Commercial 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. 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, but getting acquainted with its foibles requires practice.
Pro performance
The continuous-servo modes found on advanced compacts and semi-pro SLRs can be very effective in optimum conditions, but pro level systems offer the best all-round performance for commercial photography. Not only are they faster and more accurate, they can also track moving subjects, even when another person or object passes in front. Pro SLRs are less likely to be fooled into switching the focus onto the background if the subject temporarily drifts out of the AF area, making them perfectly suited to shooting field sports like football, where referees and other players frequently obstruct your subject.
Most professional SLRs are also equipped with predictive autofocus – a more sophisticated version of continuous-servo. In this mode the camera tracks subjects that are steadily approaching or retreating from the lens. It then predicts the focusing distance at the precise moment when the shutter opens, rather than locking the focus when the shutter is pressed, as with continuous-servo.
Cameras that feature one-shot and continuous AF typically work in conjunction with multi-area AF. Multi-area covers a much wider part of the frame than the default centre point and is designed to enable quick and easy focusing on off-centre subjects. The number of area AF points ranges from three to 99 and can often be linked to spot metering. In multi-area mode you can select the active focus point(s) manually, or let the camera select the AF area containing the object closest to the lens. Multi-area is a fantastic tool but remember that in awkward focusing conditions, the centre AF point is more likely to be successful than any off-centre point.
Problems, problems
Despite all its advancements, autofocus technology is still loaded with problems. One of the biggest limitations of cheaper cameras is that they offer a single, centre-frame AF point. Speed is another issue, even with entry-level SLRs, although by prefocusing or panning you can capture predictable fast-moving subjects.
Moreover, with the exception of pro SLRs, most continuous-servo modes are far less effective than their propaganda suggests, eating up battery power extremely fast. Another acute problem, particularly with compacts, is noise, which renders many cameras useless for wedding, wildlife and commercial photography. You may also find that your autofocus mechanism becomes sluggish when the working temperature becomes significantly colder or hotter than room temperature. Even cutting edge professional AF systems aren’t foolproof.
If you can’t focus properly, first try your camera’s AF lock facility. Focus on something else exactly the same distance away as your desired subject and activate the lock. Keeping the shutter button half depressed, recompose the photo correctly and depress the shutter button fully. Most cameras let you control whether the exposure and focus are locked simultaneously or separately.
Manual magic
Your second option is to focus manually. The human eye usually needs much less light to focus accurately than a lens, so this can be a good choice in low light. Most consumer compacts don’t offer manual focus. More advanced AF compacts let you choose the focusing distance from a set of predefined electronic steps, but this is fiddly and requires you to guess the subject distance. Further up the food chain, several prosumer compacts feature non-interchangeable lenses with manual focus rings on the lens barrel, enabling quicker, more accurate manual focusing. At the professional end, most decent interchangeable SLR lenses offer manual focusing in addition to AF operation.
Although SLR lenses generally offer the swiftest, most precise manual focusing, it can be tricky to judge whether the image in the viewfinder is accurately focused. This is because AF SLRs feature standard autofocusing screens rather than the specialist focusing screens found on manual focus SLRs. Designed to enable rapid and accurate focusing by hand, manual screens feature microprism and split-field prism focusing aids. These exaggerate poor focusing and require you to line up fractured image lines to focus correctly. Pro SLRs use interchangeable focusing screens, and autofocus models often offer a choice of replacement screens featuring various combos of split-prisms, microprism patterns, clear spots, cross hairs, grids, millimetre scales and reference circles. Pick one designed for general use or for more specific applications such as macro, astrophotography, architectural, commercial photography, moving subjects and telephoto.
If your SLR or budget doesn’t let you switch screens, there may be other manual focusing aids available to you. Some AF lenses let you fine tune focus manually after autofocusing. With certain lenses and AF SLRs, half depressing the shutter button while focusing manually tells the camera to emit a focus confirmation when it thinks you’ve nailed it.
For ultimate ‘micro-focusing’ there’s nothing like using Canon’s ‘Live-View’ via the LCD on the rear of the camera body. You can zoom into the subject either 5x or 10x and then manually focus for complete accuracy – a feature that becomes a ‘how did I ever live without this’ part of your photographic technique.
This article shows the quality and variety of techniques that customers benefit from as part of our commercial photography and architectural photography service. Call now to discuss your requirements and timescales on 079101 68536.
Copyright - Adam Coupe Commercial Photography 2008
