Q. I need an adapter to mount a SLR/DSLR lens on my astro camera.
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
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. This causes a pop-up error but the message also informs the
lens Focal Distance
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 a lens adapter. The 2D drawing however is rendered correctly,
including markings 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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