I was comparing Panasonic lenses the other day, and I wanted to know about their macro capabilities. Obviously Panasonic lists the closest focusing distance, but this doesn't tell me much about the image, since focal length is also a factor.
So I came up with this formula for calculating the size of an object, which fills the entire field of view, at a given distance (in this case the minimum focus distance).
Size = 2 ⋅ tan(FOV/2) ⋅ distance from camera
to see how I came to this formula, see the image below.
When plugging in the numbers from some lenses I came to these results:
Is the formula correct? Does this mean, if I take a picture with the first lens, I could have an object with a size of 0.076m fill the entire image?
Your geometry is a reasonable approximation for distant scenes. At close scenes, the focal point to image plane distance is significant and must be taken into account.
At 1:1, the focal point 2x the focal length from the image plane and the scene. At higher magnifications, the focal point is closer to the scene than the image plane. You can't ignore this for calculating geometries of macro and near-macro shots.