| Photography Technique – Focusing | | | | around six metres, which forces the camera to rely |
| Understanding the strengths and weaknesses of your | | | | on depth of field measurements to ensure that |
| camera's autofocus system is crucial if you want | | | | distant objects are in focus – especially important |
| pin-sharp shots for commercial photography | | | | for architectural photography. This trick works with |
| All pro commercial photographers know the ability to | | | | normal to medium focal length lenses, but not with |
| achieve pin-sharp shots is essential. It doesn't matter | | | | telephoto lenses. |
| how thrilling the subject or how inspired the | | | | Passive autofocus works like the human eye, |
| composition, even a minute hint of blur can mark the | | | | analysing an image's contrast levels to determine |
| difference between an award-winning shot and a | | | | whether it's in focus and adjusting the lens until the |
| second-rate snap. If the eyes of a portrait are | | | | lines and edges exhibit maximum sharpness. The key |
| slightly soft and the eyebrows and tip of nose area | | | | principle behind passive AF is that objects in focus will |
| sharp, for instance, the image will lack impact and | | | | have sharper edges and higher contrast. The |
| credibility. | | | | downside is that it struggles in dim light and with low |
| Most modern cameras adjust the focusing | | | | contrast subjects, and is prone to 'hunting' for |
| automatically at the touch of a button. Good quality | | | | sharper focus points after pin-sharp focusing has |
| autofocus systems can move the lens elements into | | | | been achieved. |
| position faster and more accurately than the human | | | | The advantage of passive AF is that it works |
| hand and eye can. This makes them invaluable for | | | | exceptionally well with long lenses. Many SLRs also |
| sports, documentary, natural history, paparazzi and | | | | have a built-in AF assist illuminator that shines a |
| fashion photographers and commercial photography, | | | | high-contrast pattern onto the subject, giving the |
| not to mention anyone with less than 20-20 vision. | | | | camera a target to lock onto when lighting is poor. |
| Nailing the focus for every shot is far from | | | | AF assist lamps only have a maximum range of |
| straightforward, involving considerably more user | | | | around three metres. If the beam is out of range, try |
| intervention than a point and shoot approach. This is | | | | the AF assist lamp of an external flashgun, as this is |
| because the speed, complexity and accuracy of AF | | | | likely to be more powerful. Although advanced |
| systems varies dramatically from camera to camera | | | | automatics and SLRs favour passive autofocus, |
| and in different shooting conditions. Ultimately, | | | | some take advantage of both systems, using active |
| autofocus is just a mechanical tool that's prone to | | | | AF for close-ups and passive AF for distant subjects. |
| errors and suffers limitations like any other man-made | | | | The actual mechanics that drive your autofocus |
| technology. The good news is, if you understand | | | | system also have a bearing on its effectiveness. SLR |
| how your camera's AF system works and what its | | | | lenses with built-in motor drives, for example, are |
| shortcomings are, you'll be well equipped to anticipate | | | | generally considered to be faster, quieter and more |
| and compensate for it's failings. | | | | efficient than lenses whose elements are focused via |
| Active and passive | | | | a motor inside the camera body. |
| Autofocus systems come in two different flavours | | | | Focusing modes |
| – active and passive, both of which have distinct | | | | You'll typically find two core autofocus modes on |
| advantages and disadvantages. Active autofocus is | | | | advanced cameras: one shot/single-servo AF and |
| commonly found on compacts and works on the | | | | continuous-servo/AI servo AF. Single-servo is |
| same principle as radar, emitting infrared beams that | | | | designed for shooting relatively static central or |
| bounce off the subject. By measuring the angle and | | | | off-centre subjects such as portraits, still lifes and |
| size of the reflected beams plus the time delay | | | | landscapes. You focus by half-depressing the shutter |
| between the signal emission and reception, the | | | | button and when the camera has focused it emits a |
| camera calculates the distance between the CCD and | | | | confirmation light and/or beep. Single-servo cameras |
| subject. | | | | are customarily in 'focus-priority' mode and the |
| The benefits of passive autofocus are its high speed | | | | shutter can only be released when the focus |
| and ability to work in pitch darkness. The main | | | | confirmation is displayed. |
| downside is that it's only effective at distances of | | | | |