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 photography 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 therefore the lens being considered must have a large enough Focal Distance to allow an adapter to be made. A custom adapter can only be fabricated if the lens Focal Distance is substantially larger than the camera or image train back-focus. This condition generally precludes using newer mirrorless camera lenses for this application as these lenses usually have a very short Focal Distance.
The adapter exact effective length can be calculated by subtracting the astronomical camera back-focus (including any installed accessory) from the lens Focal Distance.
  • Adapter Effective Length = Focal Distance - Camera 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, for example the Canon EF family. The Focal Distance is NOT RELATED to the lens focal length. A particular lens Focal Distance can be displayed in Build-An-Adapter by selecting that lens on the left side and the desired camera on the right side, and then simply specifying 0 for the adapter length. The lens Focal Distance will then be displayed in a pop-up window.
If the camera is the only component in the image train, the Camera Back Focus is simply the internal back-focus of the astronomical camera as quoted by its manufacturer.
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 these instructions are followed precisely and Build-An-Adapter issues a pop-up window stating that either the length entered is insufficient, or too large to preserve infinity focus, an adapter for this combination of lens and camera is probably not possible.
Please note that 3-D rendering is not available for s 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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