The Hasselblad lens was developed for 120 or 220 roll films. If I successfully attach it to a 35mm full-frame camera, how long is the lens? What is the effective focal length?
250 mm. This is because "effective focal length" is based on the full-size 36 x 24 mm 135.
When used in a medium format camera with an image size of 6 x 4.5 (a / k / a "645") with exposed dimensions of 56 x 41.5 mm, a 250 mm lens would have an "effective focal length" of about 155 mm since this is the case With an FF camera with a format size of 36 x 24 millimeters, the same diagonal field of view would arise as with a 155 mm objective.
6×4.5 (a / k / a "645") is probably the most common image size for 120/220 films. At other aspect ratios, the width of 56 mm remains constant (as this is the maximum usable width across the short side of the filmstrip), but the length progressively increases along the long dimension of the film.
80 mm = 80 mm for FF camera, "effective focal length" of 50 mm for 645 camera.
100 mm = 100 mm with FF camera, "effective focal length" of 65 mm with 645 camera.
250 mm = 250 mm for FF camera, "effective focal length" of 155 mm for 645 camera.