Autofocus on most modern cameras is pretty reliable, but sometimes it doesn't quite get it right. Occasionally this is made worse by our own technique, or by not setting our camera up in the best possible way.
Here are seven tips to help you boost your accuracy when using autofocus.
Know when to – and when not to – focus and recompose
The technique of focusing on a particular part of the scene before moving the camera to recompose the image is used widely. Many photographers do this without even thinking about it.
It saves you having to adjust the focusing point and most of the time it can work successfully. In some situations, however, it can lead to the subject not being completely sharp – so it’s useful to know when this is likely to happen.
Focus-and-recompose should be used with caution when shooting close up
Essentially, the closer you are to the subject and the wider the aperture you use, the less room you have for manoeuvre. For more distant subjects, and particularly with depth of field extending through the frame, a slight movement from the camera shouldn't matter, but as you get closer you should either use this in conjunction with a smaller aperture or be careful not too shift the camera too far from its original position.
Use particularly wide apertures cautiously
If the specific combination of your camera and lens results in focusing to be very slightly out, this will be more visible at wider apertures where depth of field is more shallow.
You're more likely to see focusing errors when depth of field is shallow
Here is an example. Let’s suppose you capture a close-up portrait with an 85mm f/1. 4 lens, and you focus on one of the subject’s eyes. If everything is as it should be, the eye should be in focus and you should start to see details behind and in front of it slightly out of focus (or at least starting to go that way). So, the bridge between the eyes, eyebrows and so on.
If your camera and lens don't combine to give you focus on the eye itself, you may end up with the subject's eyebrows being in focus but the eye you focused on being slightly out of focus. At a smaller aperture, the extended depth of field may be deep enough to mask such inaccuracies, as everything will be rendered sharply.
If you do find this to be the case…
Fine-tune your AF
The last few generations of enthusiast-to-professional DSLRs (and accessories such as Sigma's USB Dock) have offered users the choice to fine-tune the way a lens behaves on a camera body, for the reasons described above. Nikon’s AF Fine Tune and Canon’s AF Microadjustment are examples of this.
Nikon's AF fine-tune feature is now available on a number of the company's DSLRs
The correction doesn't change anything physically with the lens, but it does remember what kind of bias you would like applied to images, should you find a particular combination to be slightly off.
Use live view with DSLRs
If you use a DSLR, your camera's autofocusing is performed by an AF module that's positioned away from your camera's sensor whenever you use the viewfinder. In live view, however, focusing is performed by the main imaging sensor.
This means that focusing using the viewfinder and live view can create two slightly different images, with two slightly different points of best sharpness.
Using your camera’s live view system is the most accurate method as you remove the separate focusing module from the equation. What you see is a truer refection of what’s being captured, whether your camera is using a standard contrast-detect AF system or a mixture of contrast- and (sensor-based) phase-detect AF. Sure, it’s not always practical to use live view, and sometimes too slow, but for any tripod-based situations it certainly makes sense from the perspective of accuracy.
Dig out your manual
If you’ve become used to using a particular focusing setup on one camera for everyday shooting, it's possible you'll continue to do this when you upgrade to another – particularly if they're from the same system.
Minor refinements to autofocus systems and controls hardly grab headlines, so it’s quite possible you're not aware of what your camera fully allows. This in turn means you may not necessarily be working as well as you could be.
A quick dig around your camera's menu should reveal a few possibly focusing tweaks you may not have been aware of
At a very basic level it could be learning how an AF-ON or AE-L-AF-L button can be customised, but the more you dig into the menu or manual, the more you may discover can help. Options on current DSLRs include using a group-area AF mode instead of single points and making sure the point stays in the same place when the camera is rotated - and that's just scratching the surface.
Use your customisable buttons
If you own a relatively advanced camera you may be able to assign certain autofocus-specific functions to custom controls.
Commonly used focusing functions can often be assigned to customisable buttons
Some cameras now allow you to assign a pre-determined focusing point to such a control for easy recall, which you may find useful if you tend to rely on an off-centre point. You may also be able to use this to stop an autofocus system’s operation when it's working continuously, or to alternate between single-shot and continuous focusing modes.
Limit your focusing points
A particularly high number of focusing points always makes headlines, but having so many to hand doesn't always help.
If you tend to use a single point for focus and move it around the frame to suit your subject, you’ll already appreciate that having so many points capable of being addressed can slow you down and might mean you miss the shot.
Far better to limit the array to a smaller number of points so that you can reach the one you want faster.
You may be able to limit your AF system to a more manageable number of points
. digitalrev.com2017-5-31 03:00
