Saturday, January 8, 2011

How Do I use My Auto Focus? Part-1

There is a lot to cover when explaining auto focus, so it will take 2 installments to cover it all.

Auto Focus From The Beginning

The first auto focus SLR for general sale was released in 1978. That camera, the Polaroid SX-70 Sonar OneStep, used an active auto focusing system that sent out sound waves. Today's digital SLR's use a passive system that takes advantage of light coming in through the lens.

So, auto focus sure isn't anything new, having been around for 30+ years now.

How Does My Digital SLR Auto Focus?

It’s called SIR TTL passive phase detection - (Secondary Image Registration - SIR, Through The Lens - TTL).
It’s a pretty simple system actually. It’s used by most dSLR cameras today for that reason.

Wikipedia - GNU Free Documentation License
Here is how it works:
Light enters the lens and is reflected by the main mirror, but the main mirror (1.) is only 50% reflective.

Half of the light goes up through the pentaprism/pentamirror (4.) and to the viewfinder eyepiece (5.).

The other 50% goes on through the main mirror and gets reflected off the 100% reflective (and much smaller) secondary mirror (2.), which directs the light down through two side-by-side optical prisms (3.), to the auto focus module (6.).

A key point is that on its way to the auto focus module, each of the two optical prisms (3.) use the light coming from opposite sides of the lens, making 2 images. Those two images made from the opposite sides of the lens are then compared (secondary image registration) for similar light intensity patterns, the phase difference between the two is calculated to determine which way, and how much, the focusing motor needs to turn to achieve focus.
The camera knows when focus has been achieved because the 2 secondary images are then in phase and have no phase difference to detect.
See, I told you it was simple. ; ) But, make a note here, auto focus for video is different.

IMPORTANT: Auto Focus Notes
The way most photographers initiate auto focus is by pressing the shutter release button down 1/2 way. An option many digital SLR cameras offer is moving the auto focus function from the shutter release button to a button on the back of the camera, also known by the very inventive name - back button focus.

There are still situations when manually focusing your camera is the hot setup, because  auto focus has some limitations, like when:
  • There is little or no contrast between your focus point and the background. Like when the background and the subject are the same color.
  • When the focus point has objects in it that are at different distances from the camera. Like an animal in a cage and the cage bars and the animal are both in the focus point.
  • If a subject is dominated by regular geometric patterns. Like rows of windows in a building.
  • When the focus point has areas that are really bright and really dark. Like if the part of a subject in the focus point is half in bright sunlight and half in dark shade.
  • When background objects look larger than the subject. Like a building in the background.
  • When the subject has many fine details, resulting in little variation in brightness. Like a field of flowers, or trying to focus on a blank wall.
These limitations will also be noted in your cameras users manual.

 Focus Points
Most digital SLR cameras today have at least 3 focus points. However, more and more of the entry level cameras have as many as 11 auto focus points.

There are 2 kinds of auto focus points: vertical-type and cross-type.
Most of the focus points you see in the viewfinder are vertical-type focus points. Vertical focus points can only detect contrast in one dimension, and are not as accurate as cross-type focus points that can detect contrast in 2 dimensions. (Since vertical-type focus points detect contrast only along a vertical line, they are good at detecting the interruptions in vertical contrast caused by horizontal lines.)
The more cross-type focus points a camera has, the more accurate the auto focus is. Cameras that only have a single cross-type focus point (some entry-level cameras don't have any cross-type focus points) have it as the middle focus point. Cameras having multiple cross-type focus points array them around the middle of the focus point array. For example Nikon's new D7000 has  39 focus points, 9 of them being cross-type focus points, while Nikon's prosumer and pro cameras have 51 focus points, 15 of them being cross-type focus points.
Check your camera users manual for how to select from multiple focus points, and which points are vertical-type, or if any, cross-type focus points.

In Part-2 we'll cover - Auto Focus and Area Modes