BH2 to DSLR distance confusion

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qanunji
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BH2 to DSLR distance confusion

#1 Post by qanunji » Thu Jul 01, 2021 12:55 pm

Hi,
I've mounted my EOS 6D on a BH2 trino (NFK 2.5) with the suggested distance of 151 mm to the focal plane.
Parfocality is then off by 76 um.
When I mount the camera at 185 mm, then its perfectly parfocal.
Is that normal? what distance should I follow?
Thanks'
Ariel

hans
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Re: BH2 to DSLR distance confusion

#2 Post by hans » Thu Jul 01, 2021 6:14 pm

No experience with BH2 but there is an adjustment for mechanical tube length on the binocular head:
https://www.microbehunter.com/microscop ... 798#p90724
Not sure if the adjustment has enough range to explain the difference you see, but possibly an explanation if your trinocular head was disassembled to that extent at some point and not correctly readjusted after?

PeteM
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Re: BH2 to DSLR distance confusion

#3 Post by PeteM » Thu Jul 01, 2021 6:41 pm

Assuming you're using an OEM setup, you might double check diopter adjustments on your eyepieces to see if they are set to the zero position to begin. Get your DSLR in focus on the trinocular tube, then adjust the eyepieces to be in focus for your eyes.

If making a trinocular camera adapter, I'll eyeball the distance with live view to a couple mm and then build in some adjustment capability. Commercial setups -- even many cheap Chinese ones -- typically use a thread adjustment on the vertical tubes, secured by a setscrew once in focus. Home-brew setups can be as simple as closely slip-fit tubes with a set of thin "washers" added or subtracted to get things just right. The slip-fit tubes are then locked in place by a set screw. Some older OEM setups do the same but have the setscrew bear against a locking collar. Helicoids can also be used if lockable in place.

Scarodactyl
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Re: BH2 to DSLR distance confusion

#4 Post by Scarodactyl » Thu Jul 01, 2021 8:21 pm

Really a hrlicoid doesn't even need to be lockable. It won't slip on its own under the weight of a camera unless it is defective.

qanunji
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Re: BH2 to DSLR distance confusion

#5 Post by qanunji » Thu Jul 01, 2021 8:40 pm

PeteM wrote:
Thu Jul 01, 2021 6:41 pm
Assuming you're using an OEM setup, you might double check diopter adjustments on your eyepieces to see if they are set to the zero position to begin. Get your DSLR in focus on the trinocular tube, then adjust the eyepieces to be in focus for your eyes.
Thanks Pete, I've already did what you suggested, + checking with other eyepieces + checking with other NFK's + checking with and without BH2-CA + I'm using a helicoid.
No Change. Parfococality is at 185 mm.

hans
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Re: BH2 to DSLR distance confusion

#6 Post by hans » Thu Jul 01, 2021 9:11 pm

Not sure what the 151/185 difference would correspond to in terms of intermediate image position (are the NFK behaving similar to a lifted eyepiece with real intermediate image, or more like a teleconverter with no real intermediate image?) but seems like a pretty large difference. Have you compared images at both distances with 100X objective (most sensitive to tube length variation) to see it there is any noticeable difference in image quality?

qanunji
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Re: BH2 to DSLR distance confusion

#7 Post by qanunji » Thu Jul 01, 2021 9:41 pm

hans wrote:
Thu Jul 01, 2021 9:11 pm
Not sure what the 151/185 difference would correspond to in terms of intermediate image position (are the NFK behaving similar to a lifted eyepiece with real intermediate image, or more like a teleconverter with no real intermediate image?) but seems like a pretty large difference. Have you compared images at both distances with 100X objective (most sensitive to tube length variation) to see it there is any noticeable difference in image quality?
Thanks Hans, Just checked as you suggested.
Apart from Magnification difference of about 0.4X, and 1 exposure stop, I can't see any quality difference.

hans
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Re: BH2 to DSLR distance confusion

#8 Post by hans » Thu Jul 01, 2021 9:55 pm

qanunji wrote:
Thu Jul 01, 2021 9:41 pm
Apart from Magnification difference of about 0.4X...
Do you have a stage micrometer or some other reference so you can tell if one of sensor distances is giving the rated 2.5X magnification of the NFK?

qanunji
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Re: BH2 to DSLR distance confusion

#9 Post by qanunji » Thu Jul 01, 2021 10:21 pm

hans wrote:
Thu Jul 01, 2021 9:55 pm
qanunji wrote:
Thu Jul 01, 2021 9:41 pm
Apart from Magnification difference of about 0.4X...
Do you have a stage micrometer or some other reference so you can tell if one of sensor distances is giving the rated 2.5X magnification of the NFK?
yes, great idea.
well, at x4 objective and x10 eyepiece, FOV @ eyepiece is 4.9 mm.
camera (with NFK 2.5) - 2.6 mm @ 185 mm, 3.3 mm @ 152 mm.
can you do the math?

hans
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Re: BH2 to DSLR distance confusion

#10 Post by hans » Thu Jul 01, 2021 10:50 pm

Not sure, but possibly FOV might be different between normal eyepiece and NFK, or vary in a confusing way as the image positions are changed relative to the NFK. Probably better to measure a know distance on the micrometer in pixels at each sensor distance, then use the specified pixel pitch of your sensor to establish total magnification object-to-sensor.

Scarodactyl
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Re: BH2 to DSLR distance confusion

#11 Post by Scarodactyl » Thu Jul 01, 2021 11:03 pm

Just to be clear which viewing eyepieces are you using?

qanunji
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Re: BH2 to DSLR distance confusion

#12 Post by qanunji » Thu Jul 01, 2021 11:16 pm

hans wrote:
Thu Jul 01, 2021 10:50 pm
Not sure, but possibly FOV might be different between normal eyepiece and NFK, or vary in a confusing way as the image positions are changed relative to the NFK. Probably better to measure a know distance on the micrometer in pixels at each sensor distance, then use the specified pixel pitch of your sensor to establish total magnification object-to-sensor.
OK, the ideal should be x10 (NFK 2.5 * x4 objective)/
185mm gives x13.8, 152mm gives x 10.9.
so, the "recommended" distance is much closer in terms of magnification, but I still got the par focality discrepency.
I'll try tomorrow to adjust the mechanical length of the phototube, maybe it'll balance it.
Thanks.

hans
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Re: BH2 to DSLR distance confusion

#13 Post by hans » Thu Jul 01, 2021 11:24 pm

qanunji wrote:
Thu Jul 01, 2021 11:16 pm
I'll try tomorrow to adjust the mechanical length of the phototube, maybe it'll balance it.
The manual makes it sound like that is normally fixed with adhesive after being set, so probably not something you want to adjust casually unless all other possibilities are eliminated or it looks like it has already been messed with. I would wait a day or two and see if anyone with actual BH2 experience has suggestions as to what might cause the discrepancy, like maybe incorrect eyepieces as Scarodactyl is suggesting.

qanunji
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Re: BH2 to DSLR distance confusion

#14 Post by qanunji » Thu Jul 01, 2021 11:30 pm

Scarodactyl wrote:
Thu Jul 01, 2021 11:03 pm
Just to be clear which viewing eyepieces are you using?
the standard WHK 10x/20 L

qanunji
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Re: BH2 to DSLR distance confusion

#15 Post by qanunji » Thu Jul 01, 2021 11:31 pm

hans wrote:
Thu Jul 01, 2021 11:24 pm
qanunji wrote:
Thu Jul 01, 2021 11:16 pm
I'll try tomorrow to adjust the mechanical length of the phototube, maybe it'll balance it.
The manual makes it sound like that is normally fixed with adhesive after being set, so probably not something you want to adjust casually unless all other possibilities are eliminated or it looks like it has already been messed with. I would wait a day or two and see if anyone with actual BH2 experience has suggestions as to what might cause the discrepancy, like maybe incorrect eyepieces as Scarodactyl is suggesting.
Im talking about this section - just 3 screws and a washer:
2021-07-02_02-28-40.jpg
2021-07-02_02-28-40.jpg (58.32 KiB) Viewed 5197 times

hans
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Location: Southern California

Re: BH2 to DSLR distance confusion

#16 Post by hans » Fri Jul 02, 2021 12:02 am

Ah, I missed that you said phototube, yeah that looks a lot less invasive than the procedure for the eye tube.

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