

Congratulations, Bill.

Arizona Ken


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Doc Bob
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Arizona-Ken]
      #5077878 - 02/17/12 11:28 PM

Bill,
Excellent review !! It helps me to now understand what is important when considering purchasing an eyepiece and that my set of Sterlings ain't half bad!!

Regards,
Bob


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Jim Rosenstock
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Olivier Biot]
      #5078066 - 02/18/12 05:34 AM

A dizzying amount of information that I've just skimmed.

I like the way the information is broken down and laid out. It's instructive how an eyepiece can become a personal favorite, and still perform poorly in categories that aren't a personal concern.

I'll be spending more time with this review....and wondering how it would extrapolate to other focal lengths. Anybody else feeling a 12mm shootout?











Jim


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Joe Bergeron
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Olivier Biot]
      #5078186 - 02/18/12 08:53 AM

What happened to that poor Brandon? It looks like someone sanded the anodized finish off the barrel and added some kind of crude label? Almost as jarring as that putting green atop the Meade.

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rguasto
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Jim Rosenstock]
      #5078194 - 02/18/12 09:01 AM

The 12mm shoot out results;
(read the 24mm shoot out) ...in conclusion all the 12mm EP's show twice the mag and 1/2 the TFOV


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csa/montana
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Olivier Biot]
      #5078232 - 02/18/12 09:43 AM

Bill, an absolutely amazing Review!

This review is the ultimate for both beginners and long-time viewers; as it's in terms easily understood.

We all owe you a huge thank you for the time & effort you put into this, to benefit all of us here on CN.

Thank you, Bill!


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waskeyc
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Olivier Biot]
      #5078238 - 02/18/12 09:45 AM

And Bill hits one out of the park!

Great job Bill, fantastic review with scads of useful information.


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Jeff Morgan
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Joe Bergeron]
      #5078266 - 02/18/12 10:04 AM

Quote:

What happened to that poor Brandon? It looks like someone sanded the anodized finish off the barrel and added some kind of crude label? Almost as jarring as that putting green atop the Meade.




I was wondering the same. Obviously not current production.


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russell23
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Jeff Morgan]
      #5078277 - 02/18/12 10:12 AM

I found Bill's comments about 24mm Panoptic fully in line with what I found with the 27mm Panoptic. There is no doubt the Panoptic is sharp right to the edge with very tight pinpoint stars. However, as Bill noted in comparing the 24mm Pan to the 24mm ES and Meade, it gives a dimmer view. I found the same with the 27mm Panoptic in the sense that I felt like I was loosing light compared to other eyepieces.

Dave


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russell23
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Jeff Morgan]
      #5078280 - 02/18/12 10:14 AM

Quote:

Quote:

What happened to that poor Brandon? It looks like someone sanded the anodized finish off the barrel and added some kind of crude label? Almost as jarring as that putting green atop the Meade.




Wasn't there some discussion early in the thread in "eyepieces" forum that the Brandon being used was modified in some way for a zoom eyepiece adapter of something?

Dave
I was wondering the same. Obviously not current production.




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rdandrea
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Olivier Biot]
      #5078365 - 02/18/12 11:14 AM

Excellent work. And a great example of why I keep coming back to Cloudy Nights each day.

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BillP
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Starman1]
      #5078515 - 02/18/12 01:12 PM

Quote:


1) Comparing a 68 degree eyepiece's edge correction with a 50 degree eyepiece's edge correction would seem to be valid, given that the widefield user will be looking for definition at or near the edge of the field just like the narrower field user. However, if we assume that the edge correction of a 50 degree eyepiece is less than perfect at the edge, while a 68 degree eyepiece is perfect at the same 50 degree point in the field, but deteriorates more at the edge, then which eyepiece actually has the better correction? ...

2) Bill seems to have some confusion about rectilinear distortion and angular magnification distortion and which aberration causes what in the image. ...


3) Bill also missed the opportunity to discuss astigmatism and field curvature in the eyepieces. Field curvature's visibility might be related to the scope in which the eyepiece is used, but astigmatism is not. ....

All-in-all, a good step toward trying to be fair in comparisons. It was illuminating to see some highly-touted eyepieces come in down the list, and I applaud Bill's lack of that annoying drone about "tinted" eyepieces.




Hi Don. Appreciate the comments...including the critical ones. For #1 you are correct, a 100 deg AFOV eyepiece that shows only 60% of its FOV sharp is indeed corrected better than a 50 deg AFOV eyepiece that shows 99% of its FOV sharp. So out to the same TFOV position the 100 deg EP is better. I chose not to evaluate the eyepieces like this because I feel it makes no sense from the human perception level and how much a user will be satisfied with their eyepiece. So from a practical standpoint, IMO most observers relate how they like or dislike their eyepiece's performance based on how well it is handling its FOV. Most people will not be very satisfied with a 100 degree EP that has only 50% of its FOV sharp, but will be satisfied very much with their 50 deg AFOV EP that is sharp to the edge. So IMO it is more important to judge this aspect of performance "relative" to whatever the FOV the EP shows. This is why I did it this way. Plus IMO it is more intuitive from the visual observation standpoint as most don't look at FOVs and register TFOV points. Anyway, this is the rationale for the approach I used.

For #2 you may well be correct. I have flipped on this aspect several times. I used to hold your contentions, but then people right here on CN argued it was incorrect and I moved to a different understanding. What is the correct one? I am not sure. I was actually reluctant to make assessments on what drivers were on distortions or aberrations as in the end, we must guess because we can't separate them to assess individually since other distortions and aberrations are present. What is important though, is when you observe with these did the targets distort their shape? Did they distort their positions and angles? In truth I think that we can never separate the causes and in the end the more correct position would be to say that the eyepiece contains a mix of RD and AM which distorts the target to a moderate level or minor level, etc. The observational impact is what is important, more than the exact cause, at least this is my position as a 100% visual observer. In any reviews I do in the future, I will probably combine these distortions as potential drivers for the distortions observed. It is edifying to let others know what could be the drivers, but is of no real value IMO trying to figure out if it was 80% AMD and 20% RD, etc.

For #3, I purposely left this out as observationally what is important is is my image clear and sharp. So I simply provided what I saw, a sharp or non sharp image. I did quality in the narratives and the charts if FC was the only driver since this we can fix with a turn of the focus knob. But in the end, I didn't really feel it was important for observs to have an assessment of the degree of astigmatism I saw as in the end what is important to know is was my off axis sharply rendered, and if not could I correct it, and how did this behavior change with different focal length scopes. So this is what I strove to capture, the practical results.

I will keep these things in mind for next comparisons.

Thx.


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Starman1
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Re: Comparison of 24mm-26mm Eyepieces new [Re: BillP]
      #5078888 - 02/18/12 05:04 PM

Bill,
As has been stated to me more than once, "distortion is distortion". It's probably clearer to identify what the distortion does than whether the distortion is RD or AMD.
Generally, I've found high RD amounts to be more deleterious in terrestrial viewing and high AMD amounts to be deleterious in astronomical.
The articles on the web discussing how the eye interacts with the eyepiece are illustrative of how the presence of some pincushion distortion (negative RD) in the eyepiece actually displays less noticeable distortion to the eye than zero RD. This apparently is due to the way the eye sees, which tends to show some barrel distortion (positive RD).
AMD is a difficult distortion to tolerate in an astronomical eyepiece. Picture that globular cluster getting smaller as it nears the edge of the field. Not good. And it certainly wouldn't be tolerable to a comet hunter, sweeping back and forth.
Which is why most astronomical eyepieces have very low AMD (the Ethos has less than 1% AMD anywhere in the field). But, like the Ethos, eyepieces with some RD do not seem to produce comments about how distorted the edge of the field is [though they would if used in a terrestrial scope].
But, I've become convinced that, while AMD is not a good thing for sweeping star fields, neither is the presence of too much RD. Both forms of distortion are just that--distortion. A designer can solve for one or the other. And it is also possible to solve for neither form of distortion in a widefield eyepiece design--a state of affairs we sometimes see in eyepieces.

Astigmatism is the bane of cheap widefields, though. When no amount of fiddling with focus will focus the edge stars, it's not a good sign (pun intended).
When widefield eyepieces are used without coma correction in short f/ratio telescopes, coma can overwhelm other issues with eyepiece field edges, so I'm glad you tested the eyepieces with and without. What I take away from your test is that, if you value edge performance and you have a short f/ratio telescope, you should use a coma corrector.

Al Nagler told me a story of how, after he developed the first Paracorr, he stuck his Widefield eyepieces in the Paracorr to discover, to his horror, that they had astigmatism at the edge of the field. That led to the development of the Panoptic line.
We tend to forget that this is a developing field and that it is easier to design today, using software programs and combining that with the knowledge gained from prior experience, to a higher standard that was available 30, 40, or 50 years ago.

As a subject for a future test, looking at "Plossl" eyepieces from a variety of eras might be an example of seeing how things have changed.
I owned several Claves but sold them in favor of some TeleVue Plossls because the TeleVues, with their superior coatings, produced fewer internal reflections (I didn't notice any sharpness differences). I then sold the TeleVues to replace them with the Meade Series 4000 5-element Japanese-made eyepieces because the edge of the field was sharper and better illuminated to my eye.
Now, that last one wasn't a real Plossl, but you get my drift--eyepieces designed and produced 50 years ago might not be as good as eyepieces produced since, even if the internal design is nearly the same.

And then don't get me started about eyepiece-to-eyepiece variation in the same Brand/Model/focal length, which I've seen. Have you ever looked at an object near the edge of the field and rotated the eyepiece to see if you can see changes in the object as the eyepiece rotates? The differences may be subtle, and usually are if the scope is collimated, but they're there. We also tend to forget these are man-made products, and none is absolutely perfect. A ranking of model A over model B might switch with different samples of the same eyepiece.

Fortunately, your test tried to avoid such tiny distinctions, and I applaud you for not simply giving a 1-2-3-4 ranking on the whole lot.

You obviously took your task seriously and spent a lot of time on it. It was interesting to me to see a confirmation of several "off-the-cuff" reactions I've had to some of these eyepieces.

Next test, try the "bright star just outside the field" test. I do this when I test binoculars and it's amazing how few pass. With some eyepieces, you can not only tell there is a star outside the FOV, but also in which direction it lies. With the best eyepieces, though, when the star leaves the field of view it ceases to exist. This is one characteristic that seems to divide the top eyepieces from the run-of-the-mill.

Thanks for your test, and your further explanations.


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BillP
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Starman1]
      #5078920 - 02/18/12 05:34 PM

Quote:

Next test, try the "bright star just outside the field" test. I do this when I test binoculars and it's amazing how few pass. With some eyepieces, you can not only tell there is a star outside the FOV, but also in which direction it lies. With the best eyepieces, though, when the star leaves the field of view it ceases to exist. This is one characteristic that seems to divide the top eyepieces from the run-of-the-mill.

Thanks for your test, and your further explanations.




Actually I did do that, but I used Jupiter (Light Control section). Celestron Silvertop was the ONLY one of the bunch that showed no hint of anything. I was shocked.


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danielgolite
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Re: Comparison of 24mm-26mm Eyepieces new [Re: BillP]
      #5078940 - 02/18/12 05:49 PM

Bill,

Just also want to applaud and thank you for your work on this review. (Wow!) Your posts concerning eyepieces have always been good reading. My first 1.25 inch eyepiece was none other than a 26mm Celestron silvertop. I still have it and it was certainly great to see it included in your review even though it is long, long, out of production.


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Jason Martin
super member


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Re: Comparison of 24mm-26mm Eyepieces new [Re: danielgolite]
      #5079131 - 02/18/12 08:05 PM

Bill,

Slightly off topic, but do you think the other focal lengths of Sterlings that you have used live up to the standard set by the 25mm?


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daniel_h
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Jason Martin]
      #5079225 - 02/18/12 09:41 PM

Well done Bill, it's a quality read, & importantly something you can view many times

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BillP
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Jason Martin]
      #5079287 - 02/18/12 10:51 PM

Quote:

Bill,

Slightly off topic, but do you think the other focal lengths of Sterlings that you have used live up to the standard set by the 25mm?




I don't see why they would not. I have them all, except the 4mm. The 4mm and 6mm I have been disappointed with as they do not seem very sharp, and of course the ER is horrible. But the 25mm-12.5mm units are quite nice. The 40mm and 30mm are good also, except they are quite heavy.


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desertlens
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Olivier Biot]
      #5079494 - 02/19/12 01:48 AM

Thanks Bill for the work that was clearly involved here... It may take some time to absorb it all.

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MADRID SKY
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Re: Comparison of 24mm-26mm Eyepieces new [Re: csa/montana]
      #5080037 - 02/19/12 11:52 AM

Quote:

Bill, an absolutely amazing Review!

This review is the ultimate for both beginners and long-time viewers; as it's in terms easily understood.

We all owe you a huge thank you for the time & effort you put into this, to benefit all of us here on CN.

Thank you, Bill!




Agreed.

Kudos!


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russell23
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Re: Comparison of 24mm-26mm Eyepieces new [Re: BillP]
      #5080071 - 02/19/12 12:09 PM

Quote:

Quote:


1) Comparing a 68 degree eyepiece's edge correction with a 50 degree eyepiece's edge correction would seem to be valid, given that the widefield user will be looking for definition at or near the edge of the field just like the narrower field user. However, if we assume that the edge correction of a 50 degree eyepiece is less than perfect at the edge, while a 68 degree eyepiece is perfect at the same 50 degree point in the field, but deteriorates more at the edge, then which eyepiece actually has the better correction? ...

2) Bill seems to have some confusion about rectilinear distortion and angular magnification distortion and which aberration causes what in the image. ...


3) Bill also missed the opportunity to discuss astigmatism and field curvature in the eyepieces. Field curvature's visibility might be related to the scope in which the eyepiece is used, but astigmatism is not. ....

All-in-all, a good step toward trying to be fair in comparisons. It was illuminating to see some highly-touted eyepieces come in down the list, and I applaud Bill's lack of that annoying drone about "tinted" eyepieces.




Hi Don. Appreciate the comments...including the critical ones. For #1 you are correct, a 100 deg AFOV eyepiece that shows only 60% of its FOV sharp is indeed corrected better than a 50 deg AFOV eyepiece that shows 99% of its FOV sharp. So out to the same TFOV position the 100 deg EP is better. I chose not to evaluate the eyepieces like this because I feel it makes no sense from the human perception level and how much a user will be satisfied with their eyepiece. So from a practical standpoint, IMO most observers relate how they like or dislike their eyepiece's performance based on how well it is handling its FOV. Most people will not be very satisfied with a 100 degree EP that has only 50% of its FOV sharp, but will be satisfied very much with their 50 deg AFOV EP that is sharp to the edge. So IMO it is more important to judge this aspect of performance "relative" to whatever the FOV the EP shows. This is why I did it this way. Plus IMO it is more intuitive from the visual observation standpoint as most don't look at FOVs and register TFOV points. Anyway, this is the rationale for the approach I used.




Bill,

I really appreciate this approach. I would rather have a 50 deg AFOV eyepiece that is sharp over 45 deg than a 70 deg AFOV eyepiece that is only sharp over 55 deg. For example, when you look at the TFOV in the Orion Q70's, roughly 60-70% of the field in diameter terms is sharp, but that means in area terms roughly 50% of the field is poor and this is very aesthetically annoying.

Dave


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andydj5xp
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Starman1]
      #5080089 - 02/19/12 12:13 PM

Quote:

The articles on the web discussing how the eye interacts with the eyepiece are illustrative of how the presence of some pincushion distortion (negative RD) in the eyepiece actually displays less noticeable distortion to the eye than zero RD. This apparently is due to the way the eye sees, which tends to show some barrel distortion (positive RD).




May be I'm now confused about this but, IIRC, barrel distortion is negative RD and pincushion distortion is positive RD (straight lines are outwards curved towards the field stop). OTOH, who really cares?

Andreas


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Starman1
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Re: Comparison of 24mm-26mm Eyepieces new [Re: andydj5xp]
      #5080438 - 02/19/12 03:08 PM

No, barrel distortion is positive RD, with lines bowed outward (similar to a barrel shape), while pincushion is negative RD with the lines bowed in at the edge.

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andydj5xp
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Starman1]
      #5080602 - 02/19/12 04:32 PM

Quote:

No, barrel distortion is positive RD, with lines bowed outward (similar to a barrel shape), while pincushion is negative RD with the lines bowed in at the edge.




You are right! I stand corrected.

Andreas


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andydj5xp
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Re: Comparison of 24mm-26mm Eyepieces new [Re: andydj5xp]
      #5081489 - 02/20/12 06:23 AM

Quote:

Quote:

No, barrel distortion is positive RD, with lines bowed outward (similar to a barrel shape), while pincushion is negative RD with the lines bowed in at the edge.




You are right! I stand corrected.

Andreas




Now I stand re-corrected

According to Rutten/Venrooij, 3rd printing, page 31, Fig.4.13: Barrel (negative) distortion and Pincushion (positive) distortion.

Andreas


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Daniel Mounsey
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Re: Comparison of 24mm-26mm Eyepieces new [Re: andydj5xp]
      #5081693 - 02/20/12 10:00 AM Attachment (40 downloads)

Barrell (left) Pincushion (right)

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bluedandelion
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Daniel Mounsey]
      #5081825 - 02/20/12 11:34 AM

Nice work Bill. For those of us who are occasional visual observers, this and previous studies by you are valuable resources. Now I know which one or two EPs I might pick up - an often asked question in the EP forum.

Thanks.

Ajay


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Starman1
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Re: Comparison of 24mm-26mm Eyepieces new [Re: bluedandelion]
      #5081932 - 02/20/12 12:48 PM

Andreas,
Eek! You're right. But the issue is further confused by the discussion of distortion in lenses here:
http://en.wikipedia.org/wiki/Distortion_(optics)
in which the radial distortion coefficient K(sub n) is negative for pincushion distortion and positive for barrel distortion.
Though I do understand the idea in that book that pincushion results from a lateral (positive) stretching of the image, and barrel the opposite, I cannot reconcile that with the math that shows if the distortion coefficient is negative you get pincushion.
Perhaps this should become a different thread so not to hijack the discussion of the review of eyepieces.


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Daniel Mounsey
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Re: Comparison of 24mm-26mm Eyepieces new [Re: bluedandelion]
      #5081940 - 02/20/12 12:51 PM

One of the things I admire about Bill is his dedication. Regardless of any topics others question in a review, it takes a tremendous amount of work and time to complete this kind of essay. I personally see eye to eye on many of his impressions which remind me a lot of the glory days of Charlton Flats.

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andydj5xp
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Starman1]
      #5082281 - 02/20/12 04:13 PM

Quote:

Andreas,
Eek! You're right. But the issue is further confused by the discussion of distortion in lenses here:
http://en.wikipedia.org/wiki/Distortion_(optics)
in which the radial distortion coefficient K(sub n) is negative for pincushion distortion and positive for barrel distortion.
Though I do understand the idea in that book that pincushion results from a lateral (positive) stretching of the image, and barrel the opposite, I cannot reconcile that with the math that shows if the distortion coefficient is negative you get pincushion.
Perhaps this should become a different thread so not to hijack the discussion of the review of eyepieces.




Don,

you are right about not wanting to hijack the discussion about Bill's magnificent review.

Thus only as a short remark: the Wikipedia formula derives the position of the undistorted point x(sub u) from the position of the distorted point x(sub d). In case of barrel distortion (negative RD) the distorted point is too near to the center (too low). Therefore, it needs a positive K(sub n) to yield the position of undistorted point. And vice versa.

Andreas


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DanaJ
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Re: Comparison of 24mm-26mm Eyepieces new [Re: andydj5xp]
      #5083103 - 02/21/12 02:12 AM

Canon's excellent book "Lens Work" has photos showing barrel and pincushion distortion, as well as photos and discussion of many more optics issues. Daniel's photos show basically what they do for barrel and pincushion distortions. The text related to these:

"[...] Distortion which stretches (+) the shape in the diagonal direction is called pincushion distortion, and that which compresses (-) the shape in the diagonal direction is called barrel distortion. In rare cases with super-wide-angle lenses, these two distortion types may coexist to produce a shape which is both stretched and compressed. [... text relating to lens design, zoom lenses, aspherical elements, and diagram of subject/lens/image formation ...] Since this type of aberration is caused by refraction abnormalities of the principle light rays passing through the center of the lens, its effect cannot be reduced by stopping down the lens."

Not that much different than Wikipedia I suppose (though I had my copy of Lens Work II before Wikipedia existed).


On direct topic: Great review, Bill, and thanks for spending all the time on it, and sharing with us. Looks like it was much harder than the earlier ones, with so many different targets and qualities to examine.


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rguasto
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Starman1]
      #5083238 - 02/21/12 07:31 AM

Yes! and that is exactly what I think every time I look through an eyepiece in my telescope.

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BillP
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Re: Comparison of 24mm-26mm Eyepieces new [Re: DanaJ]
      #5084197 - 02/21/12 05:00 PM

Quote:

Looks like it was much harder than the earlier ones, with so many different targets and qualities to examine.




In a sense, it was actually a bit easier. This time I got a voice recorder and just talked out loud in the back yard in the middle of the night as I observed (wondered what neighbors were thinking ). So I didn't have to divert my attention between observing and writing all the time. Made it go much more smoothly. Then next day just sat in front of computer and transcribed my audio to notes. Have an audio editing program so was easy to just find the spots where I was talking so didn't have to listen 3 hours for a 3 hour observing session. Typically took about 30 minutes of transcribe time to catch all the previous evening's audio notes. made the whole process much more enjoyable also as I could have fun while testing instead of writing.


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Re: Comparison of 24mm-26mm Eyepieces new [Re: Daniel Mounsey]
      #5084203 - 02/21/12 05:02 PM

Quote:

Barrell (left) Pincushion (right)




In pictures of people, the left one usually happens with age to most of us


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Daniel Mounsey
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Re: Comparison of 24mm-26mm Eyepieces new [Re: BillP]
      #5086543 - 02/22/12 11:25 PM

that's very true!

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CounterWeight
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Daniel Mounsey]
      #5108195 - 03/06/12 10:57 AM

Bill your review is an outstanding effort. Very enjoyable, really liked the format, layout, and content. Given the nuances discussed it is remarkably uncomplicated and easy to read. Very well done and many thanks!

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Re: Comparison of 24mm-26mm Eyepieces new [Re: Jason Martin]
      #5125699 - 03/16/12 03:29 PM

I'm please with them all and find them consistent bewtween each other.

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Pod
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Re: Comparison of 24mm-26mm Eyepieces new [Re: BillP]
      #5177181 - 04/17/12 03:25 PM

Hi, first post. I had an account a few years ago but forgot my user name. Just to say what a fantastic review, very concise, comprehensive, well laid out and very easy to read.

Thanks

Edited by Pod (04/17/12 03:27 PM)


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astroneil
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Re: Comparison of 24mm-26mm Eyepieces new [Re: Pod]
      #5260390 - 06/07/12 03:40 PM

Hello Bill or should I say, Mr. Eyepiece.

My apologies for chiming in so late. I've been away on the frontier for most of that time.

What can I say? A most excellent and unique project, superbly illustrated and easy enough for a eyepiece duffer like myself to understand.

I now realise there's a brave new world of oculars out there. And it's nice to know that there's still some bargains to be had.

With very best wishes,

Neil.


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