The simplest and quickest way to test if your DSLR has an autofocus problem, as well as a suggestion for what to do if there is a problem, is provided below. This test can be used to identify front or back focus problems with a specific lens or camera body. For this article, I’ll be using the Nikon D800E as a reference camera, but any modern DSLR with Live View capability can be used for the same test (even entry-level DSLRs such as the Nikon D3500 have a Live View mode). Why would you want to check your camera’s autofocus? Because you will be unable to obtain critically sharp images if your camera or lenses are defective or have a calibration issue.
1) Things You’ll Need
You will need the following items for this test:
Any DSLR that has a Live View mode.
At least one lens, preferably two or three, if you want to narrow down the problem to the camera or your lenses.
A strong and solid tripod.
A flat vertical surface in a well-lit location. For example, your garage door or a wall inside your home next to a huge window will suffice.
On regular letter size paper, print either the Siemens Star Focus Chart or the Focus Test Chart. It doesn’t matter if you print it on a laser printer or an inkjet printer. Make careful to print on standard paper rather than glossy paper such as picture paper.
To maintain the concentration chart on the wall, use Scotch tape or another adhesive substance.
The installation procedure is rather simple, as detailed below:
Choose a location on your wall for the focus chart. The wall must be straight and vertical. Do not hang the focus chart too high or too low, as you want it to be directly across from the camera.
Set up the camera on your tripod and make sure it is parallel to the focus chart after the focus chart is up. Check that the camera is not slanted left, right, up, or down — it should be parallel to the wall. Check your level by looking from the side and ensuring that the lens is pointed squarely at the center of the focus chart.
Please keep in mind that I captured the image above the camera to demonstrate where the focus chart is in relation to the camera. The focus chart is just across from the camera.
The distance between the camera and the wall is determined by the focal length of the lens being tested. If you’re shooting with a 50mm f/1.4 lens, the distance between the camera and the test chart should be around 5 to 7 feet (1.5-2 meters). If you’re using a wide-angle lens, get closer; if you’re using a telephoto lens, get farther away. The idea is to go close enough to achieve a shallow depth of field. The chart should be roughly one-third to one-fourth the size of the total image. You’re all prepared; now it’s time to shoot some photos.
3) Camera Configuration
It is always preferable to have exposure constancy when assessing the autofocus accuracy of a camera or lens. As a result, I recommend using Manual mode and maintaining the same exposure for each image. Here’s a rundown of what I’d recommend for camera setup:
Change your camera’s mode to Manual.
Set your lens’s aperture to its utmost setting. Set your aperture to f/1.8, for example, if you use a Nikon 50mm f/1.8G lens.
Set the ISO to a low value, such as ISO 100.
To establish the best shutter speed, use the camera’s exposure meter. Take a few test pictures to ensure that you can clearly read the chart. In an ideal world, your shutter speed should be around 1/500 of a second. If you’re obtaining incredibly slow shutter speeds, like 1/10, it’s because you don’t have enough ambient light. Take the setup outside and do it in bright light, or add additional lighting to illuminate the focus chart. Your test will be flawed if the amount of ambient light is insufficient. It is critical that you accomplish this in a well-lit setting since the Phase Detect sensor on the camera requires a lot of light.
If your camera has it, make sure that AF Fine Tune/AF Micro Adjustment is switched off. For example, on the Nikon D800, navigate to Setup Menu->AF fine tune->AF fine-tune (On/Off)->OFF.
Set the focal point to the center of the image. The center focus point in your camera is always a cross-type sensor, hence it is the most accurate.
Disable any lens adjustments in your camera (vignetting, distortion, chromatic aberration, etc.). You don’t want anything to have an impact on the test findings.
Select AF-S/Single Servo mode on the camera.
While this test should work for both JPEG and RAW photographs, cameras add additional sharpness, colour, and other effects to JPEG images. As a result, I would advise capturing test data in RAW format.
Change the Live View mode on your camera to “Tripod” (if you have such a setting).
You can utilize your camera’s Mirror Lock-Up feature with a remote camera release to prevent camera wobble (can be very useful if your shutter speed is slow).
Now that you’ve got everything set up, it’s time to start collecting data. Make sure to properly complete the steps below, or your test results may be invalidated:
First, in Live View mode, capture the focus chart. This will serve as your “reference” image. Once you’ve activated Live View mode, zoom in to the middle of the focus chart and either half-click the shutter or push the AF-ON button on the camera’s back to force the lens to focus. Now is the time to hit the shutter button and take the photograph. Examine the image at 100% zoom and ensure that it is tack sharp in the center, where the focus point is. You accomplished everything correctly if it appears sharp. If it appears blurry, your camera did not properly acquire focus in live view mode. Repeat until you have a clear image. Keep only one sharp image and discard the rest.
The second photograph will be taken without using Live View. Turn off the Live View feature. Now, gaze through the viewfinder and rotate the lens’s focus ring until the focus chart appears entirely out of focus. We want to compel the camera to refocus. To acquire focus, half-click the shutter button or push the AF-ON button on the back of the camera and wait until you see a green mark inside the viewfinder or the camera beeps (if you have beep enabled), confirming that focus is gained. Take a chance. Rack the focus once again by rotating the focus ring until everything is out of focus, and then repeat the process. Ideally, you should take at least three photographs this way to eliminate the likelihood of an autofocus error.
Let me explain what you’re doing here before we start analyzing the data. You shot an image in Live View mode first, then an image conventionally, as if you were snapping pictures. What are we doing? All DSLR cameras rely on an autofocus sensor called “Phase Detect” that is housed inside the camera (see my article on phase detection autofocus). The Phase Detect technology is really fast, which is great news. The bad news is that for this system to work, the Phase Detect sensor and lenses must be properly aligned and calibrated. And that is precisely what we are attempting to test here. When in Live View mode, the camera’s mirror flips up, and the Phase Detect sensor can no longer be utilized to acquire focus. As a result, the camera employs a new focus approach known as “Contrast Detect” to gain focus. Because the camera compels the lens to focus back and forth until the image appears sharp, this method is quite slow in comparison. However, because focus confirmation occurs electronically via the camera sensor, it is always precise. As a result, we’re using the initial Live View photo as a reference image.
5) Examine the Data
Now that we have a sharp reference image as well as a slew of other images, it’s time to dig into the data. Import your photographs into your computer using whichever post-processing software you like. Open the first image and check that it is sharp in the center.
It is crucial to compare photographs at 100% zoom and to switch back and forth between the reference image and the other shots. If something appears to be out of focus, repeat the process to ensure that you are witnessing a focus issue. You should be getting either consistently good or consistently terrible outcomes. If the results are consistent and the photographs seem nice from shot to shot, you do not have a focus problem. You can quit testing now and take a deep breath — you’re in good shape. If you are frequently obtaining poor results, the next step is to determine whether the fault is with your lens or the camera body.
6) Which comes first, the camera or the lens?
As previously said, it is best to test different lenses when evaluating your camera’s autofocus accuracy. If you only have one lens to examine, it will be impossible to determine whether the lens or the camera is out of alignment. It is possible that it is both at times. If you have 3-4 different lenses, run the aforementioned test on all of them and see what results you obtain. If all lenses produce fairly identical results, your camera is most likely to blame for the AF problems. Still, pinpointing the specific root of the problem will be difficult because it all depends on how poorly calibrated your lenses were before you began the test. See my lens calibration post for more information.
I’ve shot with a variety of Nikon DSLRs over the previous 6 years, and none of them had a focus issue with most of my lenses. I examined two Nikon D800 samples, and both were flawless, with no signs of focus issues. The Nikon D800E that I recently received, on the other hand, plainly has a misaligned phase detect sensor, as none of my lenses can focus effectively with it. Dialing -20 in AF Fine Tune appears to be adequate for some lenses but definitely insufficient for others, indicating that the camera suffers from significant backfocus. As a result, I was able to pinpoint the issue.