What does focusing mean? What does it have to do with the formula
1 / u + 1 / v = 1 / f
u is the object distance,
v is the image distance,
f is the focal length.
Does focusing mean that both sides of the formula are equal?
Photography Stack Exchange is a question and answer site for professional, enthusiast and amateur photographers. It only takes a minute to sign up.Sign up to join this community
Both sides of the formula are always equal (for theoretical, thin lenses; for real lenses, the formula is only an approximation). What it says is where you should place the image plane (the film or the sensor) to have a sharp image.
Focusing means changing the distance between lens and film/sensor so that it matches v.
Again, only approximately because real lenses are not theoretical thin lenses; also focusing generally moves the lens so that object distance u changes a bit.
Yes, it means changing either
u+v being constant) so that a point on the subject is just a point on the sensor/film (ie,
v is the distance from the lens to the sensor).
In other words, the formula is always true, but if
v isn't the distance between the lens and the sensor the subject is out of focus. Focusing is moving the lens (or changing its focal length) to that
v is the distance between lens and sensor.
In practice, real camera lenses are not the theoretical "thin lenses" on which your formula is valid...