Focus Calibration with Teleconverters
FoCal can be used to calibrate auto focus with teleconverters (also known as lens extenders or 'TC').
Digital cameras which support telecoverters know which lens is attached and additionally know when a lens is attached with a teleconverter.
A separate calibration value is stored inside the camera for the bare lens and for lens+telecoverter combination.
Focus calibrate the bare lens normally, then as a new calibration attach the teleconverter and calibrate again.
To use an example, given a 200mm prime lens and a 1.4x TC and a 1.7x TC.
Calibrate each of the following combinations:
- Camera + 200mm (bare lens alone)
- Camera + 200mm + 1.4x TC
- Camera + 200mm + 1.7x TC
Once calibrations are complete the camera automatically recognises each combination and automatically applies the correct adjustment as needed.
The distance required for calibration should take in to account the extended focal length, for example, 1.4x TC and 200mm lens the focal length would be 280mm (see link on calibration distances below).
Using teleconverters with zoom lenses is a similar process, calibrate the zoom lens bare and as a separate calibration zoom lens with teleconverter (see link on zoom calibration below).