This entry complements the ”Zoom and add-on lenses” post below and is therefore relevant also to those who don’t have an iPhone with dual camera. The main disadvantage of using an iPhone/iPad for photo finish is that it has a wide-angle camera. You have to stand pretty close to the finish line and the relative distance to the different lanes varies quite much. This means that it is difficult to set a slice width that is optimal for all runners.
Below you can see a finish captured with the standard wide-angle camera in the iPhone. In the first image, the slice width is 6 px, in the other, it is 3 px. In the first, the runner in lane 6 looks OK, but the one in lane 1 is drawn out. In the second image lane 1 looks fine whereas lane 6 is too squeezed.
This effect becomes smaller if you move backward and use a longer lens. Since many stadiums have a stand moving backward also means moving upwards, which further improves the image. Below is a photo finish captured with the tele camera in the iPhone 7+, 8+ or X. In this case, the runners are closer in size and the angle is better. The same effect can be achieved on other iPhones and iPads through the digital zoom available in the camera preview, but with some image quality loss. You can also zoom even further by adding digital zoom to the tele lens. This together with the large screen makes the dual camera iPhones the best SprintTimer devices yet.
Note: Normally the iPhone 7+ uses both cameras when zooming in. But that is not possible when running at 240 frames per second, so you have to choose either in the finish set up.