Lens review

Canon EF-S 24 mm f/2.8 STM

6 May 2015
Arkadiusz Olech

10. Autofocus

The tested lens is equipped with the STM stepping motor drive. Its work is almost completely silent – when it is active you can hear just a delicate whirr. Still the speed is hardly impressive. Running through the whole distance scale and setting the focus takes about one second. Mind you it is not a fault of the elderly EOS 50D, used in our tests. We tested the Canon EF-S 24 mm f/2.8 STM also on the 7D Mark II and it was equally slow.

While taking sample shots with the tested lens attached to the Canon 50D the accuracy of the focusing mechanism didn’t bowl us over. In studio conditions the result was a bit better as the number of misses reached 8% - a decent result but not a sensational one, especially if you take into account the fact that you deal here with a wide angle device, not very fast aperture-wise.

It made us suspect that maybe the new STM motor finds it difficult to cooperate with such an old body as the EOS 50D. We decided to test it additionally using a much newer camera. After attaching it to the EOS 7D Mark II we set the f/2.8 aperture and took 50 photos of our resolution testing chart. After every shot the lens was defocused. It turned out perfect hits constituted only 12% of all shots. Good hits, which deviation from the perfection amounting to just 5-10%, made up 28%. The most of settings, as much as 38%, fitted a range from 10 to 15% of deviation from the perfect hit. There were 12% of huge misses, with the resolution worse than 20% from the best setting. It is clear the implementation of a newer body didn’t make the autofocus perform better.

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Canon EF-S 24 mm f/2.8 STM  - Autofocus

After attaching the tested lens to the Canon 50D it showed a very pronounced back focus tendency. The AF microadjustment by 15 units helped - after that the lens focused properly. In the case of the EOS 7D Mark II there were no such problems.

50D, f/2.8, before microadjustment
Canon EF-S 24 mm f/2.8 STM  - Autofocus
50D, f/2.8, after microadjustment
Canon EF-S 24 mm f/2.8 STM  - Autofocus
7D MkII, f/2.8
Canon EF-S 24 mm f/2.8 STM  - Autofocus