Photo Talk

I have written for club magazines over a long period. I'm helping with a magazine now, and will post the articles.

AUTOFOCUS? OR OUGHT-TO-FOCUS?

We are rather matter-of-fact about electronics these days; once you have tried several times to digest the thick-paged instruction book of your new camera, you get the message: OK, dummy, just press the button and leave the rest to me. Forget about all those fancy doo-dads and concentrate on the picture. So the tendency is to leave everything to the gods inside the thing -and concentrate on the picture.

At some point later, you sit down with a wallet of prints and have a critical look. Great stuff .... this matrix metering can really cope with all sorts of lighting situations. But wait a minute, why are some of these prints a little soft?
Can it be that the clever gnomes operating the autofocus are looking cross-eyed?? Alas, not all autofocus systems are created equal, and you are going to have to learn the niceties of the camera you have.

Try this little test. If your camera has a zoom, set it for a mid-range, say 40-50mm. Look at a suitable target about 3- 4 metres away, line up the autofocus, and tap the button. Now switch over to manual focus and turn the focussing ring very slowly while watching the focus-indicating dot in the viewfinder.

That dot will often remain lit for a surprisingly long time before it cuts off; but it was telling you that all of those lens focus settings were OK, when you know that they couldn't possibly ALL be! You are now finding out that autofocus systems work on the ZONE principle. The full range of focus is divided up into discrete zones; how many zones allotted will depend upon the quality of the camera and the system chosen by the designer.

A real cheapie point-and-shoot may have only four or five zones to cope with a range from infinity to a meter or so. SLR systems can use up to 255 zones! Obviously, the greater the number of zones, the greater the accuracy in determining focus-or rather what the autofocus mechanism considers to be acceptable to it! Further, the actual point where the autofocus mechanism driving the lens rotation comes to rest, is not necessarily EXACTLY the same every time you focus.

The remedy for this problem, when you have the time, is to look carefully at the focus setting and check it manually. If the camera is on a tripod, try setting focus to manual and noting where the focus-indicator comes on and extinguishes. The point of true focus will be somewhere about half-way between the two settings. A little experiment and practice will enable you to tell if your autofocus is getting it right every time, or if you need to do some tweaking on a tricky shot.

The next test involves a zoom lens, say 35-70mm, or even 28-135 if you have one. Set it to 35mm and focus on an object 6 metres away. Now zoom out to maximum focal length. Is the image still pin-sharp on the screen? I bet you that most of the time it won't even be close! This test tells you that it is most difficult to get an accurate point of focus when focussing with a wide-angle lens. Naturally, this does not cause worry at this focal length, because depth of field will take care of minor inaccuracies. But when you zoom out to a longer focal length, the depth of field shrinks; and what was an acceptable error at 28mm is way out at 135mm.

If your autofocus is re-focussing after the zoom takes place then your shot will be OK; but sometimes you have the focus held while you zoom out to compose the picture. That's when the nice sharp detail can go soft!

There are actually two different methods of autofocussing in general use:

• The active method relies on the camera sending out an invisible beam of infrared light, to be reflected off the subject and received back by the camera. This is usually found in the cheaper point & shoot cameras. It can be most useful in situations of low light and contrast.
• The passive method is generally used in SLR cameras, and relies upon a half-silvered mirror or prism reflecting some of the image onto a receptor; this figures out when the image is sharpest.
The passive method is generally the most accurate, unless the subject contrast is low.

Focussing problems can be found in manual-focussing zooms that go from wide-angle to telephoto. In this case, the difficulty lies in the method used by manual SLR camera to assist focus: the crossed wedges in the centre circle of the viewfinder. Most people will be aware that this is utilised by observing a vertical line on the subject which is displaces sideways by these wedges. The lens focus is turned until the vertical line runs unbroken; the lens is then in focus.

If you try and focus a wide-angle lens in this manner, you will find that with most cameras it is rather difficult to determine the exact point. On the other hand, a longer focal length will permit you to bring the line halves together with speed and precision. The problem for the camera designer is to select the optimum angle for the wedges; a steep angle suits a short focal length better, but makes it almost impossible to be accurate at a long focal length.
He compromises by favouring the longer focal length which demands the accuracy, and hopefully leaves focus inaccuracies at the wide-angle end to be taken up by a much wider depth of field. Therefore, one should get into the habit of focussing at the long end of the zoom, and then zoom back to the desired subject framing-rather than the other way round.

Another problem area lies in the inability of the autofocus mechanism to cope with rapidly moving targets. Of course, some top-line SLR cameras feature predictive autofocus which can adjust focus based on the previously stored rate of focus change. Or alternatively, a freeze-focus mode which waits for a subject in the field to come into focus, then fires the shutter at the appropriate moment.

If your camera lacks these refinements, then you can resort to the time-honoured method of focussing on some object at the same distance as the point where you wish to shoot the moving target. Then, after disengaging the autofocus, you can take the shot when your target reaches that point, without having the camera suddenly trying to refocus when you press the shutter release.

Notwithstanding all this, I still feel that autofocus is a great innovation - especially to those whose sight has slipped a little with advancing years! You just have to find its limitations, and know how to compensate for them.
Remember that inside most auto-everything cameras is lurking a fine manually-controlled machine-provided you are able to switch the appropriate functions off when they are not needed. Now you can show it who's boss!

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