Q. I need an adapter to mount a SLR/DSLR lens on my astro camera.
A. Build-An-Adapter can create lens adapters for some common SLR or DSLR photographic lenses but please be aware that there are severe constraints in designing a lens adapter for astronomical purpose and the instructions below must be followed exactly to produce a custom adapter that will work for this application.
The adapter effective length is critical to preserve the lens infinity focus when installed on an astronomical camera. The adapter effective length must be calculated by subtracting the astronomical camera, or image train, back-focus from the lens Focal Distance.
  • Adapter Effective Length = Focal Distance - Image Train Back focus
The lens Focal Distance (aka Registration Distance) is a fixed value, generally specified in mm. This Focal Distance is the same for all the lenses in a particular family, such as for example Canon EF. The value is NOT RELATED to the lens focal length. A particular lens Focal Distance can be displayed in Build-An-Adapter by selecting the lens brand name and type on the left side and the camera on the right side, and then simply specifying 0 in the adapter length field.
If the camera is the only component in the image train, the Image Train Back Focus is simply the internal back-focus of the astronomical camera as quoted by its manufacturer.
A custom adapter can only be fabricated if the lens Focal Distance is substantially larger than the camera or image train back-focus.
Once both the lens Focal Distance and the camera, or image train, back-focus have been determined, the simple subtraction above is all that is needed to determine the adapter effective length:
(Please check this answer if you need help calculating your Image Train Back Focus.)
For example if the lens focal distance is 85 mm and the image train back-focus is 38 mm, the nominal effective length for the lens adapter will be 47 mm (85 minus 38). For safety we recommend shortening this length a little (the exact amount is lens dependent but 0.5 to 1 mm is generally adequate) therefore a safer effective length would be 46 mm for this particular example. Note however that a lens adapter cannot be shorter than a certain minimum since we do need some room for the adapter body itself. The minimum value depends on the selected lens type and on the device the lens is to be used with.
If a length error window appears in response to what appears to be a reasonable effective length, there is most probably not enough room to build an adapter for this lens and image train combination.
If these instructions are followed precisely and Build-An-Adapter issues a message stating that either the length entered is insufficient, or it is too long to preserve infinity focus, an adapter for this combination of lens and camera is probably not possible.
Please note that the 3-D drawing currently does not show the bayonet section of a lens adapter. The 2D drawing however is rendered correctly, including an indication showing where the adapter effective length is measured from.
Important: lens control functions, such as aperture and focus, are not available in lens adapters designed for astronomical cameras. Manual lenses are generally recommended for these applications.

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