Page 73 - Friedman Archives

Focusing Essentials 73

3.4 FOCUSING ESSENTIALS

There’s more to focusing than you think, and with the A77 II Sony has
brought Live View image analysis front and center to enhance some
already very sophisticated focusing schemes. Many people have been
overwhelmed by the sheer number of choices and permutations. That’s
OK; I’m here for you and I’ll explain all of it.

So let’s start slowly, showing what a focus point needs to work properly,
and then expand to “how does the camera know which one to use” and
finally “under what conditions will it track a subject accurately?” and
“What do all the various focusing modes do?”.
A single Focus Point
Let's start from the basics. That semi-transparent mirror you see when you
remove the lens is there for just one reason: to siphon off some of the light
from your lens and route it UP to a special purpose Phase-Detect focusing
array. Those 79 focus points you read about in the camera’s marketing
material? They’re situated at the top of the mirror box; where the EVF and
the pop-up flash reside.
This Phase Detect Array has focusing points than any other camera ever
made (and it can be used in lower light than previous Sony models).
Figure 3-6 shows how these focus points are distributed throughout the
viewfinder:
Each one of these focus points can detect contrast – differences in light-
and-dark that are very close together – and the phase information from the
array allows the camera to know exactly how far to move the lens’ optics
(and in what direction) so it can focus quickly and accurately – usually
without hunting.

To demonstrate how a focus point works, let’s turn on just one and do an
experiment: With the exposure mode dial set to “P”, do MENU  3 
Focus Area  Center (or access it from the Fn menu if it hasn’t been
removed via customization). This turns on only ONE of the 79 focus
points – the center one – and everything outside that square is ignored.

If you were to point this square at a blank, featureless wall, the
autofocusing would get confused and “give up” (as evidenced by a flashing