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How and why to tape the pins on a Canon Teleconverter (Still Under Construction)

All Canon bodies other than the professional 1 series bodies allow autofocus with lenses with a maximum of f/5.6 or less (1 series bodies allow autofocusing with the central autofocus point up to f/8) so a common problem when using teleconverters is that autofocus is disabled. This is because canon have deemed the autofocusing accuracy to be too inacurate at the lower light levels encounted when using a lens at f/8. However, in bright situations, autofocusing at f/8 is still works reletivly well. The question is how do you trick the camera into thinking that there is no teleconverter present? Well some bright person found that if you put some tape over the three leftmost pins (when viewing the teleconverter from the lens side - see photo) the camera no longer 'knows' that there is a telconverter.

The result is a noticable lack of focusing precision, particulary if light is a little low or there is limited contrast, but for bright, high-contrast stiuations it almost works as normal.

I have used this technique with the Canon EF 400mm f/5.6 L USM lens with some success, often I have been unable to tell the difference between photos taken with and without the teleconverter. However I have noticed that the autofocus is not as accurate, when the conditions are bright it works pretty well, but when they are not you tend to end up with more out of focus pictures. This technique will work with the canon zoom lenses that have a maximum apertures of 5.6 eg. 100-400mm IS USM f/4-5.6 and 28-300mm f/4-5.6 IS USM.

Using a sigma 1.4x DG teleconverter with the Sigma APO 500mm EX HSM, taping the pins works very well, mainly due to the fact that the resultant 700mm f/6.3 lens is only one third of a stop slower than the required f/5.6. Autofocus is marginally less accurate but much more satisfactory than with the canon 400mm.

The technique will also work with the 2x teleconverter, but performance will be significantly worse. I wouldn't recommend using the technique with lens/teleconverter combinations that lower the maximum aperture to less than f/8.