Autofocus problems

Its often the case that people complain about autofocus errors on cameras. The latest being the Pentax K-5. Its interesting how often this crops up, and I think that we must probably accept that no camera is perfect. I've personally had autofocus failures at one time or another with almost every camera I've owned. The only camera that has locked on 100% of the time is the Leica X1. It does however take several seconds to do this, so there's no great benefit. 


The Pentax K-5 is no worse or no better than any other camera I've used and the problems occur in the same situations as with other cameras. Wide apertures and fast motordrives are a particular recipe for error. Slight forward or back focusing at f/1.4 is a big problem, whereas at f/11 it wouldn't be noticeable. Some of the problems however are caused by camera operation. Pressing the shutter can cause a slight change of angle of the camera, and many cameras are programmed to re-focus when the shot is taken. At something like f/1.4 this can be enough to shift the focus. 


Another problem is expecting the camera to know what you want to focus on. Multi-spot AF systems often choose what to focus on from a set of common picture situations, and if your particular image doesn't fit, the AF will just do its own thing.


In the example below an AF system will often focus on the background, and it can be very difficult to get certain AF systems to lock on to something like this.


Pentax K-5 35mm f2.4

This illustrates another problem when working outdoors which is wind blowing something so that it moves out of focus. If the focus isn't locked then the camera may re-focus and miss the shot. 

Often when I shoot Panoramas, one or more shots can be out of focus. I've found Nikons particularly bad at this. Even with AF locked (or so I thought) my D3 would often decide to misfocus occasionally. Usually when I was trying to take the pictures quickly. I've had the same problem with Canon cameras.

A very frustrating form of these AF errors is when the camera happily locks on and beeps to indicate a successful focus and all you see is a huge blur in the viewfinder. What on earth it thinks its focused on I can't imagine. 

I try to minimise the problems by just using one centre spot point and I always try to find a spot  that has a lot of contrast to give the system every chance. If I've got time I'll maybe do it once or twice just to check. For an important picture I'll often take more than one shot, re-focusing every time.

Of course there are situations where you just have to trust the system as you don't have the time to do anything else. We can probably all remember the Canon 1D Mk III saga, which never actually seemed to get completely fixed. This was in a high speed camera designed for sport and news photography and must have caused enormous problems. 

I've always taken the attitude that things like AF are just an aid and experience has taught me that they are far from 100%, so I don't really expect that. Metering and Image Stabilisation are also aids that occasionally give unexpected results and should not be regarded as infallible. 

I'm as fond of helpful technology as anyone, but I'm often glad that I learnt photography on much more basic cameras. I'm usually aware of situations that can cause problems and I will often ignore the technological aids in those situations. Using fast lenses at wide apertures and macro lenses I will always use manual focus. Personally I find macro photography without a tripod very difficult. I have seen it done successfully, but thats very rare.

As long as we bear in mind that that our cameras can only do so much, then we can enjoy the benefits of what they offer us. 

David