Been spending a lot of time on here!
- Nov 27, 2011
- Reaction score
- St. Louis
- Can others edit my Photos
- Photos OK to edit
Since we can "free lens" on a camera, the flange length is not fixed to one number by the lens (if it were, "freelensing" would be impossible because of the new flange length).The focal length of the lens is the distance from the lens nodal point to the film/sensor when the lens is focused at infinity. The lens focal length does not have a fixed relationship with the lens's projected image circle. Two lenses of the same focal length can project different image circles. However on the same camera they will have the same angle of view.
So since the length (50mm) is the distance from the nodal point (that is fixed?) to the sensor (which is not fixed), we cannot say that a lens is 50mm? We can only say that a lens is 50mm at a given distance from the sensor or film?
When you focus a lens on subjects progressively closer to the camera the lens moves away from the sensor whether you're freelensing or not. Focal length as inscribed on photographic lenses is the distance from the lens nodal point to the film/sensor when the lens is focused at infinity. That's true if the lens is physically mounted to the camera or not. It's true if you hold the lens and point it out the window and focus on a piece of paper in your other hand.