Manual Focus

21/11/09

MANUAL FOCUS

So why would you want to manually focus you new and very expensive DSLR, when the magic chip inside the camera, which numerous boffins have spent zillions of hours perfecting, will do it for you ? Well, despite what Mr Canon, Mr Nikon and others like to tell you, autofocus is not yet perfect, and one of the main reasons for that is you - yes that’s right you.

Problems generally begin to occur in low light situations. Clearly, as with all photographs taken with autofocus it is very important that the autofocus system can lock on to the the point in the image that you, the photographer, wish it to. So there you are in the gloom, you’ve identified your subject, pointed the camera at the aspect you wish to focus upon, and half depressed the shutter to activate the autofocus. Off goes the autofocus system, and at this stage a couple of things may happen, (1) it will begin to hunt - backwards and forwards goes the system trying to lock on, and eventually either you or it give up with the resulting frustration, or (2) moments after half depressing the shutter, you hear that comforting beep which tells you that lock on has been achieved, and without a moments hesitation down goes the shutter. After all the boffins at Mr Canon, Mr Nikon etc, have just told you that focus has been achieved, and who are you to question the boffins.

Back home you download the image to your computer, and what do you find - it’s not quite as sharp as you expect. No surprise there then. The likely reason, the camera has not focussed on the point in the image that you had selected. During it’s hunt to focus, it’s not found your point, but it’s found one either to the left, to the right, slightly in front, or slightly behind. Result, out of focus, and it’s your fault for becoming conditioned to the Beep, and upon hearing that little sound you’ve depressed the shutter without a moments thought. If you must rely on autofocus in these circumstances, then don’t automatically react to the Beep. Take a moment or two and check that focus has been achieved at the point you wish to be in focus.

Hence why in the above such circumstances, ie: low light, many photographers switch immediately to manual focus. It once again puts you the photographer in charge of the camera, it enables you to be as certain as you can be that focus has been reached on the subject point that you wish.

Therefore to summerise :

Manual focus disables the camera's built-in automatic focus system so you can focus the lens by hand (*). Manual focus is useful for low light, macro or special effects photography. It is very important when the autofocus system is unable to get a good focus lock, e.g. in low light situations. Note that some digital cameras allow you to manually focus only to a few preset distances. Higher-end digital cameras allow focusing using the normal focus ring on the attached lens, just like in conventional photography.
 
(*) In digital cameras, manual focus is sometimes implemented on a fly-by-wire basis, whereby the manual inputs to focus in or out are relayed to the autofocus system which effects the change in focus.