One of the biggest problems photographers face in the era of autofocus cameras are lenses that don't focus just exactly right. The problem, known as front or back focus (depending on how the lens misses) causes the point of focus to fall in front of or behind where it is intended to be. Now, thanks to the Internet, there are instructions on how to build yourself a lens calibration chart for minimal cost.
The 'GhettoCAL' is the brainchild of photographer David Liang, who has detailed instructions on is own website. For the basics, one simply prints off the chart, cuts it to the size of a backing (use a board or foam), and tapes it down while making sure that it is completely flat. In addition, one will need a few paperclips and a ruler. The final cost: next to nothing.
As for the problem itself, here's how to go about finding out if you have a mis-focusing lens vs. some other problem. The good news: it only takes a few seconds and a few pictures to determine if a lens isn't focusing properly.
To get started, set up some objects/find some that are about evenly spaced. As a suggestion, a set of walk lights spaced a few feet apart will be perfect for a long telephoto lens, you'll want to find something more closely spaced for a shorter optic. Now, using the walk light example, pick a light in the middle of the row on which to focus, shooting from nearly head-on. By focusing on a middle light, you can determine whether the lens is front focusing (in focus area is in front of intended focus point) or back focusing (actual focus is behind intended focus). Se focus on the center point (you'll see why this is useful in a second).
Next step: mount the camera on a tripod to eliminate the possibility of camera shake and then shoot a series of pictures at different focal lengths and repeat the test a few times.
View slideshow: How to tell if a camera lens is not focusing properly
Now, pictures taken, it's time to see what came out on the computer (those tiny camera LCD screens are just too small to determine anything meaningful). Upload your pictures and then blow them up to 100% size so that you can see even the smallest detail. Now the benefit of the center AF point selection becomes obvious in that, when first viewed at 100%, you will be transported right to the center of the image (and thus the center AF point). From there, see if the image appears in focus. If it is, the signs look good, so start looking at the other pictures the same way. If they all look good (you repeated the test several times for validity), your lens works.
However, if your pictures seem soft, one can tell whether the issue is the focus or the optics in a few seconds. If the in-focus area of the image is somewhere other than where it was intended to be, you have a mis-focusing lens. If nothing is is focus, it's the optics. Now, if you're lucky, your camera has an AF microadjust feature that will allow you to compensate for a mis-focusing lens. Just be sure to read up on how to use the AF adjust as using it incorrectly will create more problems than it will solve. If this is your problem, here is where you use your GhettoCAL chart to fix the problem.
As a final suggestion, test your lens as soon as possible as many retailers, even the reputable ones, have short windows in which to make a return.
Good luck with your purchases!
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