let's begin this conversation talking a 35 mm film. a frame of 35 mm film is 24 mm x 36 mm. the diagonal is 43 mm. but is we made the diagonal circular and fit the film inside it for full coverage, it's now 50 mm. any lens that has a focal length equal to the diagonal of film used is considered as normal. hence a 50 mm lens is considered normal for a 35 mm camera. it renders an image equivalent to eyesight. this is true for digital cameras as well as long as they have a full frame sensor. cameras with smaller sensors have a magnification factor that could 1.3 or as much as 1.6.
any lens with a focal length less than the diagonal is considered wide angle. also any lens with a focal length greater than the diagonal is considered telephoto. if one had a full frame camera, a 35 mm wide angle, a 50 mm normal and a 135 mm telephoto, all that they would need with practice is business cards. the obvious; standard lens, what you is what you get. again the obvious, a wide angle has shows more than normal. once again, a telephoto would bring distant objects closer. to all of the above "whoopee."
if you choose a subject, let's say a building 30 feet away, and photograph it with a 50 mm lens, it's just a rendering. if you changed lenses to the 35 then it would show much more of the surroundings. so what? but what if you stepped up closer to the house and made it the same size in the viewfinder as the 50? now how about using the 135 mm telephoto and moving back to again make the building the same size as the 50 mm? so what's the point? the point is you can make the house the same size with all three lenses by you physically moving. again why bother? a wide angle lens makes the background recede whereas the telephoto lens compresses it it with the subject. think of the classic explosion scenes in movies where the hero is almost blown up. no where close to danger, long lens. okay real life. any car salesman would tell you that half of the sale is getting the prospect on the lot. suppose you have a nice house that you want to sell. unfortunately it's a little closer to a major highway than you care for. wide angle lens will make the background look much further away. beautiful background, great bring it closer with the 135 mm and stepping back.
three different lenses and three different interpretation of the same scene. but there's more.
Depth of Field.
depth of field (dof) is the area of sharpness that occurs 1/3 and 2/3's behind the point of focus. several factors effect dof such the focal length of the lens, distance focuses and the working aperture selected. for this discussion we will only deal with the focal length of the lens selected. we have chosen three lenses in the beginning; 35 mm wide angle, 50 mm normal and the 135 mm telephoto. basically, the shorter the lens (35 mm) the greater the dof whereas the longer the lens (135 mm) the more shallow the dof.
kindly indicate if you would like for me to go one with this.