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 optical path.
(If you are designing a SLR or DSLR lens adapter for an astronomical camera, please check this answer.)
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 camera CCD.
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 Optical Distance, thus:
  • 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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