Photography Technique - Focusing

By Author: Adam Coupe
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Photography Technique - Focusing

Understanding the strengths and weaknesses of your camera's autofocus system is crucial if you want pin-sharp shots for commercial photography

All pro commercial photographers know the ability to achieve pin-sharp shots is essential. It doesn't matter how thrilling the subject or how inspired the composition, even a minute hint of blur can mark the difference between an award-winning shot and a second-rate snap. If the eyes of a portrait are slightly soft and the eyebrows and tip of nose area sharp, for instance, the image will lack impact and credibility.

Most modern cameras adjust the focusing automatically at the touch of a button. Good quality autofocus systems can move the lens elements into position faster and more accurately than the human hand and eye can. This makes them invaluable for sports, documentary, natural history, paparazzi and fashion photographers and commercial photography, not to mention anyone with less than 20-20 vision.

Nailing the focus for every shot is far from straightforward, involving considerably more user intervention ...
... than a point and shoot approach. This is because the speed, complexity and accuracy of AF systems varies dramatically from camera to camera and in different shooting conditions. Ultimately, autofocus is just a mechanical tool that's prone to errors and suffers limitations like any other man-made technology. The good news is, if you understand how your camera's AF system works and what its shortcomings are, you'll be well equipped to anticipate and compensate for it's failings.

Active and passive

Autofocus systems come in two different flavours - active and passive, both of which have distinct advantages and disadvantages. Active autofocus is commonly found on compacts and works on the same principle as radar, emitting infrared beams that bounce off the subject. By measuring the angle and size of the reflected beams plus the time delay between the signal emission and reception, the camera calculates the distance between the CCD and subject.

The benefits of passive autofocus are its high speed and ability to work in pitch darkness. The main downside is that it's only effective at distances of around six metres, which forces the camera to rely on depth of field measurements to ensure that distant objects are in focus - especially important for architectural photography. This trick works with normal to medium focal length lenses, but not with telephoto lenses.

Passive autofocus works like the human eye, analysing an image's contrast levels to determine whether it's in focus and adjusting the lens until the lines and edges exhibit maximum sharpness. The key principle behind passive AF is that objects in focus will have sharper edges and higher contrast. The downside is that it struggles in dim light and with low contrast subjects, and is prone to 'hunting' for sharper focus points after pin-sharp focusing has been achieved.

The advantage of passive AF is that it works exceptionally well with long lenses. Many SLRs also have a built-in AF assist illuminator that shines a high-contrast pattern onto the subject, giving the camera a target to lock onto when lighting is poor. AF assist lamps only have a maximum range of around three metres. If the beam is out of range, try the AF assist lamp of an external flashgun, as this is likely to be more powerful. Although advanced automatics and SLRs favour passive autofocus, some take advantage of both systems, using active AF for close-ups and passive AF for distant subjects.
The actual mechanics that drive your autofocus system also have a bearing on its effectiveness. SLR lenses with built-in motor drives, for example, are generally considered to be faster, quieter and more efficient than lenses whose elements are focused via a motor inside the camera body.

Focusing modes

You'll typically find two core autofocus modes on advanced cameras: one shot/single-servo AF and continuous-servo/AI servo AF. Single-servo is designed for shooting relatively static central or off-centre subjects such as portraits, still lifes and landscapes. You focus by half-depressing the shutter button and when the camera has focused it emits a confirmation light and/or beep. Single-servo cameras are customarily in 'focus-priority' mode and the shutter can only be released when the focus confirmation is displayed.

In continuous-servo mode the camera adjusts the focus continuously while the shutter button is half-depressed, until you fully depress it. Unlike single-servo, the shutter can be released at any time, a feature known as 'release priority'. Continuous-servo is designed to track moving subjects for example in sport photography, but getting acquainted with its foibles requires practice.
Adam Coupe is pro commercial photographer specialising in commercial photography, architectural photography and architectural interior photography for a wide range of organisations that need to project their products, people or brand in a vibrant way see for the full architectural photography and commercial photography portfolio : http://www.adamcoupe.com