lens – Hasselblad 250 m / m mounted on my Nikon full frame camera … what is its effective focal length?

The focal length of a lens is a measure of its power of magnification. The focal length of a lens remains unchanged despite everything. In other words, swap an objective between different cameras of different formats and the size of the objects remains unchanged.

That being said, what changes is the angle of view. Think of a movie projected on a screen, we substitute different sizes of screen during the projection of the film. The size of the projected image of the objects does not change, but the screen size change encompasses different image sizes (formats).

The Hasselblad format would be 6 cm by 6 cm. The actual image on the frame of the film measures 56 mm by 56 mm. The diagonal measurement (angle-angle) of this format is 80 mm. If we mount an 80 mm, the lens offers a viewing angle of 53 ° diagonally and 38.5 ° horizontally.

Mount the same lens on a 35mm full-frame camera and this lens projects a 24 x 36mm image. The diagonal measurement of this rectangle is 43 mm. Since this format is smaller, only the central part of the projected image of the lens can be used. In other words, the size of the screen is reduced. Now the viewing angles are reduced to 30.3 ° diagonally and 25.4 ° horizontally.

Mount 100 mm and the viewing angle changes to 24.4 diagonally and 20.4 horizontally. Mount a 250 mm and diagonal angle of view will be 9.9 ° diagonally and 8.2 ° horizontally.