Eyepieces, photoeyepieces and Teleconverters; what are the similarities and differences? Should we care?

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Re: Eyepieces, photoeyepieces and Teleconverters; what are the similarities and differences? Should we care?

Post by Pau »

Try to achieve at least 1.5X and to focus .at the position where the eyepiece diaphragm is expected to be placed

I'll try it nex week
Pau

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Re: Eyepieces, photoeyepieces and Teleconverters; what are the similarities and differences? Should we care?

Post by rjlittlefield »

Vignetting is definitely a problem with the macro lens approach. A little ray tracing shows why. All the light rays come from the back of the objective and project in more or less straight lines to fill the field stop of the eyepiece. That means they spread out from a small hole to fill a big one. If you remove the eyepiece and instead place a macro lens above the tube, then the light rays from the objective form their image in the usual position, but the light rays then continue on to spread over an even larger area before they reach the macro lens. To avoid vignetting, the entrance pupil of the macro lens must be large enough, and close enough to the front of the lens, that all those spreading-out rays can still make it through the entrance pupil. If the pupil is too small or too far back, then only light rays for the center of the image can make it through, and then you get more or less severe vignetting.

Reviewing my experience from 2006, I was using an 80mm f/4 bellows macro lens. The entrance pupil of that thing is 20 mm wide (=80/4), and it's located pretty close to the front lens surface. I was running at 2.5X in the relay system, so the 80mm FL lens must have been about 115 mm above the intermediate image. With the intermediate image located 150 mm from the objective, and the macro lens 115 mm above that, a 20 mm entrance pupil could only cover about 20*150/(150+115) = 11 mm diameter of the intermediate image. But then at 2.5X, that 11 mm of intermediate image became 26 mm at the camera sensor, which did not quite fill the camera sensor. I have just now gone back and looked at the original images, and sure enough there are dark corners. At lower magnification, the 80mm lens would have been farther back. That would have captured a smaller part of the intermediate image, which combined with the reduced magnification would have produced severe vignetting.

To avoid the vignetting it would be necessary to introduce what's called a "field lens", near the intermediate image, to redirect the light rays so they could all pass through the entrance pupil of the macro lens. The following comes from pages 70-71 of "Practical Optical System Layout" by Warren J. Smith (1st edition, 1997):
FieldLens.jpg

What the figure calls the "Eye lens" would be the entrance pupil of the macro lens. The diagrams are not to scale for the setup we're discussing, but I hope the concept gets across. These sorts of field lenses are commonly found in relay lens chains. The book's Figure 2.11, not reproduced here, shows that in a periscope, every other lens group is serving as a field lens whose main job is not to refocus the light, but rather to redirect already focused light so that it goes through the next refocusing lens instead of continuing sideways to splat into the sides of the periscope tube.

I hope this helps, at least conceptually. The whole situation is a bit of a Catch-22: vignetting sucks but field lenses are hard to get right also. I can't recall ever going to the trouble to use a field lens of my own selection.

--Rik

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Re: Eyepieces, photoeyepieces and Teleconverters; what are the similarities and differences? Should we care?

Post by seta666 »

rjlittlefield wrote:
Sat Mar 27, 2021 2:22 pm
Vignetting is definitely a problem with the macro lens approach. A little ray tracing shows why......
Thanks again, it seems to me that the macro lens approach is more of a curiosity, not a very practical one.

So going back to relay lenses, would it be possible to use something like a 2.5x photoeyepiece at another magnification by changing the distance ratios objective>eyepiece>camera while keeping the system parfocal? Same principle I use with Nikon SC eyepiece.

The image circle of 2.5x relay lens that covers full frame should also cover an APS-C sensor well at 1.66X for Nikon/Sony and at 1.56x for Canon. I understand this would bring new aberrations but I do not think these should be worst than with the viewing eyepiece approach.

2.5x photo eyepieces are quite common and affordable while things like NFK 1.67X are both rare and expensive, maybe this could be another solution.

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Re: Eyepieces, photoeyepieces and Teleconverters; what are the similarities and differences? Should we care?

Post by viktor j nilsson »

seta666 wrote:
Sun Mar 28, 2021 7:04 am
rjlittlefield wrote:
Sat Mar 27, 2021 2:22 pm
Vignetting is definitely a problem with the macro lens approach. A little ray tracing shows why......
Thanks again, it seems to me that the macro lens approach is more of a curiosity, not a very practical one.

So going back to relay lenses, would it be possible to use something like a 2.5x photoeyepiece at another magnification by changing the distance ratios objective>eyepiece>camera while keeping the system parfocal? Same principle I use with Nikon SC eyepiece.

The image circle of 2.5x relay lens that covers full frame should also cover an APS-C sensor well at 1.66X for Nikon/Sony and at 1.56x for Canon. I understand this would bring new aberrations but I do not think these should be worst than with the viewing eyepiece approach.

2.5x photo eyepieces are quite common and affordable while things like NFK 1.67X are both rare and expensive, maybe this could be another solution.
https://krebsmicro.com/NFK_lowermag/index.html

This is the only test I've ever seen.

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Re: Eyepieces, photoeyepieces and Teleconverters; what are the similarities and differences? Should we care?

Post by seta666 »

viktor j nilsson wrote:
Sun Mar 28, 2021 8:44 am
https://krebsmicro.com/NFK_lowermag/index.html
This is the only test I've ever seen.
Now I remember seeing this test years ago; I did not use microscopes back then so I just forgot about it, thanks.

I still wonder how low you can go before image quality degrades and whether it would perform better at higher magnifications because of effective aperture or not.

At 1.67X the NFK 2.5X is working at 66% of its designed magnification, at 2X it would be working at 80%
A Nikon CF PL2X ( Were these designed for full frame cameras?) working at 80% of its designed magnification would be working at 1.6X

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Re: Eyepieces, photoeyepieces and Teleconverters; what are the similarities and differences? Should we care?

Post by Ichthyophthirius »

seta666 wrote:
Sun Mar 28, 2021 7:04 am
So going back to relay lenses, would it be possible to use something like a 2.5x photoeyepiece at another magnification by changing the distance ratios objective>eyepiece>camera while keeping the system parfocal?
Hi,

I still think it's possible to use the NFK2.5x together with a focal reducer. Reduces the magnification and maintains parfocality. You bring a focal reducer between projective and camera and reduce the distance between camera and projective in the process.

It definitely works: viewtopic.php?f=25&t=25345&p=156047 Since I wrote this, a lot of focal reducers ("Speedboosters") have become available for cameras and there are also many available for astrophotography. I think it's worth investigating although I don't pursue it myself at the moment; I fully satisfied with the various afocal and direct projection solutions I use.

Regards, Ichty

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Re: Eyepieces, photoeyepieces and Teleconverters; what are the similarities and differences? Should we care?

Post by rjlittlefield »

seta666 wrote:
Sun Mar 28, 2021 7:04 am
rjlittlefield wrote:
Sat Mar 27, 2021 2:22 pm
Vignetting is definitely a problem with the macro lens approach. A little ray tracing shows why......

To avoid the vignetting it would be necessary to introduce what's called a "field lens", near the intermediate image, to redirect the light rays so they could all pass through the entrance pupil of the macro lens.
Thanks again, it seems to me that the macro lens approach is more of a curiosity, not a very practical one.
For my own curiosity, I tried the field lens approach. It turned out that placing a Raynox DCR-250 on the microscope tube end eliminated vignetting in all cases I tried, using a Canon 100 mm f/2.8 IS USM stopped even down to f/16. Magnification on sensor ranged from roughly 1.17X down to 0.25X, depending on distance from camera to microscope, refocusing the macro lens to match. Surprisingly, I saw no curvature of field or other serious image defects in a quick look. I did not try pixel-peeping with a finely detailed subject. Results were similar with the Raynox either way around.

I tried also with an ITL-200, large end down; that helped but not as much since I could only stop down to f/4 without vignetting. Theory indicates that the field lens should be around 100 mm focal length in this case, so it makes sense that the DCR-250 at 125 mm FL would work better.

--Rik

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Re: Eyepieces, photoeyepieces and Teleconverters; what are the similarities and differences? Should we care?

Post by seta666 »

Ichthyophthirius wrote:
Mon Mar 29, 2021 4:39 am
I still think it's possible to use the NFK2.5x together with a focal reducer. Reduces the magnification and maintains parfocality. You bring a focal reducer between projective and camera and reduce the distance between camera and projective in the process.....
Thanks, another interesting approach; a focal reducer was something I did consider for the MP-E on the APS-C sony cameras but kind of seems more practical just to use a teleconverter instead of eyepiece plus focal reducer in this case; certainly useful for those who need a corrective eyepiece.
rjlittlefield wrote:
Mon Mar 29, 2021 9:23 am
For my own curiosity, I tried the field lens approach. It turned out that placing a Raynox DCR-250 on the microscope tube end eliminated vignetting in all cases I tried, using a Canon 100 mm f/2.8 IS USM stopped even down to f/16. ....................
The good old raynox DCR-250 never stops to amaze me; so this should be placed where exactly? I did a quick test and I could manage to make it work both with the canon 60mm at 1.5X and olympus 80mm and 1.4X, no distortion but some field of view curvature and extra CAs.

I was looking for both parfocality and 1.5X magnification range. That meant placing the raynox around 30mm further away than where the sensor goes for direct projection while the macro lens was not that far away from the raynox, maybe same 30mm or so. So I do not know if I am using it as a close up lens, field lens or both are the same.. :D If the raynox was were the eyepiece is supposed to go magnification was 1X aprox

Maybe I test it against the Nikon SC as projection lens and compare the results

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Re: Eyepieces, photoeyepieces and Teleconverters; what are the similarities and differences? Should we care?

Post by rjlittlefield »

seta666 wrote:
Mon Mar 29, 2021 12:32 pm
So I do not know if I am using it as a close up lens, field lens or both are the same.
They are not the same concept. Ideally a field lens goes exactly at the location where an image is focused. In that position it serves to change the general direction of each beam so that all beams can go through the next lens, but it does not change focus or magnification. So, as you observed, "If the raynox was were the eyepiece is supposed to go magnification was 1X aprox".

The farther the Raynox is located away from the focus plane, the less it acts like a field lens and the more it acts like a closeup lens. In your arrangement it is doing parts of both concepts.
this should be placed where exactly?
"Like all lenses, it should be placed where it does exactly what you want, assuming that such a place exists."

This answer may sound tongue-in-cheek, but I mean it seriously. I was interested mainly in checking out the concept of a field lens to avoid vignetting. So I placed the Raynox as close to the ideal position as I could, and got only 1.17X maximum magnification versus 1.0X with the macro lens alone. Sticking with that approach, if I wanted 1.5X magnification then I might mount a 1.4X teleconverter behind the macro lens, or add some extension behind the macro lens to increase the magnification of that part of the system. Placing the Raynox farther back, as you did, may accomplish the same goal in a different way. It is all a matter of tradeoffs, which will surely be different with your lenses versus mine.

--Rik

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Re: Eyepieces, photoeyepieces and Teleconverters; what are the similarities and differences? Should we care?

Post by seta666 »

rjlittlefield wrote:
Mon Mar 29, 2021 6:57 pm
"Like all lenses, it should be placed where it does exactly what you want, assuming that such a place exists."
At the end this is where I am trying to get at.. around 1.5Xish magnification whenever I need that extra reach
Sticking with that approach, if I wanted 1.5X magnification then I might mount a 1.4X teleconverter behind the macro lens, or add some extension behind the macro lens to increase the magnification of that part of the system.
Using a field lens, plus macro lens plus teleconverter seems like an overkill, too much glass each of them with its own issues
; adding extention would make the phototube quite tall, maybe it is still OK but I am getting used on how minimalistic the set up looks with direct projection.

At the end I have many options to choose from:

- With a 4/0.13 (or even a 10/0.25)I could get to that 1.5X just by adding extension tubes, as its low NA is less affected by it
- For other lenses a 1.5X TC would be just perfect only if my trinocular head would allow it, I could just use tubes instead whenever I needed it (may test it when I get the BH2-M42 adapter).
- The nikon SC eyepiece seems to work OK at 1.8X if extreme corner performance is not that important but its f18mm field number does allow less than that
- The Raynox plus macro lens combo also seems to work OK but I did notice some more CA´s compared to the nikon SC, still worth testing it too.
- Afocal could work well too but so far I am getting a hot spot and I do not know if it is because of the Konica or because how reflective Sony sensors are.
- I do not own any photoeyepiece but would love to test a CF PL2X at lower than rated magnification

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