Is there a use for a 25x eyepiece?

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don1357
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Is there a use for a 25x eyepiece?

#1 Post by don1357 » Tue Dec 07, 2021 2:09 am

I mean other than manufacturers throwing them in there to claim bigger numbers? Does anybody actually uses them?

Scarodactyl
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Re: Is there a use for a 25x eyepiece?

#2 Post by Scarodactyl » Tue Dec 07, 2021 2:36 am

There might be niche uses where detection is the critical factor vs resolution. Leica makes 40x eyepieces for their macroscopes/stereos which seems nuts to me but there you have it.

apochronaut
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Re: Is there a use for a 25x eyepiece?

#3 Post by apochronaut » Tue Dec 07, 2021 3:13 am

Almost no use these days. Niche.
In the past, up until the era of wide field eyepieces with good to excellent eye relief , high power eyepieces had some value coupled with low power high N.A. objectives as a way of getting reasonably high magnification plus a long working distance. You could for instance use a 10X .30 apochromat with 25X eyepieces for a well resolved 250X image with the working distance of a 10X objective. Field of view and eye relief expectations were consistent with the optical techmology of the time, so the low eye relief was accepted as a fact of life.
Now, all microscope eyepieces must deliver better performance to be accepted and an average 25X design won't measure up. There is no incentive for any company to design a high eye relief wide field 25X eyepiece because few would be sold but there were/are a few that are close to lower power eyepieces in performance. Not cheap. Some were included as accessories with some stereo mics too but only for niche uses. Nowadays too, there is a broader selection of l.w.d. objectives , so using the working distance of a low power objective, ramped up with a high power eyepiece makes less sense.

Those included in cheaper kits today are there to fulfill an illusive promise of unreasonable magnification. They perform poorly, not just due to the field and eye relief issue but mainly because of the 1000X rule.
A microscope's optical system cannot magnify more than 1000X the N.A. of the objective, otherwise there is no further detail observed and often less. So, a 25X eyepiece mated to average quality achromats, is limited to use with a 10X .25 or at most a 20X as long as it is a .50 or greater N.A. objective.
Almost all buyers of those scopes aren't really looking for 250X with a 10X objective. They are looking for 1000X with a 40X objective or 2500X with a 100X objective, both of which will be of terrible image quality.

Scarodactyl
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Re: Is there a use for a 25x eyepiece?

#4 Post by Scarodactyl » Tue Dec 07, 2021 5:03 am

I guess the other thing is that high end scopes typically have mag changers as an option, which is probably an easier sell (and simultaneously a lot more expensive than a pair of eyepieces).

jfiresto
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Re: Is there a use for a 25x eyepiece?

#5 Post by jfiresto » Tue Dec 07, 2021 10:29 am

While 25X is pushing it, a pair of 20X eyepieces are handy with the Wild M7 models. They effectively double the zoom range, split into two overlapping, lower and higher ranges. With the M7's 0.07 NA and limited, 3.1X maximum zoom, empty magnification should start with ca. 1000 * 0.07 / 3.1 = 23X eyepieces.
-John

apochronaut
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Re: Is there a use for a 25x eyepiece?

#6 Post by apochronaut » Tue Dec 07, 2021 12:19 pm

20X on some stereos have been more common and you can see their potential inclusion in the target N.A.s of many diascopic microscope objective classes. 40-45X elite objectives are usually .85 N.A. There were quite a few 60-63X apo and fluorite types between 1.25 and 1.4 N.A.

I have a fondness for 15X -16X eyepieces, within which a wider apparent field can be accomodated in a 23.2mm tube than with a 10X. With a 10X, the limit is 22mm and that is stretching it, most are 20mm. Bringing a 15X out to it's maximum field, yields the equivalent of about a 24mm field in a 10X.
A 60X objective and 10X/20mm eyepiece will have a 20 F.N. or about 330 microns roughly at 600X. A 40X objective with a 15X/16mm eyepiece, will have a 16 F.N. but yield a 400 micron field roughly at 600X , plus slightly more working distance. Fluorite and apo objectives especially benefit from the use of 15X objectives, where their higher N.A.s allow for more magnification.

With older apochromat systems, 15X eyepieces were the default due to the wider f.o.v., in a time when fields were generally restrictive. The 15X could be used with all objectives, only overstepping the resolution barrier of a 90X 1.3 N.A. apochromat by 3.8% but since apochromats are not as tightly bound to the 1000X rule as achromats, in practical terms : nothing. During the 2nd W.W., Spencer issued regular delistings of cat.#'s as production facilities and materials usage switched over to wartime production. At one point, the only compens eyepiece available for the apochromat system was a 15X.

Greg Howald
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Re: Is there a use for a 25x eyepiece?

#7 Post by Greg Howald » Tue Dec 07, 2021 1:46 pm

I use 15x alot, 20x sometimes and 25x makes a nice little paper weight.😃 Greg

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patta
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Re: Is there a use for a 25x eyepiece?

#8 Post by patta » Wed Dec 08, 2021 2:36 pm

I think in the past - before digital cameras - those eyepieces were useful, to clearly resolve the smallest details.
With the 10x eyepiece, I can't really see what the objective is capable of. So higher mag eyepieces.
Like, in amateur astronomy, people use routinely 6mm or even 3mm eyepiece; they will be the equivalent of like 50x or 100x eyepieces! It is easier to see things when are large, even if that's empty magnification.

Today to resolve the smallest things we use digital camera. For visual use, don't need max possible resolution, better something bright and confortable like the 10x eyepiece.

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Re: Is there a use for a 25x eyepiece?

#9 Post by apochronaut » Thu Dec 09, 2021 12:35 am

The expectations and results are very different when using a telescope than they are using a microscope.

The analogy that I like most that explains empty magnification and why it is useless to pursue it is this one. Imagine that you enter a movie theater and on the screen is a man wearing a shirt. On it are buttons but the holes in the buttons that the thread passes through to sew the buttons to the shirt are not visible on the screen. You can walk as close to the screen as you like and therefore magnify those buttons but you will never see the holes.

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