Which is better for afocal - Pentax SMC 40mm pancake or Canon EF 40mm pancake ?

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Hobbyst46
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Which is better for afocal - Pentax SMC 40mm pancake or Canon EF 40mm pancake ?

#1 Post by Hobbyst46 » Tue Sep 10, 2019 12:37 pm

Hello all,
This is a possibly a tough and problematic question but I will try.
Background:
My finite 160mm microscope with an aberration-correcting 8X or 10X eyepiece.
And a Canon EOSM mirrorless camera, equipped with an EF adapter, so the focal flange is about the same as an EOS DSLR (45-46mm).
Now using a 50mm/1.8 Canon prime lens (by means of EOSM/EF adapter), thickness 36mm, with 49mm filter thread, that is used for connection to the photo tube..
Shooting on Manual mode all the way.

Yet, thinking about upgrade by means of a pancake lens instead of the 50mm prime.
Rational for replacement:
1) the pancake is supposed to provide less aberration correction on its own, so to less interfere with the corrections given by the
objective+eyepiece (is this true ?)
2) the pancake is thinner, so easier to adjust the image above the eyepiece (is this true ?)

Assuming that the above are true and worthwhile to do, I consider two pancake lens options:
a) Pentax SMC-M 40mm/2.8, K mount; thickness 18mm; available from eBay, not locally; average asking price on eBay 110-150$, likely no great deals on that; plus an K/EF adapter (~10$). It is also possible to get an K/EOSM adapter, and dispose with the EOSM/EF adapter, to retain the focal flange distance.
b) Canon 40mm/2.8, EF mount; thickness 22mm; available locally, new - ~190$; used - ~ 150-170$; plus an 52mm-49mm (<10$).

Money and availability are important, yet, maybe one of these pancakes is optically better than the other!
I would appreciate all comments/suggestions.
Zeiss Standard GFL+Canon EOS-M10, Olympus VMZ stereo

MicroBob
Posts: 1402
Joined: Sun Dec 25, 2016 9:11 am

Re: Which is better for afocal - Pentax SMC 40mm pancake or Canon EF 40mm pancake ?

#2 Post by MicroBob » Tue Sep 10, 2019 1:30 pm

Hi Doron,
as far as I understand it the reason for a pancake lens is this: The eyepiece has a certain point where the image is formed. The camera objective has a certein point where this image has to be projected ideally (somewhere in the middle). These points can be brought in one place much easier when a hígh eyepoint eyepiece is combined with a pancake lens.
When everything fits you should get an evenly illuminated image without dark corners when you stop the camera objective down. You only need a longer exposure. If the corners darken, you don't have the right point. Does this hapen with you 50mm camera objective?

How important is the camera objective quality? I'm not sure. Zeiss used simple designs in alanlog times. On the other hand side an acquaintance of mine (who should know!) held the opinion that the quality of the camera objective is indeed immportant.
I would assume that your two options are about equal.

As far as I know the camera objective doesn't mess with the colour correction of the microscope components.

Bob

Hobbyst46
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Joined: Mon Aug 21, 2017 9:02 pm

Re: Which is better for afocal - Pentax SMC 40mm pancake or Canon EF 40mm pancake ?

#3 Post by Hobbyst46 » Tue Sep 10, 2019 2:44 pm

MicroBob wrote:
Tue Sep 10, 2019 1:30 pm
When everything fits you should get an evenly illuminated image without dark corners when you stop the camera objective down. You only need a longer exposure. If the corners darken, you don't have the right point. Does this hapen with you 50mm camera objective?
Thanks, Bob

Just ran the test. The 50mm/1.8mm, when stopped down to f=4.0, there are no dark corners in the picture (or in the live view). Further stopping down, to f=5 and beyond, increases the dark corners in the picture (NOT in the live view image). At f=8 the bright image circle is fairly small relative to the dark surrounding in the picture (NOT in the live view image).

To clarify: when I focus (with the microscope) in order to get the image, even if the setting in the live view is f=8, the screen is uniformly lit. Yet the picture shows a bright circle within black frame formed by the corners.

When I first mounted the camera, I let the aperture stay at f=1.8 or f=2, to gather light.

Re-thinking: the Canon EF lens apparently, being an electronic lens, only stops down upon triggering the shutter release. In other words, to verify that there is no darkening of corners when connecting the camera, one MUST have the aperture closed already. So an absolutely manual lens must be used. That rules out any Canon EF lens, whereas the Pentax is fine. Does it make sense ?
Zeiss Standard GFL+Canon EOS-M10, Olympus VMZ stereo

Hobbyst46
Posts: 2117
Joined: Mon Aug 21, 2017 9:02 pm

Re: Which is better for afocal - Pentax SMC 40mm pancake or Canon EF 40mm pancake ?

#4 Post by Hobbyst46 » Tue Sep 10, 2019 4:14 pm

Hobbyst46 wrote: Re-thinking: the Canon EF lens apparently, being an electronic lens, only stops down upon triggering the shutter release. In other words, to verify that there is no darkening of corners when connecting the camera, one MUST have the aperture closed already. So an absolutely manual lens must be used. That rules out any Canon EF lens, whereas the Pentax is fine. Does it make sense ?
MicroBob wrote:...
So, one step further. I own a totally manual MD-50mm/1.7 prime lens (from a Minolta film camera) and an EOSM-MD adapter. Its front lens is relatively close to the mechanical barrel rim. By virtue of the manual aperture, I stopped down. Mounted the camera+lens on the microscope and adjusted the distance from the KPL eyepiece until for f values between 1.7 and 8 of the aperture there is no darkening of the corners. The manual lens allows such adjustment without actually taking pictures. I guess that it is now an acceptable setup.
The question is, will a pancake lens - Canon EF (provided that I can achieve such setting somehow) or Pentax SMC (which has manual aperture so can be set-up in the same manner as the Minolta lens above) - provide any advantage in image quality ?
Zeiss Standard GFL+Canon EOS-M10, Olympus VMZ stereo

MicroBob
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Re: Which is better for afocal - Pentax SMC 40mm pancake or Canon EF 40mm pancake ?

#5 Post by MicroBob » Tue Sep 10, 2019 5:02 pm

Hi Doron,
you have to carry out the test with closed aperture and I would recommend to use a taken picture as the live view image may be "optimized" in a way.
When you set f8 on the EF lens it will close down in the moment of exposure and the test can be carried out as good as with an old manual lens. Manual pancake lenses are just used because there are so few pancakes at all, new and old.
The old Minolta MD is a really good lens and there will be little to improve by choosing something else. You may be able to find a very expensive lens (e.g. Leica) that is a slight bit better. That Canon pancake will probably be not better. So the Minolta MD fits from the matching of exit and entrance pupil and if it gives you the right framing you are done. There was also a Minolta MD 45/2.0 which is built quite short.


Bob

Hobbyst46
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Joined: Mon Aug 21, 2017 9:02 pm

Re: Which is better for afocal - Pentax SMC 40mm pancake or Canon EF 40mm pancake ?

#6 Post by Hobbyst46 » Tue Sep 10, 2019 5:44 pm

Thanks a lot, Bob. Accepted.
Zeiss Standard GFL+Canon EOS-M10, Olympus VMZ stereo

Scarodactyl
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Joined: Sat Mar 03, 2018 9:09 pm

Re: Which is better for afocal - Pentax SMC 40mm pancake or Canon EF 40mm pancake ?

#7 Post by Scarodactyl » Tue Sep 10, 2019 9:15 pm

I got my canon pancake (cancake?) for 90 bucks on eBay, I suspect they can be had cheaper yet. It is a bit annoying that it is all electronic but it works.
I haven't tried stopping it down. I did put live view at 5x on an edge area and move the camera and lens up and down over the eyepiece, and didn't see any noticeable change in the image or abberations as I did. Is the distance really that critical?

Hobbyst46
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Joined: Mon Aug 21, 2017 9:02 pm

Re: Which is better for afocal - Pentax SMC 40mm pancake or Canon EF 40mm pancake ?

#8 Post by Hobbyst46 » Tue Sep 10, 2019 10:05 pm

Scarodactyl wrote:
Tue Sep 10, 2019 9:15 pm
I got my canon pancake....I haven't tried stopping it down. I did put live view at 5x on an edge area and move the camera and lens up and down over the eyepiece, and didn't see any noticeable change in the image or abberations as I did. Is the distance really that critical ?
the question is, what happens when you move the lens when it is stopped down to f=5.6-8, say, and shoot every few mm (of distance between the front element of the camera lens and the eyepiece). From the image files, would the corners appear dark, and would the dark area depend on the distance ? if not, the advantage of the pancake over the thick prime lens would be obvious.
Zeiss Standard GFL+Canon EOS-M10, Olympus VMZ stereo

Scarodactyl
Posts: 377
Joined: Sat Mar 03, 2018 9:09 pm

Re: Which is better for afocal - Pentax SMC 40mm pancake or Canon EF 40mm pancake ?

#9 Post by Scarodactyl » Tue Sep 10, 2019 10:45 pm

Ah, I can see how that would make a difference. I'm not sure when I would want to ever stop it down though.

MicroBob
Posts: 1402
Joined: Sun Dec 25, 2016 9:11 am

Re: Which is better for afocal - Pentax SMC 40mm pancake or Canon EF 40mm pancake ?

#10 Post by MicroBob » Wed Sep 11, 2019 12:39 am

when setting this up in stopped down mode you get the theoretically right setting. How important this is in practical use? It is probably easier to set it up according to the theory than to determine how much of a problem it is to set it up differently. :D Especially since you have the pancake!

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