One of the most frustrating things that can happen in photography is seeing a perfect moment and snagging a shot, only to get back to your computer and find out the picture was barely out of focus. As a wedding photographer, I can't count how many times my couple has been composed perfectly with that magic moment that only lasts one frame, and my autofocus drags away from their eyes. To be fair, there is a lot of human error that goes into focus problems. However, did you know the problem doesn't always lie in the user?
Probably not all photographers are aware that every lens bought from their manufacturer isn't always perfect right out of the box. In fact, when it comes to focusing, most new lenses are shipped with an acceptable amount of variance in their auto focus. For your average picture, these small differences are probably pushed aside. But if you're interested in getting your shot tack sharp every time as far as the camera is concerned, you may want to pay attention.
With many cameras these days, it's possible to test your lenses and make micro adjustments to your auto focus without having to send your lens back to your manufacturer. Unfortunately, this feature is not included in every camera. If you want to check to see if your camera has this feature, I've included a list below from some of the major manufacturers. If your camera does include a micro adjustment feature, the option to make adjustments can be found in one of the custom function menu options. If you own another model and find issues, you'll have to send your lenses in for adjustments.
There are several ways to test the accuracy of a lens's auto focus. Each manufacturer has a slightly different way they advise for their own lenses, but for today, I'm going to talk about an easy do it yourself option. All you need is a measuring tape, a tripod, a little bit of space, and some contrast.
When measuring your autofocus accuracy, it's important to run any test in an evenly, well lit area. Your camera looks for contrast when it focuses, so make sure your test subject is easy to pick out. Ideally, all you want is a small piece of paper or a small flat object. If you want something professional, there are focusing chart options which can be purchased for this exact thing. For my test, I placed a piece of white paper on a dark wooden table near some soft window light.
When you set up your camera, make sure it is stable on a tripod. The camera should be placed at a distance approximately 50X your lens focal length away from your subject. In other words, if you are shooting with a 50mm lens, you multiply 50mm x 50 = 2.5 meters away. This is not a set rule that needs to be followed. If you primarily use a lens from a certain distance from a subject, then you may want to make any micro adjustments for that specific distance.
Once your camera is set, place the measuring tape right next to your piece of paper or whatever point of contrast you've chosen. Your camera should be at an angle from the measuring tape so that it is easy to tell where the focal plane is landing. Open up your f-stop to the widest option of the lens and put your focal point right on your subject. Take several sets of test shots.
While it is possible to zoom in on your LCD, for more accurate results, you may want to view your images on a computer screen.
Your shots should fall into one of three categories: front focused, back focused, or correctly focused. Note that these images are cropped in to clearly show results.
Front focused- the focal plane lies in front of your point of contrast.
Back focused- the focal plane lies behind your point of contrast.
Correctly focused- your focal plane lies evenly on your point of contrast.
If your lens does seem to be consistently focusing incorrectly open up your custom function for micro adjustments and add or negate value to correct the focus problem. If you are front focused, you will need to add value. If you are back focused, you will need to negate value. The first time you adjust, you will probably need to guesstimate the value adjustment needed and then retest until you are satisfied with the results.
As of 2013, cameras that have the auto focus micro adjustment feature include: