I. CAMERA FEATURESAND OPERATION 10
Shutter Release
Techniques:
Half-Pressvs. Mash
focusmanually until you are “in the ballpark,” then try using AF for fine-tuning.
Additionally, thisis an instance where the distance range selector switch on “white”
lenseslike the EF 70-200mm f/2.8L USM can reduce the length of time the AF system
spendshunting for focus, if your subject distance happens to be beyond the range
stated on the lensswitchboard.
In One-ShotAF, it can take as long as a full second or more for the camera to achieve
focuscompletion in extremely low light without a focusing aid. Even with a Speedlite
featuring an AF-Assistbeam, low-light AF will be slower than bright-light AF. Thisis not a
design flaw or manufacturing defect; it’srelated to the sensitivity of the AF sensor.
Remember thatthe AF system is always passive, even when using an AF-Assist beam;
AFsearch in low light can be faster with an AF Assist beam than without one, but bright
lightAF speed will always be faster with a readable subject.
In AI Servo AF, the camera samplesthe AF detection data at varying rates of frequency
depending on the lightlevel. The brighter it gets, the higher the sampling rate and
therefore AFperformance improves. But as light levels drop off, the sampling rate
decreasesand a point is reached where the tracking ability of the AF system is
diminished. In other words, it’sunrealistic to expect AI Servo AF to track fast-moving
subjectsas well in low light as it does in bright light.
Again, you maynot always have control over lighting conditions, but it is important to
understand thatthere are limitations to AF performance according to the level of
available light.
Another factor thathas a lot to do with AF performance is shutter release technique. In
order to achieve the greatestpossible performance from your EOS camera, it isessential
to pressthe shutter button halfway and hold it there until the instant you are ready to
shoot, especiallyin AI Servo AF with moving subjects. No other way is nearly as effective
in termsof maximizing AF performance.
There are two issuesat stake here: the first is focus tracking and the second isa
phenomenon known as“release-time lag” or “shutter lag.” In One-Shot AF, pressing the
shutter button halfwaylocks the focus. When the camera is set for AI Servo AF and the
shutter button ispressed halfway, the AF system begins tracking the subject. If you
simplypress the shutter button down without waiting for the AF to function, the shutter
release mightlock up or be delayed in One-Shot AF. Under the same conditions, the
shutter willrelease without focusing in AI Servo AF. This is true with or withoutthe use
ofan AF-Assist beam. Conversely, pressing the shutter button halfway and holding it
there untilyou are ready to shoot allows the focusing system to lock focusin One-Shot
AFor start tracking the subject in AI Servo AF. Pressing the shutter button down all the
wayonce focusing has been established will release the shutter in the shortest possible
time, therebyincreasing the odds of capturing “the decisive moment.”