Q. How can I figure out the adapter length required?
A. The Adapter Effective Length
is the net distance the custom adapter will add to the
(If you are designing a SLR or DSLR lens adapter for an astronomical camera, please check
If you need an adapter consuming the minimum possible amount of back focus, just enter
0 for the Adapter Effective Length
to let Build-An-Adapter calculate the shortest possible
adapter that will connect the devices selected. The actual, calculated, length will be displayed
in the length field after the Build button is clicked. The length is displayed in inch or mm
according to the unit selected. The calculated value may or may not actually be zero; the minimum
length is constrained by the specific mechanical features required for this particular adapter.
Always check the 2D drawing displayed on the Build page for a visual confirmation of how the
effective length is measured.
When designing a custom adapter for a photographic lens
or for the camera side of a field flattener or reducer, it is important to note that a minimum length adapter will
almost certainly be inadequate: to reach focus or achieve a flat image field, a flattener,
reducer, or lens must be located at a precise Optical Distance
in front of the
The recommended Optical Distance
for a reducer/flattener is also known as
the Metal Back Distance
and the value is generally supplied by the device manufacturer.
To determine the Adapter Effective Length
for such optical system, the
Image Train Back Focus
needs to be calculated precisely and subtracted from this
- Adapter Effective Length = Optical Distance - Image Train Back focus
For example, if you have a flattener that requires an Optical Distance
of 55 mm and you want
to connect a camera having 17 mm of back-focus to that flattener, your Image Train Back Focus
is simply 17 mm and you will need a custom adapter with an Effective Length
of 38 mm
(55 minus 17) to end up with the correct spacing for that flattener.
If there is a filter wheel and/or other accessory in our image train between the reducer/flattener
and the camera, the thickness of each accessory should also be taken into account in our
calculations. The Image Train Back focus
is then the sum of these components
thicknesses and back-focus, then as before we use the formula
above and subtract the total Image Train Back focus
from our Optical Distance
Additionally, if there is any glass filter installed between the reducer/flattener
and the CCD (perhaps inside a filter wheel), the effective length may need a small correction to take
into account light diffraction through the filter glass: each 3 mm of glass thickness INCREASES
the Optical Distance
by about 1 mm and the correction must be ADDED in the formula,
therefore increasing the Adapter Effective Length
by the same amount to compensate for
the glass correction.
In our sample image train with the 17 mm back-focus camera, if we install a filter wheel
that has a thickness of 25 mm equipped with 3 mm thick glass filters, our total
Image Train Back-Focus
sums up to 42 mm (17 plus 25) and the Adapter Effective Length is
now 14 mm (55 minus 42, PLUS 1 mm for the filter correction).
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