If you’ve ever cursed your DSLR’s focusing system for missing a shot, don’t worry — you’re not alone. However, it’s far more likely that out-of-focus photos are the result of user error and unfamiliarity with the wide array of autofocus set-up options than the camera’s performance itself.
Focusing performance can be improved by something as simple as reading the camera manual, and, naturally, the more you use your camera, the better you’ll become at judging the type of conditions in which the autofocus will struggle.
But there are some factors that have an affect on the camera’s focusing speed and accuracy which are less obvious. Here are 6 things you probably didn’t know about focusing…
1. Autofocus performance can be improved via firmware
Ensure that your digital camera is running the latest firmware update. In addition to fixing bugs, manufacturers frequently include AF speed and performance upgrades as part of a firmware release.
Take the example of the Canon EOS 5D Mark III’s autofocus. When Canon launched the 5D Mark III, its centre AF point sensitivity was only effective with lenses that had a maximum aperture of f/5.6.
This was a concern if you intended to use a teleconverter, as the maximum effective aperture of a lens and teleconverter combination could be more than f/5.6.
Add a 1.4x teleconverter to a 400mm f/5.6 lens, for example, and you effectively end up with a 560mm f/8 lens.
However, Canon followed the 5D Mark III’s release with firmware version 1.2.1, which enabled f/8 autofocus with the centre AF point, as well as improving the speed of focus acquisition with a Canon Speedlite’s AF-assist beam.
2. Your camera and lenses are likely to suffer from front focus and back focus problems
There may be times when the camera confirms it has locked focus on a subject, but that subject appears out of focus in the final photo.
This is because some camera and lens combinations suffer from either front focus (where the photo is in focus in front of the focus point) or back focus (where the photo is in focus behind the focus point).
To improve focus accuracy, manufacturers include Autofocus Micro Adjustment (AFMA) as a menu option in many DSLRs.
AF Micro Adjustment enables you to compensate for front focus and back focus errors. You can make your own autofocus calibration test to help with the process, or use dedicated autofocus calibration software such as FoCal.
You’ll need to calibrate the AF for every combination of camera and lens you own, as well as saving AF Micro Adjustment settings for both the widest and longest settings on a zoom, as focusing errors are usually different at each focal length.
3. Focus shift causes focusing errors at small apertures
The image you see in a DSLR’s viewfinder is always displayed at the lens’s maximum aperture — such as f/4 on a 17-40mm f/4 zoom – even if a smaller aperture has been set on the camera.
This ensures that the camera’s phase detection autofocus sensors are exposed to the maximum amount of light available.
It also means that the depth of field will be at its narrowest, making it easier for the autofocus system to differentiate between areas that are in focus and areas that are out of focus.
If you’ve set a smaller aperture on the camera, the lens will be ‘stopped down’ to this aperture when you take a picture.
But even though a lens may be accurately focused at its widest aperture, it may suffer from slight back focus or front focus problems at smaller apertures — an effect known as ‘focus shift’.
If you’ve set a very small aperture, then the increase in depth of field can be enough to compensate for the shift in focus. However, at wider apertures the focus shift can be noticeable.
Prime lenses that have very wide maximum apertures such as f/1.2 and f/1.4 are the most susceptible to focus shift.
So, it often pays to check the focus at the ‘real’ aperture before you take a photo by using a camera’s depth of field preview button. Live View mode makes it easy to check for focusing errors too.
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