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ASI camera + Nikon lens - does the spacing have to be perfect?

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#1 joelin



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Posted 15 February 2020 - 04:40 AM

So I want to put my ASI camera with my Nikon lens...with the combination of adapters or spacers I have, I might be 1-2mm too short or too long of the 55mm distance I need. 


Is it better to be too long or too short?


I also wanted to confirm, the back focus distance of Nikon and Canon DSLR lenses is 55mm correct?

#2 Coconuts



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Posted 15 February 2020 - 06:59 AM

You want to focus on the Flange Focal Distance (lens flange to focal plane).  You also want to get this right to a fraction of a mm. 


The FFD of Nikon F mount DSLRs is 46.5 mm; that of Canon EF and EF-S mount DSLRs is 44.0 mm. 




You should try and match this exactly, subtracting, in each case, the sensor to front plate depth of your camera.  It's best to buy an adapter specific for your lens type and camera.  ZWO offers Nikon and Canon adapters for their full-frame ASI cameras (17.50 mm sensor depth from M54 x 0.75 mm tilt plate):





A diagram for these is shown below.  Note that they make the adapter length slight less (~ 0.2 mm) than a simple FFD - 17.5 mm calculation would indicate; that is to allow a bit of forward movement of the tilt plate to allow for tip-tilt adjustment.  If you don't need to adjust the tilt plate, a thin shim between the tilt plate and the camera body can be added.  That would be 0.2 mm for the Canon adapter, and a different value (no idea why) of 0.18 mm for the Nikon adapter.  0.2 mm would work fine for either case.


Now the above doesn't take into account any use of a filter between the lens and the camera.  Introducing a filter shifts the enses focal plane further back, and requires that a shim be added to increase the adapter length.  A good rule of thumb for this is that the shim (which can go between the tilt plate and the camera body) should be one third the thickness of the filter.


And there is one last twist to consider.  Many camera lenses lack a manual aperture adjustment ring.  While there are tricks you can try to set the aperture on a DSLR and then switch to the astrocamera, the best solution is to purchase a dedicated adapter that is also a USB ASCOM lens controller.  This allows software control of both aperture and focus.  The best option here, at least for Canon lenses, is made by Astromechanics:




Note that these adapters assume that you use about a full mm of the tilt plate to camera body adjustment.  A gasket of some sort will be required in that case to prevent any stray light from reaching the sensor.  I am eagerly awaiting the release of their new Canon full frame lens controller for my new ASI6200MC; this is due out in the next few weeks.


Camera lens adapters.JPG


Hope this helps!


All the best,




#3 james7ca



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Posted 15 February 2020 - 08:03 AM

You can be short but you never want to be too long. Too long and you won't be able to focus at infinity. Too short and you should be able to use the focus adjustment on the lens to make up the difference (assuming that the lens has a way to manually focus -- if it has internal focus with no manual adjustment then the lens can't be used for scope-based astrophotography when using a dedicated astro camera -- that kind of lens will have to be used with a Nikon camera body so that it can auto focus).


So, given that the sensor to flange distance on a Nikon F-mount  lens is 46.5mm (NOT 55mm) you what to be slightly under that spacing. Note, if you put 55mm of spacing between the lens and the sensor you won't be able to focus at infinity. You also don't need to be within fractions of a millimeter on the spacing (if you try to reach exactly 46.5mm you may end up not being able to get perfect focus -- if you happened to be a fraction of a millimeter too long). As long as you are a little short you should be okay. That said, any lens that uses so-called floating lens elements will want to be used pretty close to the Nikon specified lens flange to sensor distance of 46.5mm.

Edited by james7ca, 15 February 2020 - 09:29 AM.

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#4 Iamhondo



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Posted 16 February 2020 - 01:37 PM

You already have some good advice above. (Yes, definitely not 55mm and definitely don't be too long.)


I just have a "tip-in" suggestion. If you get the ZWO lens adapter for EOS you can use Canon EF lenses. With an inexpensive Nikon/EOS adapter, you can attach Nikkor lenses (Nikon-F mount) to the EOS adapter.


So instead of buying 2 ZWO adapters ($40-50 each), you only need the EOS adapter and Nikon/EOS adapter (about $20).


More options!




#5 Coconuts



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Posted 16 February 2020 - 05:07 PM

Joe: Good point, but bear in mind that two bayonet connections in series increases the risk of camera/sensor tilt, to which fast / full frame camera lenses are very sensitive.  It helps, especially for heavier camera lenses, to support them in a scope ring with a split plastic sleeve.  More on that theme here:




All the best,



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