The AF test is one of only two sets performed in the Popular Photography Test Lab that doesn’t use the HMI lights described earlier. (The other set is performed on our lens bench, which has its own light source.) The AF test uses an array of six 1000-watt Altman tungsten lights controlled by sliding dimmer control from Lighting Methods Inc. and powered by a Theater Technologies Inc. power supply.
The test is performed using only the center AF point, which is typically a high-sensitivity cross-type sensor or, in some cases, a dual cross-type point (essentially two cross points overlaid and working in tandem). This presents a best-case scenario for a given camera and should be interpreted with that in mind. The great advantage of this method is that we are able to compare any DSLR to another, as they are all on a level playing field.
Ultimately, the AF testing is a way to track a manufacturer’s products over time. Because we use the same lens (whenever possible) on all the different bodies from a given manufacturer, readers can make informed decisions about which body might be best for them within a given line. Plus, it serves another purpose of product testing in general—to keep manufacturers on a path of innovation and improvement, with the hope that photographers will ultimately have better tools at their disposal. With a general sense of how a given body will focus, you can also make loose comparisons between brands. While it doesn’t represent an absolutely perfect way to compare one DSLR’s focusing capabilities to another’s, we think it’s the best way possible given the number of variables that go into AF.
Whenever a camera being tested has an optical viewfinder, we test that finder’s accuracy and magnification. The accuracy test compares what is framed in the finder with the actual resulting image recorded by the camera. Most finders do not show you everything that will be captured, though some offer 100% accurate framing.
To perform our accuracy test, we frame our resolution target precisely on the framing guidelines of the target in the camera’s finder and capture an image. We then use the ruler tool in Photoshop to compare the size of the captured image to the size of the whole frame, and calculate a percentage.
To measure magnification, we shoot an image of the resolution target through the finder with a 50mm f/1.4 lens on the camera being tested, using another camera also outfitted with a 50mm f/1.4 lens. We then remove the camera being tested and shoot another image of the resolution target from the exact same position that we shot the image through the finder. Then we use the ruler tool in Photoshop to compare the size of the image shot through the finder with that of the other image to get the magnification factor.
Our testing doesn’t end with the lab. We also take every camera out into the field and shoot with it to see how the meter responds to various scenarios, how well the continuous AF functions, if the claimed number of images per burst is true, how the ergonomics of the camera, how easily you can access necessary functions in the menu system in a real-world scenario, and much more. The only way to fully understand a camera is to use it extensively, and that is what we do.