Originally posted by jatrax I think the accepted 'Normal' lens for 35mm film is 43.3mm (diagonal of 24 x 36mm) and the 1.5 crop of that is 28.8mm so a 28mm lens is as close as it gets. I know 50mm was always consider 'Normal' but that's not the way the math works out.
Maybe someone else can explain it better or correct me if I'm wrong, that's just the way I learned it.
I think it is missleading to clasify wide / normal or tele lens by their focal lengths. Instead, it is way better to use their Angle of View (AoV) value.
The human vision has great visual ability in the central region (about 20° to the left and 20° to the right from the central axis (our nose)). Away from the center, our visual ability decreases dramatically, such that by just 20° off-center our eyes resolve only one-tenth as much detail. At the periphery, we only detect large-scale contrast and minimal color:
http://cdn.cambridgeincolour.com/images/tutorials/camera-eye_detail1c.jpg
So, a normal lens should have arround 35°-45° AoV in order to equal our eyes "AoV". (a 50mm lens on 135 format (24x36mm) has an AoV of 40°...
Finally, AoV is a function of format and focal length. I mean, AoV depends on the size of the negative (photosensitive material) and te focal length used.
In APS-C sensors (23.6mm x 15.6mm) you can see this AoV vs FL (Focal Length) chart made at:
Field Of View Calculator
FL X AoV Y AoV
25 50° 32’ 17" 34° 39’ 29"
26 48° 49’ 30" 33° 24’ 4"
27 47° 13’ 5" 32° 13’ 44"
28 45° 42’ 25" 31° 8’ 6"
29 44° 17’ 10" 30° 6’ 40"
30 42° 56’ 46" 29° 9’ 4"
31 41° 40’ 52" 28° 14’ 53"
32 40° 29’ 10" 27° 23’ 56"
33 39° 21’ 18" 26° 35’ 56"
34 38° 16’ 55" 25° 50’ 35"
35 37° 15’ 54" 25° 7’ 44"
See this article for lot more info:
Cameras vs. The Human Eye
** when referring to AoV, always using the horizontal (X axis) value.