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.
For a practical example how to calculate an adapter length please check this video.

Noting that a custom adapter designed at minimum length may not be adequate for a photographic lens, or for the camera side of a field flattener or reducer: 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 sensor.
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
In the simplest example, if we have a flattener requiring an Optical Distance of 55 mm and we want to connect a camera having 17.5 mm of back-focus to that flattener, the Image Train Back Focus is simply 17.5 mm and we will need a custom adapter with an Effective Length of 37.5 mm (55 minus 17.5) to end up with the correct spacing for the flattener.
This example, although simple, is not very realistic as image trains generally incorporate several different accessories inserted in front of the camera. Here is a more complete example:

We have a reducer requiring 86 mm of Optical distance, and we have both a filter wheel and an off-axis guider inserted in the image train between the reducer and the camera. Such devices clearly consume back focus so the thickness of ALL these accessories must be taken into account in 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.
For this more complete example we end up with an adapter effective length of 46.5 mm (86 - 10 - 12 - 17.5).
Additionally, if there is any glass filter installed between the reducer/flattener and the camera (eg. in the 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 example image train above, if our filter wheel is equipped with 3 mm thick glass filters, our corrected Adapter Effective Length is now 47.5 mm (86 minus 39.5 PLUS 1 mm for the filter correction).


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