Differences between zoom and focus

Ideal convex lens imaging formula: 1/u + 1/V = 1/F, where U is the object distance, V is the image distance, F is the focal length

The word "zoom" refers to changing the focal length F. Only the focal length of the zoom lens can be changed. The focal length of the fixed focus lens is fixed.

The word "Focus" or "Focus" is used to changing the image distance from V, that is, changing the distance from the lens light center to the negative plane. Except for some low-end dummies with no focus mechanism, the image distance cannot be changed. All lenses, regardless of Fixed-focus zoom, can change the image distance.

The conventional saying "Focus on XXX" refers to changing the image distance V through "focusing, the distance from the scene XXX to the lens U and the lens focal length F meet the imaging Formula 1/u + 1/V = 1/F, that is, the scene xxx can be clearly imaged on the negative film.

Turn: http://www.douban.com/note/132423490/