That circle-with-a-line mark on top of a camera is the focal plane mark: it shows exactly where the sensor sits, so you can measure focus distances from the right point — which matters for macro work, cinema focus pulls, and lens testing.
That odd, tiny mark you often see on the top of cameras — a circle with a line through it — is the focal plane mark, and it shows exactly where the sensor or film sits inside the body.
You can see an example in the photo I’ve included at the top of this post. You won’t find it on every camera model, but it’s included on surprisingly many. And it has been around for decades.
It’s known by various names: focal plane mark, sensor plane mark (on digital cameras), film plane mark (on film cameras), or image plane mark.
It’s denoted by a Greek phi symbol Φ.
It shows precisely where the image sensor (or film frame) is inside the camera (because, obviously, you can’t see that directly from the outside).
The focal plane mark shows the location of the image sensor or film frame inside the camera.
It can be useful for precise or repeatable focusing distances in macro photography or for a series of shots taken from a consistent distance and perspective.
Situations where the focal plane mark is useful
Most photographers can and do ignore it most of the time. But there are times when the focal plane mark can be very useful.
The most obvious is when you need very precise or repeatable focusing distances and need to break out a measuring tape. Macro photography is a common example, but it’s also helpful when you need a series of shots taken from a consistent distance and perspective.
It’s also the reference point for the focus distances printed on lenses. When a lens says its minimum focus distance is 0.3m, that’s 0.3m from the focal plane mark, not from the front of the lens. That’s worth remembering with macro work, because the working distance you actually get — glass to subject — is always shorter than the number on the barrel, and a long lens with a hood on it eats up a surprising amount of that measurement.
Another situation where I find it very useful is when setting up a camera on a specialized panorama tripod for spherical panorama shots.
Specialized tripod heads like those offset the mount point to the camera so that it rotates around the lens’s no-parallax point — the center of the lens’s entrance pupil, which sits inside the lens rather than back at the focal plane. That greatly reduces the perspective change from shot to shot and makes it much easier to align images when stitching.
You’ll see that point called the nodal point almost everywhere panoramas get discussed, and on the packaging of some heads too. It’s the wrong name. A lens does have nodal points, but they’re elsewhere on the optical path and they have nothing to do with parallax — the point you’re trying to rotate around is the entrance pupil.
