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ChrisR
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 Posted: Sat Sep 09, 2017 2:48 pm Post subject: |
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| All-Ex wrote: | In the bottom line, you cannot do a shooting with infinity corrected microscope objectives using the D850
Can you do it with finite ones? |
Other way around: With Infinites it should be possible to an extent, using a camera lens as tube lens, to focus.
Finites use no camera lens, therefore there is no controllable focusing mechanism.
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All Ex
Joined: 20 Jul 2015
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| Posted: Sun Sep 10, 2017 3:54 am Post subject: |
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I do agree that the topic I was intending initially was about focus shifting that D850 performs.
On the other hand, I found very exciting, inspiring and very interesting the turn that the conversation took and I followed it with interest.
About Chris`sR respond, you mean to focus the tube in other than the infinity point, if so what would be the function to estimate the steps for that?
And another thing what about the 200mm FL, in the Nikon AF range of lenses?
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All--Ex
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ChrisR
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| Posted: Sun Sep 10, 2017 4:39 am Post subject: |
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| Quote: | | About Chris`sR respond, you mean to focus the tube in other than the infinity point, |
Yes
| Quote: | | if so what would be the function to estimate the steps for that? |
Though you can work things out easliy based on a Thin lens mode, or a Thick lens model with estimations,
I wouldn't suggest that anything other than a trial.
Step as close as you can, then see what happens.
| Quote: | | And another thing what about the 200mm FL, in the Nikon AF range of lenses? |
Many many camera lenses have been used as tube lenses. Primes, and zooms at the long end, generally work well enough to use. As I said, it may be the case than only internal-AF-motor lenses will work.
As I said, there are zero options, off the shelf, for Finite objectives.
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Lou Jost
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| Posted: Sun Sep 10, 2017 5:32 am Post subject: |
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| Quote: | | As I said, there are zero options, off the shelf, for Finite objectives. |
Actually, some astronomy photographers use an automated extension tube which extends electronically. Very expensive though. I first learned about it somewhere on this forum. But I don't remember under what topic. If I recall correctly, the total change in length of the tube was small.
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Lou Jost
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All Ex
Joined: 20 Jul 2015
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| Posted: Sun Sep 10, 2017 6:29 am Post subject: |
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Sorry for the delay, I`m engaged in measuring the magnifications in several set ups for future shootings.
| CrisR wrote: | Though you can work things out easily based on a Thin lens mode, or a Thick lens model with estimations,
I wouldn't suggest that anything other than a trial.
Step as close as you can, then see what happens |
It seems to be a pain, let's hope that the outcome will worth all that procedure.
We can make other brands to shoot stills with Nikon, but I don`t know any AF Nikon lens (except for the beast portrait one) with 200mm FL to have an internal motor to do the focus shifting the D850 is capable for.
Except if we follow what Lou Jost said, .but that is another thing.
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Lou Jost
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mawyatt
Joined: 22 Aug 2013
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Location: Clearwater
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| Posted: Sun Sep 10, 2017 6:46 am Post subject: |
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| Lou Jost wrote: | | Quote: | | As I said, there are zero options, off the shelf, for Finite objectives. |
Actually, some astronomy photographers use an automated extension tube which extends electronically. Very expensive though. I first learned about it somewhere on this forum. But I don't remember under what topic. If I recall correctly, the total change in length of the tube was small. |
Lou,
These are called Crayford Focusers I believe.
Here's the Meade version that I have on my old 10 & 12" Schmidt Cassegrain Telescopes.
https://www.meade.com/meade-zero-image-shift-electronic-micro-focuser.html
Would be interesting to adapt one of these Crayford type focusers for stacking. If the standard DC motor was replaced with a small stepper, then the standard controllers from Cognisys (Stackshot), Wemacro or MJKZZ could be employed for automated stacking without the use of a focus rail.
Best,
Mike |
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rjlittlefield
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| Posted: Sun Sep 10, 2017 8:13 am Post subject: |
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| mawyatt wrote: | | These are called Crayford Focusers I believe. |
This should be a viable approach.
The best place for such a device will be between objective and tube lens. In that position, the rear lens will remain focused at infinity. The effect of the focusing extension will be to adjust distance between subject and objective, just like the focusing stage of a microscope. Being located in an infinity section of the optics, the added extension will have little effect on image quality, other than increasing or introducing vignetting.
You could also stick the focusing extension between the tube lens and camera. In that position, its major effect will still be to adjust distance between subject and objective. However, by extending the rear lens by the minimum thickness of the focuser, it will also force the rear lens to focus closer than infinity. That will introduce a certain amount of spherical aberration by forcing the objective away from its design point. The same as with AF motor focusing, the added aberrations will be small with low NA objectives, then becoming significant with high NA.
Helicon now makes a device they call Helicon FB Tube. It is an automated extension tube that comes in Canon and Nikon mounts. It's not clear to me whether the device has small enough focus steps for use with microscope objectives on the front, but if it does, that could be a handy device.
--Rik |
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mawyatt
Joined: 22 Aug 2013
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| Posted: Sun Sep 10, 2017 12:21 pm Post subject: |
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| rjlittlefield wrote: | | mawyatt wrote: | | These are called Crayford Focusers I believe. |
This should be a viable approach.
The best place for such a device will be between objective and tube lens. In that position, the rear lens will remain focused at infinity. The effect of the focusing extension will be to adjust distance between subject and objective, just like the focusing stage of a microscope. Being located in an infinity section of the optics, the added extension will have little effect on image quality, other than increasing or introducing vignetting.
You could also stick the focusing extension between the tube lens and camera. In that position, its major effect will still be to adjust distance between subject and objective. However, by extending the rear lens by the minimum thickness of the focuser, it will also force the rear lens to focus closer than infinity. That will introduce a certain amount of spherical aberration by forcing the objective away from its design point. The same as with AF motor focusing, the added aberrations will be small with low NA objectives, then becoming significant with high NA.
Helicon now makes a device they call Helicon FB Tube. It is an automated extension tube that comes in Canon and Nikon mounts. It's not clear to me whether the device has small enough focus steps for use with microscope objectives on the front, but if it does, that could be a handy device.
--Rik |
Would be an interesting experiment for sure!! Seems the needed adapters shouldn't be too hard to interface to the objective and tube lens for the objective to tube placement. I recall these Crayford Focusers use a dc motor and pulse the motor. This should work for our use but probably won't have the precise incremental movement desired for stacking. Replacing with a fine step stepper motor and controlling with Stackshot or other controller would be ideal.
Maybe someone will follow up on this and give it a try?
Best,
Mike |
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