Sigma 4.5 mm f/2.8 EX DC CIRCULAR FISHEYE HSM
10. Autofocus
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The Sigma 4.5 mm is equipped with a silent HSM motor. It works really fast but the credit shouldn’t go only to the mechanism. The second reason is the distance scale per se, which contains points for 0.135, 0.2 meter and infinity only. The motor requires less than 90 degrees movement of the focus ring to run through the whole scale. It’s not a difficult task and even without an HSM the body plus the lens team would deal with it quickly enough.
The question arises whether an HSM motor is really indispensable here. You can even go further and question the necessity of existence the autofocus mechanism itself. With 4.5 mm focal length and f/2.8 when you set the sharpness at 0.4 meter you get the depth of field from 0.2 meter to infinity. On stopping down the lens to f/5.6 we don’t have to think about the sharpness at all. It’s enough to set it at 20 cm and we get a sharp picture from 10 cm to infinity…
Speaking about autofocus misses or potential front and/or backfocus tendencies is also a bit unfounded here.
