Understanding Camera Autofocus

One of the most difficult features to understand when it comes to digital cameras is the autofocusing system.

Back in the old days (Oh, No! Here he goes again.) focus was done manually either on a fresnel focusing screen or by a split-image rangefinder system. So long as your eyesight was good, you could depend upon getting nicely focused images. But as some of us got older we needed glasses and our ability to focus quickly and accurately suffered.

And then came autofocus. How wonderful. The camera focused itself. But, and it’s a big but, what did the camera focus on?

So if you’ve progressed beyond point-and-shoot photography where the camera default focus is on whatever is closest to it and nearest the centre of the frame, you’re going to want to know how to use your automatic focusing system.

Focusing systems are different from one camera manufacturer to another and even from one model to the next. In general, when you’re in automatic mode (on some cameras changing the shoot mode changes other parameters like focusing methods) your camera focuses as above on whatever is closest and nearest the centre. This works for 99 per cent of amateur shots of the family around the Christmas tree.

But what if you want to focus on Aunt Sally who is sitting halfway down the Christmas dinner table? The easiest way is to point the camera so that Aunt Sally is centred in the frame and hold the shutter half way and then recompose your image. The press the shutter all the way down.

Or, on most digital cameras you can change the number of focusing points (this is how digital cameras focus by using points that can be turned on as a group or individually).

Shooting with my Nikon D-300 I shoot with all 51 points active most of the time. But, when it comes to Aunt Sally (or shots in a crowded room) I change the number of focusing points from 51 down to one. That one is by default dead centre in the frame and will allow me to again hold focus by focusing and then holding the shutter button down halfway.

The Nikon will allow for continuous focusing (as in sports mode on some camera) or locked in focusing on one object.

But, on some cameras you can move the focusing point off of centre so that it is no longer necessary to recompose.

Now on a Flickr discussion group I blew an answer about whether or not metering can be set to follow the focusing point around. Most folks when they set focus to one point are still using evaluative (Canon’s terminology) or matrix (Nikon) metering. You can set the metering to follow the single point.

The big issue I have with all this is it’s all so geeky. As a professional news photographer I would have missed the shot by the time I got the camera setup. That’s why it’s important to understand your cameras controls based on the type of photography you do. If you’re an artist (or a camera geek), this stuff might be useful (even fascinating).

For much of the special event photography I shoot I do it on program mode (Gasp. I know! Me and Joe Buissink.) with matrix metering and single shot mode and I only go to single point if I’m shooting through a crowded room. And guess what: My percentage of shots in focus and sellable go way up. And, in my world, that’s what counts.

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3 thoughts on “Understanding Camera Autofocus

  1. “For much of the special event photography I shoot I do it on program mode (Gasp. I know! Me and Joe Buissink.) with matrix metering and single shot mode and I only go to single point if I’m shooting through a crowded room. And guess what: My percentage of shots in focus and sellable go way up. And, in my world, that’s what counts.”

    Hi Peter — so you are in shooting in P mode *and* autofocus? Because you mention you only go to single point “if I’m shooting through a crowded room”.

  2. Okay I’m got again. Single point of focus is still auto 🙂

    I’m just making the decision on what the camera focuses on by going to single point as opposed to multi-point focus where the camera often focuses on someone else than my chosen subject in a crowded room.

  3. Yeah that’s what I meant. I always just use the middle single focus point about 99% of the time.

    I usually use the multi-point/”camera decides” mode when I am taking group shots of people. 3 or 5 people lined up.

    Whenever I try using single focus point on one of the eyes of the most forward person, the rear people get out of focus. I guess I should decrease my aperture?

    Go to F/8, F/11?

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