Important Introduction
Understanding focus modes is key to getting the photo that you want in a hurry – it’s the difference between the photo being in focus and out of focus. You’ve probably struggled to focus in the past and not been entirely sure why it just won’t lock on exactly what you want it to, but don’t fear, this post will tell you everything you need to make sure you never lose focus again.
One Shot / AF-S
This is the simplest of all of the focus modes and it does exactly what it says on the tin – it focuses for one shot. You would typically shoot on this mode when shooting a subject that’s not moving as the camera will only focus once when you depress the shutter button halfway. This is the mode that you would use if you wanted to use the focal lock of the camera, to focus on the subject and then move the camera, to recompose the frame. The camera will not focus again on this mode until you lift up the shutter button and depress it again.
AI Servo / AF-C
This is often referred to as continuous focus because it will focus when you partially depress the shutter, but will monitor movement in the frame and make any necessary adjustments in focus between the shots, without the need for removing your finger from the shutter button. This mode is useful if you’re going to be shooting moving subject, such as at a marathon and other sporting events. You would not be able to use the camera’s focal lock to recompose a shot in this mode, you&
#039;ll find that the camera will continually try to focus.
AI Focus / AF-A
This is probably the least understood mode and it’s actually a mixture of the 2 modes above. When the camera has only slight movement it will act as if it’s on One Shot / AF-S mode and will allow you to use the focal lock feature. When the camera detects movement, the focus mode will start acting like AI Servo / AF-C mode instead and track the subject. This may sound like the best focus mode to use, but I typically like to set to one of the other modes as I usually know what i’m shooting and what to expect. This mode does come in handy though when you’re shooting still objects that are like to move without much notice, such as a bird on a perch. 
If you have any questions, please feel free to leave a comment and I’ll get back to you.
