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All About Focus

by Brooke Tasovac (follow)
Blog (339)      Articles (157)     
Image taken by Chris Smith

In photography, an image that is in focus will be referred to as clear or sharp. An image thatís not in focus will appear to be blurred, distorted or soft. Most of the time having a sharp image is preferable, unless youíre aiming for creative use of blur.

Focusing is done by locking on to a fixed point within the frame before the shutter is pressed to take the photograph. Most SLR cameras allow the user to choose Auto Focus or Manual Focus. Even though itís best to use manual for the other camera settings, Auto Focus is actually more commonly used by most photographers.

Why? Because it means the photographer can concentrate on composing an image and not risk missing a moment because they are trying to lock focus manually. When you use manual focus you are adjusting the lens ring, which is slower than when Auto Focus varies itself to suit the action happening in front of the camera, using the AF points.

What are AF points?

AF points are the rectangles arranged in a pattern (based on the rule of thirds) that flash when you press the shutter button. Auto Focus uses all of the available AF points to establish what area to focus on and it doesnít always focus on the subject you want. If there are a lot of things the camera can focus on within the depth of field it can become confused.



auto_focus

To override this you can select just one AF point (but only when shooting in manual, TV or AV modes). Check your camera manual about how to change the AF point on your camera from auto to a point that you choose. You can do this before you compose your shot or while looking through the viewfinder.



manual_af_points

You should choose your AF point based on the position of the subject or object youíre shooting. A good rule of thumb for photographing people is to focus on the eyes for a single person or the person in the centre of a group photo.

For landscapes choose the area where you want to draw a personís eye when they look at the photo (just be sure the depth of field is wide enough that the foreground and background are both sharp).



choosing_af_points
My SLR was confused by this photo - the autofocus kept switching between the water and my daughter. By choosing a certain AF point the focus was set completely on her.



choosing_af_points
I changed the AF point to the centre for the second shot so the water jet was in focus.

Whether you choose to use all your AF points or only one, you must choose a drive mode. Auto Focus can be used in one of three different drive modes. When the right mode is chosen, focus will generally be sharp on the subject you are shooting.
1) One Shot (for stationary subjects)
2) Al Servo (for fast moving subjects)
3) Al Focus (for subjects that might switch between a stationary and moving positions). This is a feature only available on a Canon SLR, not a Nikon.

With Al Servo and Al Focus the camera will refocus whenever the subject moves which makes it easier to capture subjects such as people or animals.

When should manual focus be used?

Although manual focus is not ideal for photographing moving subjects or fast paced events, there are certain situations where it is needed such as:

1) Subjects that are backlit, especially in bright sun
Sun flare can look beautiful in photos but it throws off the light meter and can make it hard for a photographer to lock focus. In this situation manual focus would be the best option, as well as maybe using a filter or a lens hood to prevent light from directly hitting the lens.

2) Macro photography
This is because the depth of field is so narrow in macro photography that the focus point can be thrown off by even the tiniest movement from the photographer or another source such as the wind blowing a single stalk of grass. Manual focus is necessary for this style of photography too.

3) Any scene where the depth of field may change
For example, I once set up my camera to take a self-portrait but as I moved from behind the camera to in front of it, the focus point was thrown off and the final image was blurry. By selecting manual focus instead, the final shot looked exactly as I originally set it up.

Still not sharp enough?

Sometimes even when a photo is focused properly it may not be as sharp as you want it to be. This could be for one of several reasons:

1) Your diopter needs to be adjusted

The diopter is the tiny wheel next to the viewfinder on an SLR that helps the photographer have a clear line of sight when looking through the viewfinder. It should show the AF points in focus.

If the view is even a little bit unclear it may be the reason why your photos are less sharp than they should be.

2) Your shutter speed is too low

Motion wonít be sharp unless the shutter speed is high enough to freeze it. Bump your shutter speed up for moving subjects.

3) Camera shake

If youíre shooting in low light or using a heavy lens, camera shake from your hands or the movement of the mirror within the camera might create some blur.

To prevent this use a tripod and a remote shutter release, or turn on the Image Stabilisation feature on your lens (if it has this option).

Back-button focusing is another technique professional photographers use to reduce camera shake. Turning on back-button focus is something that differs between camera brands and models.

4) Your depth of field is too shallow

When you select low aperture numbers youíre essentially telling the camera ďI only want a small area of this photo to be in focus.Ē If your depth of field isnít wide enough to fit in everything youíre trying to photograph, then the areas outside where the AF point is aimed won't be as sharp.

5) Youíre using an unsuitable AF mode
I often forget to switch my AF mode when I shoot. I use Al Servo when I photograph my daughter so that Iím always able to capture her while sheís moving and then forget to switch it for other subjects. Remember to adjust it at the start of every shoot.

Once youíve tried all these things, you can sharpen photos even more during post-processing. If you want to do this, you should shoot in RAW rather than JPEG. JPEG files have a lower resolution than RAW files, and are less likely to be sharp. JPEG images can look very pixelated after theyíve been compressed.

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