Canon RF100mm F2.8 L MACRO IS USM Instructions Manual page 11

SA Control Ring
When not using the SA control ring, return it to the central position (where it clicks) then slide the SA control
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ring lock switch to the LOCK position.
Using the SA control ring will change the angle of view and the exposure.
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So, after using the SA control ring, you will need to adjust your composition.
The SA control ring effect will differ depending on what you are shooting (focusing distance, distances
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between subject and background or foreground, etc.). The effect increases as the focusing distance gets
shorter than infinity (∞), and is at its maximum at 0.50 m/1.64 ft. The effect diminishes as you get closer than
0.50 m/1.64 ft., and vanishes at 0.26 m/0.86 ft.
The SA control ring effect will change according to aperture setting. The lower the f/number (larger aperture
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hole), the greater the effect, reaching its maximum when the lens is wide open. Conversely, the higher the
f/number (smaller aperture hole), the less the effect. For that reason, shooting in aperture-priority AE mode is
recommended.
Quickly turning the SA control ring may result in delayed focus.
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Autofocus (AF) can be used when the SA control ring is being used, but focus may be hard to achieve with
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certain subjects. If that happens, switch to manual focus (MF). Note that you may have to manually adjust
focus by enlarging the image, as the guide frame for the focus guide may not display properly.
Some subjects may cause focus to be lost when the SA control ring is used. If that happens, refocus the
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subject.
This lens reaches its maximum resolving power when the SA control ring is in the central position.
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