jcolwell Offline Upload & Sell: On
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Canon recommends 50 times the focal length (30m) and LensAlign recommends half of that (15m). If you add an Extender, you should use the 'extended' focal length in these equations; i.e. with a 1.4x Extender, it would be 44m (Canon) and 21m (LensAlign).
I test my 500/4L IS bare and with 1.4x and 2x Extenders with the LensAlign II target at a distance of about 12m, which is the most I can easily accomplish indoors. I do a few spot checks on more distant targets outdoors, and it's always been fine.
If you have a prefered or typical subject distance, then use that; otherwise, pick something convenient and later do a few spot-check tests at other distances.
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