BH2 to DSLR distance confusion
BH2 to DSLR distance confusion
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
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
Re: BH2 to DSLR distance confusion
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?
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?
Re: BH2 to DSLR distance confusion
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.
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.
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Re: BH2 to DSLR distance confusion
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.
Re: BH2 to DSLR distance confusion
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.
Re: BH2 to DSLR distance confusion
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?
Re: BH2 to DSLR distance confusion
Thanks Hans, Just checked as you suggested.hans wrote: ↑Thu Jul 01, 2021 9:11 pmNot 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?
Apart from Magnification difference of about 0.4X, and 1 exposure stop, I can't see any quality difference.
Re: BH2 to DSLR distance confusion
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?
Re: BH2 to DSLR distance confusion
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.
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Re: BH2 to DSLR distance confusion
Just to be clear which viewing eyepieces are you using?
Re: BH2 to DSLR distance confusion
OK, the ideal should be x10 (NFK 2.5 * x4 objective)/hans wrote: ↑Thu Jul 01, 2021 10:50 pmNot 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.
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.
Re: BH2 to DSLR distance confusion
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.
Re: BH2 to DSLR distance confusion
the standard WHK 10x/20 LScarodactyl wrote: ↑Thu Jul 01, 2021 11:03 pmJust to be clear which viewing eyepieces are you using?
Re: BH2 to DSLR distance confusion
Im talking about this section - just 3 screws and a washer:hans wrote: ↑Thu Jul 01, 2021 11:24 pmThe 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.
Re: BH2 to DSLR distance confusion
Ah, I missed that you said phototube, yeah that looks a lot less invasive than the procedure for the eye tube.