Friday, October 17, 2008

AF Area-Or How To Place Focus Where You Want It

A familiar scenario.

It may have happened to you that you came across a sight worth taking a picture of, you framed your subject, half pressed the shutter, made sure everything was set, and fired away. And you got a nice picture to show around.

However, that scenario not always plays itself that way.

You may have come across a sight you wanted to take a picture of, you framed the subject, half pressed the shutter and...the camera focused on something else. This happens especially if youre shooting in low light or low contrast conditions.

But why does it happen?

Meet the focusing sensors.

Your Sony Alpha DSLR (and every other camera brand DSLR) has focusing sensors.

The focusing sensors are those little stripes in the viewfinder that light up red when you focus with them. NOT to be confused with the thin black stripes on the very corners of the frame, those are to compose when shooting in widescreen mode (16:9 format)

Those focusing sensors are the camera's aid to correctly focus automatically on your subject. They work by detecting contrast, or how strong is the difference of the light values in your picture. Try shooting the blue sky with nothing else in frame and then with a subject in frame, like a leaf. Since the blue sky wont offer another light value for the sensors to compare, they will get confused and wont focus, but if you got another subject, they got something to compare against and one of them will focus.

When a sensor detects more contrast above all others, thats the one that turns red and you got focus lock, and thats the one the camera uses.

The camera's hard choice.

The problem is that sometimes the sensor the camera picked isnt the place where YOU want the focus to be at. The camera may have focused on the farthest sensor to the right and you actually wanted to use the upper left corner sensor.

This can ruin your shot and potentially make that you lose the moment.

The camera has to guess which sensor to use depending on which sensor is covering the subject. Most of the time it will guess correctly, but there will be times when you need to intervene directly.

AutoFocus technology has improved a lot since it was introduced in 1985, but there are still times where it wont do what you want it to.

This is why the Sony Alphas have a function called AF Area.

Know your AF Sensor count.

Before moving on to what AF Area is and how to use it, lets first list the AF sensors available to the Sony Alphas so you can know how many you have at your disposal.

Alpha 100: 9 focusing sensors, 8 lines and 1 center cross sensor.

Alpha 200: 9 focusing sensors, 8 lines and 1 center cross sensor.

Alpha 300: 9 focusing sensors, 8 lines and 1 center cross sensor.

Alpha 350: 9 focusing sensors, 8 lines and 1 center cross sensor.

Alpha 700: 11 focusing sensors, 10 lines and 1 center cross sensor.

Alpha 900: 9 focusing sensors, 8 lines and 1 center cross sensor plus 10 assist sensors.

By lines I mean the single line sensors around the viewfinder, the center cross sensor is the middle one, the square shaped sensor, its called center cross because its made of two sensors that run parallel to each other, this sensor is designed for apertures of 2.8 and larger. This sensor has more advantage over the others to acquire focus due to its double capacity to detect contrast.

From what Ive read about the A900, the 10 assit sensors are available when you use Wide AF (this setting will be covered in the next section). Its featured also in one of the videos about the A900 I posted in this thread.

AF Area explained.

The AF Area is a function that allows you to tell the camera to pick which sensor to use or you tell it which sensor YOU want to use.

There are three options you can chose with this function:

Wide: The camera has all the sensors available to pick from. In this mode you can compose a group shot and the camera will pick a sensor to focus with.

Spot: In this mode the camera will ONLY use the center cross sensor, nothing else.

Local: In this mode you can select any of the sensors in your viewfinder by moving the Controller (A100/200/300/350) or the Multi selector joystick (A700/900).

If you press the center button of the Controller or press the center of the Multi selector, it will force the camera to use the center cross sensor. This is a handy feature if you need a focus sensor picked NOW or to override a wrong sensor choice made by the camera.

Have it in mind that by doing this, you skip the part of half pressing the shutter, since the camera will achieve focus lock. In this case, press the shutter completely after achieving focus lock.

How to change AF Area selection.

A100: Turn your Function (Fn) Dial to Focus, half press the button in the dial and use the controller or the control dial to browse and select AF Area. There you will have the 3 options displayed.

A200/300/350: Press the Function (Fn) Button, select the AF Area window and select your desired option.

A700/900: There are two ways to access AF- Area with these cameras.

Menu: To select which AF-Area setting to use by going through the Menu:

Menu->Recording (camera icon) menu 3->AF Area->Wide, Spot or Local

Using the Fn button: Press the Fn button and using the Multi selector move to the third box of the second row from the top. That will be the AF Area window. You can either change it while using Quick Navi and the control dials or by pressing the center of the Multi selector to go to Exclusive screen.

Uses for the AF-Area settings.

This is a quick guide of scenarios in which you can use the 3 settings of AF-Area.

Wide: When taking group shots or shooting a moving object. The Wide setting will give you plenty of space to move and there will be a sensor that can be used if your subject goes to the frame's edges or the center.

Spot: Portraits, if youre shooting centered portraits, or product shots, this one will spare you the time the camera will take to chose a sensor to use. If youre shooting something in the center, this is the sensor to pick.

Local: When your shooting pictures where the focus has to be precisely at one point in particular, you use Local. It will allow you to set the focus anywhere you want within the sensor range. If youre shooting a portrait and you want the focus to be in the person's eyes and one of your sensors is over them, you chose this one.

The only drawback from this setting is that it takes longer to set (although Im talking of seconds or faster than that) than Wide or Spot. I suggest you use this on subjects you can take your time to shoot or when doing pictures with focus placed in a specific place.

The good news is that the Controller or Multi selector are pretty quick to use and set which sensor to use.


The Sony Alpha DSLRs do a good job of guessing the focus point of your pictures most of the time, the problem is that sometimes the system will not be able to resolve the situation and youll have to intervene.

Now that you know what AF-Area is and what are its possibilites, you can overcome those times where AF wont find a focusing spot or when you want to place the focus on a particular point in the frame, due to artistic or practical intentions. You may want to blur a part of your subject and place the focus someplace else, or you may want to avoid the hassle of changing the sensor each time or waiting for the camera to do it, or you just want the camera to do it itself.

Whatever your intentions when taking a picture, now you have another piece of know-how that will improve your control of the camera and maybe make the difference between a shot that looks good but its not what you want and a shot that is what you wanted in the first place.

Practice with this setting if you havent, it may help you on those situations where the human input is more accurate than the camera's.

Until next time.

No comments:

Post a Comment