What are higher-magnification projective eyepieces good for?

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Scarodactyl
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What are higher-magnification projective eyepieces good for?

#1 Post by Scarodactyl » Wed Jun 19, 2019 8:39 pm

This is a general question that I've been running up against now and then. Obviously 1.6x or 2.5x projective eyepieces of whatever brand are in much demand because they are great for directly adapting a dslr by projecting right onto the sensor. But these are in the minority of projectives, with plenty of 5x, 8x, 10x etc out there on the market. From my understanding these were used for projecting images onto larger and now-obsolete film formats. It makes sense that they made a lot of them, but what I don't understand is what they could use used for now, and why it is that so many seem to be sold. Maybe it's an illusion, but it seems like there are enough of them up on eBay at non-bottom-of-the-barrel prices that somebody must be using them for something. Can they be paired with a reducing lens similar to an afocal setup? Or is there some other usecase?

abednego1995
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Re: What are higher-magnification projective eyepieces good for?

#2 Post by abednego1995 » Thu Jun 20, 2019 1:58 am

"Well, why not get a 10x rather than a puny 2.5x when you can!"

I think that's the rationale behind it, and many would eventually come down to lower mags when they realize what resolution is :-)

Cheers,
John

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Re: What are higher-magnification projective eyepieces good for?

#3 Post by MicroBob » Thu Jun 20, 2019 10:58 am

I think they are plenty on the market because they were ordered in sets by customers who didn't know what they precisely need, so they made a tick behind everyone on the order list. That they are offered for real money doesn't mean that they sell - have you checked the completed listings on ebay?

Bob

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Re: What are higher-magnification projective eyepieces good for?

#4 Post by apochronaut » Thu Jun 20, 2019 1:45 pm

In older systems higher mag. eyepieces were used for larger format film but with small format sensors, they aren't really needed.

Eyepieces have changed though. Most visual eyepieces in use with oo systems have high eye relief, so are intrinsically projective and with a 20mm or so f.o.v. when used in a photo tube of the correct length , they can be arranged to cover the relatively similar long width of an APS-C sensor quite perfectly.

I see a lot of images posted on this forum with lateral ca, many of them using low power relay lenses supplied by the maker of the microscope, or add on lenses chosen based on the magnification rather than the corrections. Lateral ca is unacceptable.

Using a 10X optimally corrected eyepiece providing a flat, full field image capture with an optimal image circle, seems a somewhat better overall choice.

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Re: What are higher-magnification projective eyepieces good for?

#5 Post by Scarodactyl » Thu Jun 20, 2019 5:23 pm

apochronaut wrote: Eyepieces have changed though. Most visual eyepieces in use with oo systems have high eye relief, so are intrinsically projective and with a 20mm or so f.o.v. when used in a photo tube of the correct length , they can be arranged to cover the relatively similar long width of an APS-C sensor quite perfectly.
That's interesting--all you have to do is get the sensor into that high relief zone with no intervening optics? How high does the eyepoint have to be? It's very intriguing, but given how far into the camera the sensor is on a dslr I'm kind of curious how it would work in practice, and which systems would be good targets for it (how recent are we talking in terms of eyepieces?). Do you have any examples of this kind of setup?
apochronaut wrote:I see a lot of images posted on this forum with lateral ca, many of them using low power relay lenses supplied by the maker of the microscope, or add on lenses chosen based on the magnification rather than the corrections.
Is the lateral CA typically from the projective rather than the optics themselves? Certainly mismatching corrective eyepieces is going to cause problems, but not everyone's using apochromats either.

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Re: What are higher-magnification projective eyepieces good for?

#6 Post by apochronaut » Thu Jun 20, 2019 10:16 pm

Here are some pictures through various eyepieces. Most of them are designated for photo. At one time or another I have used them all to take pictures. The problem is, I can't remember which is which and I seem to have misplaced the order in which they were taken. Perhaps one of the posters can help me out and tell me what the magnifications of them are. The 10X should be easy because apparently they would cause reduced resolution.

I do have a list though: Vickers 10X comp., Olympus FK 6.7X, Wild 6X K H, Olympus NFK 5X LD ,AO 180, Kyowa SP 2.5X, Nikon CF Photo 5X, AO 1054, Olympus FK 5X. All the pictures were taken through exactly the same system, only the eyepiece was changed. The eyelens to camera sensor was kept as close to the same as possible.

edit. have added the order.
Attachments
Olympus FK 5X
Olympus FK 5X
DSC02974 (1024x575).jpg (232.69 KiB) Viewed 8323 times
Vickers 10X compens
Vickers 10X compens
DSC02973 (1024x575).jpg (231.19 KiB) Viewed 8323 times
Olympus NFK 5X
Olympus NFK 5X
DSC02972 (1024x575).jpg (243.5 KiB) Viewed 8323 times
AO 180 10X W.F.
AO 180 10X W.F.
DSC02971 (1024x575).jpg (234.09 KiB) Viewed 8323 times
Wild 6X K  H
Wild 6X K H
DSC02970 (1024x575).jpg (212.61 KiB) Viewed 8323 times
Last edited by apochronaut on Fri Jun 21, 2019 3:16 pm, edited 2 times in total.

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Re: What are higher-magnification projective eyepieces good for?

#7 Post by apochronaut » Thu Jun 20, 2019 10:20 pm

and the rest.
Attachments
AO 1054 10X W.F. focusing
AO 1054 10X W.F. focusing
DSC02978 (1024x575).jpg (247.29 KiB) Viewed 8322 times
Nikon 5X CF Photo
Nikon 5X CF Photo
DSC02977 (1024x575).jpg (187.99 KiB) Viewed 8322 times
Olympus FK 6.7X
Olympus FK 6.7X
DSC02976 (1024x575).jpg (246.04 KiB) Viewed 8322 times
Kyowa SP 2.5X
Kyowa SP 2.5X
DSC02975 (1024x575).jpg (192.07 KiB) Viewed 8322 times
Last edited by apochronaut on Fri Jun 21, 2019 3:17 pm, edited 1 time in total.

MichaelG.
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Re: What are higher-magnification projective eyepieces good for?

#8 Post by MichaelG. » Fri Jun 21, 2019 6:53 am

apochronaut wrote:Here are some pictures through various eyepieces. Most of them are designated for photo. At one time or another I have used them all to take pictures. The problem is, I can't remember which is which and I seem to have misplaced the order in which they were taken. Perhaps one of the posters can help me out and tell me what the magnifications of them are. The 10X should be easy because apparently they would cause reduced resolution.

I do have a list though: Vickers 10X comp., Olympus FK 6.7X, Wild 6X K H, Olympus NFK 5X LD ,AO 180, Kyowa SP 2.5X, Nikon CF Photo 5X, AO 1054, Olympus FK 5X. All the pictures were taken through exactly the same system, only the eyepiece was changed. The eyelens to camera sensor was kept as close to the same as possible.
A very encouraging set of images !!
... which would suggest that, contrary to the usual dogma, finely matching comp [K] to the objective manufacturer's formulation might not be essential.

May I ask the question: What objective did you use ?

MichaelG.
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Re: What are higher-magnification projective eyepieces good for?

#9 Post by MicroBob » Fri Jun 21, 2019 8:11 am

Hi together,
for testing objective-eypepiece combinations I would suggest an object micrometer. It is flat and has highest contrast, so a mismatch is easily visible in form of colour fringes. One has to check whether there is a cover slip glued on top of the scale, otherwise it has to be added e.g. with a drop of immersion oil.
This test also shows planarity of the image.
The chinese object micrometers cost just 10€ shipped and work very well.

Bob

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Re: What are higher-magnification projective eyepieces good for?

#10 Post by Hobbyst46 » Fri Jun 21, 2019 12:30 pm

Although I cannot link the results to the objectives, I prefer photos 2970, 2971 and 2978 over all others for resolution, and somewhat also for CA.

I believe that FK should give different results from NFK. My problem with the Olympus FK 5.0X objective is that, on a trino with the FK in the photo tube, I
could not get parfocality with the bino eyepieces, regardless whether they were Zeiss WF10 or Olympus WHK18 or WHK20.
Zeiss Standard GFL+Canon EOS-M10, Olympus VMZ stereo

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Re: What are higher-magnification projective eyepieces good for?

#11 Post by apochronaut » Fri Jun 21, 2019 2:18 pm

Michael G. The objective used was an American Optical 40X .80 corrected for a .18 coverslip. The slide is a very old diatom slide with a .22 coverslip, so there is probably a small amount of spherical aberration.

MicroBob. That is a better idea, yes but then there are those that will come forward suggesting diatoms as the best test subject. I did this as a quick evaluation, primarily of resolution. I do have some sorts of small micrometer test slides. I often use an old wild boar tusk section for this type of test too. It has quite defined black and white striations, although quite irregular.....less 2 dimensional than a micrometer, though.

Hobbyist46. I found my list! The three eyepieces you identified are Wild 6X K H, AO 180 10X W.F.( standard eyepiece for a series 100 microscope) and an AO 1054 10X W.F. focusing eyepiece for reticle use. Both of the latter as well as the Vickers and Wild are actually visual eyepieces. The AO 180 10X, upon closer inspection seems to have a defect. I will have to check it over, since it is showing an assymetrical uncharacteristic ca on the right side; almost nothing on the left. I have about a dozen of these. I just happened to pick the wrong one out of the box. It's performance should match the 1054, or even be superior in all ways. Probably, the field lens is not seated accurately.

I have adjusted each frame to match the largest original crop( Olympus FK 6.7X). In this way, the resolution and ca differences will show up more.
In the same order as before.
Attachments
Olympus FK 5X
Olympus FK 5X
DSC02974 (1280x779) (1024x623).jpg (266.17 KiB) Viewed 8247 times
Vickers 10X comp.
Vickers 10X comp.
DSC02973 (1280x780) (1024x624).jpg (265.25 KiB) Viewed 8247 times
Olympus NFK 5X LD
Olympus NFK 5X LD
DSC02972 (1280x799) (1024x639).jpg (255.95 KiB) Viewed 8247 times
AO 180 10X W.F.
AO 180 10X W.F.
DSC02971 (1280x799) (1024x639).jpg (280.04 KiB) Viewed 8247 times
Wild 6X K H
Wild 6X K H
DSC02970 (1280x821) (1024x657).jpg (285.83 KiB) Viewed 8247 times
Last edited by apochronaut on Fri Jun 21, 2019 3:11 pm, edited 1 time in total.

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Re: What are higher-magnification projective eyepieces good for?

#12 Post by apochronaut » Fri Jun 21, 2019 2:25 pm

4 more.
Attachments
AO 1054 10X W.F. focusing
AO 1054 10X W.F. focusing
DSC02978 (1280x750) (1024x600).jpg (263.57 KiB) Viewed 8247 times
Nikon 5X CF Photo
Nikon 5X CF Photo
DSC02977 (1280x695) (1024x556).jpg (282.68 KiB) Viewed 8247 times
Olympus FK 6.7X
Olympus FK 6.7X
DSC02976 (1280x719) (1024x575).jpg (226.46 KiB) Viewed 8247 times
Kyowa SP  2.5X
Kyowa SP 2.5X
DSC02975 (1280x853) (1024x682).jpg (353.36 KiB) Viewed 8247 times

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Re: What are higher-magnification projective eyepieces good for?

#13 Post by MichaelG. » Fri Jun 21, 2019 2:31 pm

apochronaut wrote:Michael G. The objective used was an American Optical 40X .80 corrected for a .18 coverslip. The slide is a very old diatom slide with a .22 coverslip, so there is probably a small amount of spherical aberration.
Thanks for the detail ... I find it helps [when trying to understand what effects I am seeing, and why]

MichaelG.
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Re: What are higher-magnification projective eyepieces good for?

#14 Post by MichaelG. » Fri Jun 21, 2019 5:02 pm

.

There is an interesting little detail on these images, which is portrayed quite differently by some of the eyepieces:
Nikon 5X CF Photo : Crop with Region of Interest
Nikon 5X CF Photo : Crop with Region of Interest
IMG_3075.JPG (117.33 KiB) Viewed 8225 times
The question therefore is: Which one is portraying it most accurately ?

MichaelG.
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apochronaut
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Re: What are higher-magnification projective eyepieces good for?

#15 Post by apochronaut » Fri Jun 21, 2019 10:04 pm

Having used all of these for various photos or sets of photos over time, I have been using the Wild 6X K H eyepiece more and more with objectives that are for a 160mm system. Sometimes, with certain 160mm objectives, the Nikon 5X CF Photo presents with a more aberration free image across the field.
Generally, with the infinity systems that I use mostly, one of the focusing 10X focusing reticle eyepieces that AO made( there are about 7 of them with different cat. #'s) seems to be the best, mainly due to the lack of distortion at the periphery of the field. The Wild 6X did well with this 40X .80 objective in a 200 mm infinity system and probably has the sharpest on axis resolution but I have a feeling that with a variety of subjects that the 1054 would probably edge it out in overall performance. Perhaps I can start a thread and do a head to head test later, with a better test subject.


I have been generally disappointed with all of the Olympus eyepieces, probably due to the fact that they seem to be designed for fairly short tubes( 125mm) and the systems I work with are greater than 150mm, mostly 200. That's why the Olympus eyepieces perform so poorly in this test and why one cannot judge a photo eyepiece by it's magnification.
They are all purpose built. For DIY ers, experimentation is the key to success, unless you can copy a known quality set up.

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Re: What are higher-magnification projective eyepieces good for?

#16 Post by MichaelG. » Fri Jun 21, 2019 10:44 pm

apochronaut wrote:Michael G. The objective used was an American Optical 40X .80 corrected for a .18 coverslip.
I've just been looking at your previous comparison of 40x objectives:
viewtopic.php?t=4420

... That #1323 looks a mighty fine lens

MichaelG.
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Re: What are higher-magnification projective eyepieces good for?

#17 Post by Scarodactyl » Sat Jun 22, 2019 12:41 am

apochronaut wrote:Sometimes, with certain 160mm objectives, the Nikon 5X CF Photo presents with a more aberration free image across the field.
Is there a particular reason you're going with the 5x instead of a 4x or 2.5x? Do the lower mag ones produce vignetting and/or lower quality? Wider FoV is a pretty big priority for me when adapting a camera, though I suppose to some extent that's becsuse I am typically working with stereo systems and there isn't resolution to spare.

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Re: What are higher-magnification projective eyepieces good for?

#18 Post by apochronaut » Mon Jun 24, 2019 4:26 pm

There is no absolute relationship between using a lower magnification eyepiece and resolution. The images I took pretty much show that; where in fact probably the 2nd most highly resolved image was taken through a 10X eyepiece and the most highly resolved image was through a 6X eyepiece. The lowest magnification eyepiece of the lot a 2.5X looks pretty impressive in it's original image as captured on the sensor but once cropped to give an equally magnified image as the largest one taken in the set up, it reveals the defects in it's image. There is also no direct relationship between the magnification marked on the eyepiece and the image size it will produce, as also evidenced by the images I provided.
Within any particular series of eyepieces that have similar corrections, one magnifying less will theoretically have better resolution but then it has to be magnified or cropped in order to provide an equivalent magnification. It ultimately comes down to the lens design and how much the image needs to be magnified or cropped afterwards.

I'm sure there are all kinds of lenses that can be used as photo relay lenses, reverse lenses, enlarging lenses. Many of these have high repute, such as certain Raynox copy lenses but I have set my systems up to receive eyepieces, mainly because that's what the photo tubes I have were designed to receive and of the 10 or 15 of them covering 2.5X,4X,5X,6X,6.7X,8X and 10X, the ones I go to most are 5,6 and 10X. It has to do more with the design of the entire system and achieving distortion free edge to edge sharpness, rather than whether the relay lens magnifies too much. If I can get an image that is distortion free ,very close to what I am seeing through my eyepieces, that covers the entire f.o.v. and is reasonably parfocal while shooting through my mediocre 20 mp camera, I'm happy. Unfortunately, any of the lower magnification eyepieces I have tried in my systems have provided poor results. It is especially a problem with high magnification objectives.

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Re: What are higher-magnification projective eyepieces good for?

#19 Post by Scarodactyl » Mon Jun 24, 2019 6:22 pm

apochronaut wrote:There is no absolute relationship between using a lower magnification eyepiece and resolution.
Certainly not on this system, but you have resolution to spare with this objective. I assume corrections are the same (or close to it) from one eyepiece to another in the same series, the only difference basically being resizing and cropping the image. But stereo systems don't always have resolution to spare--see recent calculations suggesting the Wild M7 has about ~36mp of resolution. In a case like that I wouldn't want to be applying a heavy crop before the picture is even taken, even aside from it being annoying to have a mismatch between what's visible through the eyepieces and what the camera sees.
apochronaut wrote:There is also no direct relationship between the magnification marked on the eyepiece and the image size it will produce, as also evidenced by the images I provided.
Obviously they aren't comparable between different series, but I'm only talking within the same Nikon series. If the 5x produces good images, I'd assume a lower magnification one would produce similar quality with lower magnification, hence the question.
It may be I'm misinterpreting your tests above, but it looks like two of them have an FoV double that of the others. I would think that that the broader FoV would be a lot closer to what is visible through the eyepieces than the 2x magnified ones. I know sometimes the optics are capable of a much broader FoV than you see through 10x eyepieces but I'd assumed it was uncommon.
apochronaut wrote:Unfortunately, any of the lower magnification eyepieces I have tried in my systems have provided poor results.
This is what I was most curious about. Do you mean that within the same series, lower powered projectives give poorer results? Do you think it's something inherent to their design, or maybe just that they aren't cropping the edges where a mismatch in correction might be more obvious, or something else?

I will say, by the way, I do appreciate that you've handily answered the question in the title of the thread--they might all have some utility in the right context.

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Re: What are higher-magnification projective eyepieces good for?

#20 Post by patta » Tue Aug 18, 2020 10:02 am

Here my late thoughts about the issue..
summary: it's all about correct spacing.

1) Why low-power photo eyepieces have problems
I believe that is due to their focal length and the correct positioning of eyepiece and sensor. High NA objectives are very sensitive to the misplacement of the "virtual image" position right under the eyepiece; and while 10x eyepieces from most manufacturers are designed with the virtual image more or less in the same place (+- few mm), that is not the case of low-mag photo eyepieces.
A 2x eyepieces has a focal length of about 80mm!! If I want to place an 80mm lens for projection to give 2x magnification, it should be placed 120mm above the virtual image; and the sensor has to be 240mm above the lens. That's quite a stack! I won't venture in this construction and instead, I pop in the 2x eyepiece in the place of a 10x and also keep the same camera distance.., as result, to reach focus the virtual image is forced well down in the tube, and the objective need to work in a far-from-optimal distance; giving a poor image.
This thinking may explain the Apochronaut observation that the magnifications marked on the eyepiece and those obtained on the sensor are mismatched: photo eyepieces were designed each for its own specific system, with different adapters and working distances, and good luck for us to find right vintage brochures with the needed specifications. No correct spacing, no stated magnification.

2) Why 10x or so eyepieces are useful
a) As written above, the 10x is easier to use because popping it into the eye tube already matches more or less the right positioning; and having the 10x a short focal length, the camera can be mounted on it at resonable distances.
b) Yes a 10x will give a lot of empty magnifications and heavily crop the image. So what? Do we really need 36 Megapixel, superwide images? Most of the time, I'm interested only into a couple of small details at the centre of the image, so anyway I'll enlarge and crop it, until the limit resolution that the objective had to offer.
And unless we have a Planapo objective, with a sample perfectly flat and horizontal, a wide-field image will be good in the centre but poor at the corners, so I'll crop it anyway.
c) Despite their not-so-low price, high-mag eyepieces are probably the cheapest way to compensate the chromatic aberration that we get in direct projection (unless I have to buy 20 eyepieces to find the right one).
d) a nominal 10x eyepiece can be (ab)used as a practical 5x: just put the sensor nearer.

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Re: What are higher-magnification projective eyepieces good for?

#21 Post by abednego1995 » Tue Aug 18, 2020 1:10 pm

I'm nodding my head off.

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Re: What are higher-magnification projective eyepieces good for?

#22 Post by apochronaut » Tue Aug 18, 2020 3:05 pm

Most plan objectives made well enough to be called and accepted as fully plan do not differ in corrections or flatness of field significantly, across the stated field of whatever eyepiece they are made to be used with. This is the case whether they are achromat or apochromat but obviously apochromats will have a higher degree of overall freedom from ca and most likely higher resolution.

The tests I did above were mainly done to show that individual manufacturers use differing corrections for photo eyepieces and differing image pick up points. All of those first two sets of widely variable images, irregardless of the eyepiece magnification were made at the same pickup point. Only the AO eyepieces, all 10X, were working at a location that had been pre-determined to capture an entire 20mm field onto an APS-C sensor used in an AO 34mm parfocal system . There are resolution differences , revealed fairly well once the images are all cropped to the size of the one that had maximum magnification at that point but it isn't exactly magnification dependent. The AO 180 and Vickers Compens, both 10X resolve about as good as any of the others, with the possible exception of the Nikon, which despite it's great on axis performance leaves a little to be desired laterally, so yes with that one and that one only in this limited example you might be able to crop out undesirable ca and obtain a decent image of specific details but in order to achieve a representative photo of the field in question, you would have to go back to a lower power objective's field to crop out of and that might entail a loss of resolution due to the lower N.A. Not always is a Nikon or Wild 10X .45 objective available.
I sometimes assist new microscopists with the basics of setting up a photo system and there is a preponderance of an interest in capturing as much of the field to the sensor as ie s possible. A lot of people want to photograph what the microscope is seeing.

All those eyepieces combined cost only about 150.00, since they mostly came in joblots or very cheap out of surplus. I thought the real relationship of photo eyepiece magnifications and resolution might be of interest.
A fact I discovered while posting is that the right side of the image made by the AO 180 10X had an undue level of ca. I didn't think it mattered for the purposes of the post. That eyepiece should be fully corrected with the 40X # 1323. It turned out that the eyepiece I randomly plucked to use was defective....had a delamination and likely partly separated doublet, so on axis it probably was reasonably representational but not off axis and of course the magnification was accurate. It hardly shows up when used visually ...it was pulled from an eyepiece tube but photographically, it is evident.

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Re: What are higher-magnification projective eyepieces good for?

#23 Post by patta » Wed Aug 19, 2020 10:17 am

Yes, the tests are very interesting, showing that resolution is not related to magnification, and the eyepieces all look so different!
And I agree that, if possible, is obviously preferable to take the photo of the widest field. That's not my case as I have no plan objectives...

But I found another big mistake/confusion in my previous post, about magnification, which may be the core misunderstanding of this thread:

Nominal magnification vs Reproduction Ratio
The "magnification" inscribed on the eyepiece (like, 10X) is defined in analogy with the visual eyepieces, and is an angular magnification.
For example, a 10x eyepiece, has a focal length FL = 250/10 =25mm (250mm is the standard); and it magnifies by 10x the angle of the chief rays, w.r.t the chief rays in the tube. In visual observation, our eye is used as an afocal system, and this angular magnification is what matters.

But this magnification is a different thing than the reproduction ratio that we get in eyepiece projection.
The eyepiece projection lens takes the intermediate image below, at the field stop, and projects it up to the sensor; like a reproduction/enlarging lens.
The ratio between intermediate image diameter / image on sensor diameter is the linear magnification and that is what matter for eyepiece projection photography; this ratio does not depend directly on eyepiece focal length (or its nominal magnification) but instead on the relative distances of image and sensor. If I put the sensor a bit more far away, the image on it gets larger - while the eyepiece nominal magnification remains the same.

As example, in the tests described above, the eyepiece AO 10x has a field stop ( = intermediate image diameter) of 20mm; this image disk gets projected to an APS-C sensor size, which has diameter 27mm (limits for no vignetting). The reproduction ratio is then 27/20 = 1,35. The projection is working at 1,35X, despite the nominal eyepiece magnification is 10x.
As another example: I can use the 10x eyepiece to project the image up to the ceiling, two meter away... and get a 100x reproduction ratio.

Eyepieces with different nominal magnification may be used, but the reproduction ratio will be still dependent on the sensor positioning, rather than on the 2x or 10x label.
Low-power eyepieces (like, 2x) have the advantage of projecting the light at a narrower angle, which may be good for the sensors, and more working distance to insert filters. Maybe they can have less aberration, due to narrower angular field of view... but that's not so obvious, as illustrated by the previous tests.
Do low-power eyepieces give a larger field? It depends: If my objective/tube are maxed at 20mm field, then a wideangle 10x may be already enough to project it wholly onto the sensor.. Personally I have (only) a 15x wild photo eyepiece, with a small 10mm field stop, but that is enough to cover the small focused area of my non-plan objectives; lower mag/ wider eyepieces will add some more context, but blurry, so I stay content whith the slightly-cropping 15x.
If instead you have 30+mm of usable flat field.....

For which working distance / reproduction ratio my 15X projection eyepieces was optimized for? Who knows? Enlarger lenses for film usually declare the optimal magnification, but that is not the case for eyepieces.. Probably this old 15x was designed to project the 10mm field onto a medium format plate, 70mm diameter, giving a 7x reproduction ratio.
Being it an eyepiece, it has been labeled 15x (angular magnification) but if we think of it as an enlarger lens, it should be called a 7x or 7:1 (its optimal linear magnification).
I use this lens with the sensor nearer than was intended for, to project the 10mm field onto the 27mm APS-C disk, 2,7x reproduction ratio, not optimal but still decent.

A more "modern" 8x proj eypc with 20mm field, designed for use with 35mm film, would have been optimized for a reasonable 43/20 = 2,15x reproduction ratio.

Summary: a 10x eyepiece is not a 10:1 enlarger lens; it can instead be (ab)used as a 1,35:1 enlarger with good results.

Please be tolerant when I write obvious or wrong things, I'm realizing only now all those. Edit: corrected the definition of eyepiece magnification :oops:
Last edited by patta on Thu Aug 20, 2020 4:05 pm, edited 6 times in total.

patta
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Re: What are higher-magnification projective eyepieces good for?

#24 Post by patta » Wed Aug 19, 2020 12:07 pm

Which would be the "optimal" projection eyepiece, in terms of price/performance? A 2x or a 10x?
Here a crude argument.

For full format (35mm) cameras, the optimal lens is the 50mm (wide consensus). 85 or 135mm may be a bit sharper, but more expensive. Below 50mm, lenses are wide-angle and have all sort of problems.
The 35mm film/sensor needs a 43mm image diameter; if we scale down the optimal 50mm lens to cover the 20mm field of the microscope (instead of 43mm) we get:
Focal length eyepiece = 50 *(20/43) = 23mm
Whith the 250mm definition, this eyepiece will be called 250/23= 10,9x

Thus a 10,9x eyepiece is the optimal choice to cover a 20mm field.
Longer eyepieces (5x, 2x) may be sharper but more expensive.
Shorter eyepieces (15x, 20x) will need to be wide angle and struggle to cover properly the whole 20mm.

For larger fields (like, 30mm) longer eyepieces become preferable.

Yes, the eyepiece design is different than a camera lens, but more or less. Long FL eyepieces won't need to be so large as corresponding camera lenses, so the optimum may move upwards.

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