I am missing some good pictures because often when I go to take a picture, the camera doesn't respond. If I "unfocus" the picture, the camera will then adjust the focus and take the picture. I have to do this quite often. It will take pictures on manual focus. It tends to happen with my 18-55mm lens more than my 75-300mm lens, but happens with both. Thanks
It's too dark or not enough contrast. Anyway it is how they work. Nothing wrong. You need more light.
And an often overlooked way to get more light in such a situation is an external flash unit mounted in the hot shoe. Several of Canon's speedlites can be programmed to shine an "autofocus assist beam" in the near infra-red; and if the subject isn't too far away, this can often allow focus to be achieved. Because that light is invisible, or nearly so, it can be used even in situations where you don't want (or are not allowed) to use the "white" beam of the flash.
On some (most?) cameras the on-camera flash can also be used as an autofocus assist beam. But since it is visible (though dimmer than the full power of the flash), it can be used only in situations where flash photography is appropriate.
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