Testing the Accuracy of Your DSRL Camera's Autofocus
Automatic focusing is a great help in ever-changing environments you find yourself in as a travel photographer. It wouldn't be a mistake to say that we use automatic focusing 99% of the time. For best results, we need the autofocus to be both fast and reliable. While the former depends on the quality of the lens, the latter may get inaccurate with time. If you noticed that the focus on many of your images is slightly off, you may run a quick autofocus test for your camera before rushing to the camera repair shop. In this article, we will explain how to check the accuracy of your camera's autofocus at home, while our next article will provide some tips on camera autofocus calibration.
First of all, make sure that you are focusing correctly and exclude the possibility of camera shake: select one focus point, place it carefully against the subject, lock the focus, and do not forget to follow the shutter speed vs. focal length rule. If after all the above steps your photos are still not razor sharp, the lens of your DLSR may need some calibration. A simple way to determine whether the autofocus of your lens is accurate is to test it on a focus chart.
A variety of easy-to-make DIY charts are available for free download online. We tried the following:
- Invisicord.com focus chart: once downloaded, it can be printed on two sheets of regular A4 paper, that are easy to assemble in less than a minute. The downside of this chart is that it is rather flimsy and may not be very accurate.
- Andy's Handy Focus Calibration Chart has to be printed on A4 paper, glued onto cardboard, and then cut out according to instructions. This chart takes about 20 minutes to make. As a result you get an accurate free-standing focus target.
Once your focus chart is assembled, it's time to test the lenses:
- Stand the focus chart on a level surface.
- Set your camera on a tripod and make sure it's level and pointing into the center of the focus chart.
- The recommended distance between the camera and focus chart has to be 50 times the focal length of the lens (for example, if you are testing a 50mm lens, the distance between the camera and the chart has to be 2.5m). We noticed, however, that it is easier to determine where the sharpness area is when focusing at a closer distance, as it is more visible on the chart.
- Select the widest possible aperture of your lens (the smallest f-number).
- Switch the lens to Manual focus, manually focus at infinity, then switch the lens back to Autofocus, let the camera focus automatically by half-pressing the shutter button, and take a photo.
- For more accurate results, take several photos by repeating the previous step.
- Download the resulting images to your computer, enlarge them to 100% and examine them carefully.
- Repeat the above steps for each lens that you use on your camera.
Once the images are downloaded and enlarged on the screen, compare the sharpness of the focus chart to the sharpness on the ruler on the right side. If your camera focused accurately in the middle of the chart, the chart in the resulting image will be sharp and the sharpest number on the ruler will be O. If the number in focus is continuously located on the ruler in front of the chart, your lens has some front-focusing issues. However, if the sharpest number on the ruler is behind the chart, then your lens tends to back-focus.
Since most lenses used at their widest aperture produce some visible fringing, such fringing can help to determine the accuracy of focusing of the lens: the numbers on the ruler that are out of focus in front of the chart will have purple fringing, while those on the back of the chart will produce some green fringing. The number that has no fringing (the one that is clearly black) is the one in focus.
If after running this test you determine that your lenses focus accurately - congratulations, no adjustment is needed. However, if one or several of your lenses continuously show front-focusing or back-focusing issues, camera calibration may be needed. Some cameras (for example, Canon 5D mark II and mark III) have focus micro-adjustment setting that can be adjusted for each lens separately. We will discuss how to adjust the accuracy of your lenses using this setting in our next article. If your camera does not have this feature, you may have it serviced in the nearby camera repair store.
So get your focus charts ready, test your camera's autofocus and share the results with us!
- Shutter Speed vs. Focal Length Rule for Razor Sharp Photos
- 7 Steps to Clean Your DSLR Camera’s Sensor Yourself
About the Author
Nadia is passionate about the visual aspect of life and the personalities she meets along the way. Living in the heart of Paris, Nadia never stops exploring the City of Light through her camera and readily shares her knowledge and love for photography.