Canon EOS 750D Instruction Manual page 213

If focus is not achieved with the shooting conditions on the preceding
page, set the lens focus mode switch to <MF> and focus manually.
If you use AF with any of the following lenses, focusing may take longer
or correct focus may not be achieved.
EF50mm f/1.4 USM, EF50mm f/1.8 II, EF50mm f/2.5 Compact Macro,
EF75-300mm f/4-5.6 III, EF75-300mm f/4-5.6 III USM
For information on discontinued lenses, refer to the Canon website.
If you shoot a peripheral subject and it is out of focus, aim the center AF
point or zone over the subject to focus, focus again and then take the
picture.
The AF-assist beam will not be emitted. However, if an EX-series
Speedlite (sold separately) equipped with an LED light is used, the LED
light will turn on for AF-assist when necessary.
In magnified view, focusing may be difficult due to camera shake. Using
a tripod is recommended.
Using AF to Focus (AF Method)
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