In "One Shot" mode, the camera uses "Focus Priority"
In "AI Servo" mode, the camera uses "Release Priority"
One Shot mode is meant for non-moving subjects. "Focus Priority" means that when you press the shutter button (completely) the camera prioritizes the focus as more important than the moment. It wont take a shot if the lens is in auto-focus mode unless it can confirm being able to achieve focus.
AI Servo mode is meant fo moving subjects (subject to camera distance is changing). In this mode the camera must continuously try to re-focus. But as this is used for action/sports, where the "decisive moment" of when you take the shot is all important... the camera prioritizes the moment (when you press the shutter release button) as being more important than focus. It WILL take the shot when you fully press the shutter button even if the shot was not focused.
This "release priority" (even when not in focus) isn't necessarily a problem, because sports photographers will half-press the shutter to keep the camera continuously focusing as they track the action... and at the moment when they completely press the shutter to take the shot, the subject would already be in focus.
When you switch the lens to manual focus mode, then the camera no longer cares about focus because ... that's the photographers job to focus. By going to MF mode, the camera is willing to take the shot immediately.
There is a focus ring on your lens and you can manually focus it (unless the lens is somehow jammed or broken).
Your camera likely came with the EF-S 18-55mm f/3.5-5.6 IS II. This has a basic direct drive DC motor. It's not particularly fast, nor quiet, and it's less reliable. The newer EF-S 18-55mm f/3.5-5.6 IS STM (note the "STM" suffix) is the updated version. It has updated optics so it's just a bit better (although the old lens was pretty good already so this wont be a huge change) but the STM focus motor is much quieter and a bit faster.
If you see the letters "USM" in the lens name, those lenses use Ultra-Sonic Motors (USM). There are a few different types of USM that Canon uses, but they are usually pretty fast and snappy (there are exceptions... a few USM lenses are not known for their speedy focus).
If you see the letters "STM" in the lens name, those are "STepper Motor" technology (STM). They are also fairly fast and smooth focusing lenses, but usually not quite as fast as the USM motors. But they are known to be extremely quiet (part of the reason they were introduced is to make motors that were so quiet that you wouldn't hear the focus motors if recording video and using the internal mic. They are nearly completely quiet.
The USM motor lenses typically have a clutch that provides a gentle amount of friction on the focus ring. This gives you full-time manual focus capability because if the lens motor tries to turn while you're trying to manually focus, the clutch will just let the ring slip without resulting in damage.
The STM motors are electronic "focus by wire". There is no mechanical linkage from the focus ring on the lens barrel to the focusing elements inside the lens. It's simply an electronic input. The lens notiices you turning the focus ring so it tells the motor to adjust the focus. Since it's electronic "focus by wire", you can rotate the focus ring at any time without risk of damage. (BTW, it will only operate when the lens is attached to the camera, powered up, and you need to wake up the focus system (usually half-press the shutter wakes up the focus system.).
The direct drive motors (lenses that don't say "USM" or "STM") do NOT allow for "full time manual focus". You should flip the switch to manual focus to disengage the focus system before attempting to manually focus the lens. If you fight the focus motor, it is possible to damage this type of lens.
But also, the EF-S 18-55mm f/3.5-5.6 IS II doesn't have a particularly high build quality... some lenses are built like tanks...this is not one of those lenses.
Tim Campbell
5D III, 5D IV, 60Da
