Several of the photographic formulæ in ƒ/Calc involve the distance to the subject. Technically, this distance is from a special optical point within the camera’s lens to the subject. When the distance from the camera to the subject is much larger than the size of the camera, it doesn’t much matter what point on the camera you measure from. Generally speaking, subject distance measurements only have to be accurate for macro photography. One definition for “macro lens” is one that can focus closer than 10 focal lengths. If the subject is farther away than that, the maximum error will be less than 10%, too small to matter.

The special optical point I mentioned above is called by various names such as front nodal point or anterior principal point, but knowing what it’s called doesn’t help you to find out where on the lens this is. If you’re lucky, it’ll be marked on the lens body, usually with a symbol. (Beware if you find that symbol on the camera body instead: it probably refers to the location of the imaging plane.) This is common with macro lenss, but not universal, and doesn’t help when using extension tubes or bellows to turn normal lenses into macro lenses.

If this point isn’t marked on the lens or called out in the documentation for the lens, you have little chance of figuring out where it is on your own. On a zoom lens, this point can actually move within the lens as the focal length changes! The best you can do when applying ƒ/Calc’s answers in such a situation is to carefully examine the results in the viewfinder before tripping the shutter.