P-40 “Aleutian Tiger” / P-51D “Dakota Kid”
captured by D5 / 70-200f4 AFS
It’s so simple, so in the background and it’s so important, our plane of focus though is overlooked and misunderstood. And that’s only natural because when we think of our sensor, it’s always worrying about dust and not math. But when it comes to getting subjects in focus, either movement or depth of field and much more, you gotta think about the film plane (it effects flash exposure too!). With it being fall, many photographers are out photographing flocks of birds and I’ve been receiving emails asking why are some of the birds in focus and not others. Most suspect their camera’s AF is not working when in reality, it’s just a function of our plane of focus.
P-40 “Aleutian Tiger” / P-51D “Dakota Kid”
captured by D5 / 70-200f4 AFS
I’ve used these photos to try to illustrate why some birds (feathers or metal skins) might be sharp and others out of focus. In this case, the film plane (sensor in the D5) is traveling the same speed forward as the two aircraft (top photo). In the top photo, when the film plane speed is matched with the two aircraft, they are both sharp. When the P-40 in the foreground changes up its forward speed, it is no longer matching the film plane speed so it is now out of focus (middle photo). When you’re panning your camera to freeze the forward motion, the film plane speed must match that of the subject. When that flock of birds (or single bird for that matter) forward motion is matched by your panning, everything is sharp. When it’s not, they are not sharp. And in the case of a flock of birds, the odds of all the birds flying the same speed as your panning is pretty slim so some will be out of focus. It has nothing to do with camera AF performance but math. This is just one aspect of the film plane you need to understand and when you understand them all, you can move your photography forward!
