The page you’re reading tells you how to capture the images you’ll need. This page tells you what to do after that.
Decide what aperture to use for the test. In general, you’ll want to use the widest stop you can, but the required target size for short and fast lenses may force you to compromise. Go to this page, and find your camera format. Click on the link for that camera. You’ll be directed to a page with graphs on it. Find the curve for the aperture you want to use, and see what the minimum distance is for the focal length of your lens.
Now go to this page, find your format again, and click on the appropriate link as before. This time, you’ll be sent to a page that shows the size of a 410 mm Siemens Star target on your sensor at the minimum target distance. In a perfect world, you want to see at least 150 pixels. If the target is going to be a bit too small, you can go ahead and use the 410 mm target anyway, down to about 100 pixels. Otherwise you’re going to need to use a larger target. How much larger? By the ratio of 150 pixels to the indicated target size. If you see the target size listed as 75 pixels, you’re going to need a target that’s (150/75)*410 = 820 mm high. You may decide that you can’t practically use a target that big, and choose to pick a smaller f-stop for the testing, which will allow a smaller target.
Find a place for the target where there is even light. Direct sunlight is fine if it’s not coming from right behind where you’ll put the camera. Open shade is great. Light from an overcast sky is as good as it gets.
Put the target on the easel, facing approximately where the camera will be. Tape it down if it’s at all windy. Walk away from the target to a bit over the minimum distance, and set up your tripod. If the camera is not perfectly square to the target, that’s nothing to worry about.
Set the camera to create raw files. This test won’t work well with JPEGs.
Set the lens to the desired f-stop, and set the camera to its base ISO setting. If your camera has electronic first curtain shutter (EFCS), turn it on. Set the shutter speed so that the white background of the target is a stop or two away from the point where the raw file will be clipped. You will usually find that this requires exposure compensation of minus one or minus two stops from what the camera’s meter says.
Put the target in the center of the frame, and focus on it manually. Don’t try to use autofocus for this test. It’s not sufficiently accurate, and some cameras change focus position when switching between focus modes. Focus at the stop that you will use for taking the picture so that focus shift is not a problem. Take your time and get the focus right. If your camera has focus peaking and you set it to minimum sensitivity and maximum magnification, the Siemens Star can be a good focusing target.
Using the camera’s self-timer or remote release, make an exposure with the target centered. In addition, without changing the focus setting, adjust the tripod to make images with the target near all four corners, and optionally, in the center of the top, bottom, right, and left edges of the frame. That will give you either five or nine images.
Center the target, refocus, and repeat the capture process on or two more times, to make sure that you have at least one series which has the image with the target centered well-focused.