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Intro to Autofocus

In-camera autofocus can be 99% accurate with breathtaking speed, but doing so requires some basic knowledge and input from the camera operator. Unfortunately, hitting a focus target right all the time requires a little more effort than point and shoot. But with some careful practice and knowledge of your camera’s ability, high rates of perfection may be obtained.

In-camera autofocus can be 99% accurate with breathtaking speed, but doing so requires some basic knowledge and input from the camera operator. Unfortunately, hitting a focus target right all the time requires a little more effort than point and shoot. But with some careful practice and knowledge of your camera’s ability, high rates of perfection may be obtained.

Focus Points

Today should be the last day you snap a photograph without selecting a focus point. The fact is, no matter how great a camera’s autofocus, a computer will never be able to predict where you want to focus. Generally, cameras are trained to focus on objects towards the center and closer to the foreground. When shooting at narrow apertures, with small sensor point and shoots, or on wide angle lenses, there is usually enough depth of field to still obtain a decent focus, even if the camera misses a few feet. However, with great quality lenses that allow apertures of 2.8 and wider, large sensor DSLRs, or close up telephoto shots, it is common to see a depth of field of merely a few inches. At these settings, even focusing on a person’s nose may leave their eyes a soft blur. I will never forget my first close up portrait with my new 50mm f/1.8 lens set at 1.8 without selecting a focus point. As my subject turned sideways, my in-camera autofocus system declared her shoulder to be the target. Unfortunately, her stance put her shoulder about 8 inches in front of her eyes, well beyond the 4” of depth of field I had to work with. As a result, my stunning picture (which still looked decent on my 3” camera screen) was horribly out of focus when viewed later on a full size monitor. This marked the last day of auto focus point selection for my camera, and the beginning of a new focusing thought process. Most cameras have more than 7 focus points, each user selectable, with at least the center focusing point allowing for cross-type focusing. By placing the subject in front of one of these selected points, you will guarantee correct camera focusing at this position. Often I will simply select a center focus point, focus my subject in that point, and then recompose my frame. I will go into greater detail with a how to lesson on setting focus points.

Drive Modes

Most DSLRs have at least 3 drive modes: A single shot, a continuous focusing mode, and an AI mode that switches from single to continuous when the camera detects a moving subject. For everything that is either non-moving or moving perpendicular to you, a single shot focus will yield the best results. In this mode, the camera will focus and hold the setting until you release the shutter. This mode is especially useful for recomposing after focusing. During continuous focusing, the camera will never stop measuring focus and will continually track the object as it moves. When objects are moving towards or away from the camera, continuous focus tracking must be engaged to maintain focus as the subject moves. Knowing your subject will help determine which drive mode to engage and will help guarantee accurate focus in all types of shooting.

The Lens

Not all lenses are created equal and this is really an understatement. There are many factors of the lens that may influence focus from motor speeds, quality of glass, and maximum aperture. Most new lenses have an internal focus motor that moves the lens elements to focus on the object you select. As you move up in quality, lenses will commonly function with high speed and almost silent precision. Inexpensive lenses will often yield clunky, loud focus with mixed results. Obtaining high quality lenses will assist your focusing ability and will yield better results. Poor quality lenses will also have sharpness issues at some focal lengths. As a result, it may be common to see muddy images even after selecting your focus points perfectly. This effect is most commonly seen on the edges at extreme focal lengths and at wide apertures. The aperture settings are important to pay attention to when focusing. With less expensive lenses, a maximum aperture of f/3.5 eases the pain of missed focus by ensuring a wide range of area is always in focus. When the camera misses focus by a small amount, the affect is almost indistinguishable and the picture will appear in focus. During close-up shooting or wide open apertures when the depth of field is very narrow, missing your focus by only a couple of inches may be the difference between a usable and unusable photo. Beginner lenses such as the 50mm 1.8 are a great way to practice accurate focusing as they allow shallow depth of fields making it easy to see the area you have focused on.

While it is easy to consider auto focus to be fully automatic, there are many key elements to understand and to ensure the camera functions correctly. Knowing these and practicing with focus points will help to take the guess work out of focusing and ensure better photographs every time.

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