CALCULATING LENS FOCAL LENGTH

1) f = v x (D/V) or 2) f = h x (D/H)

f : focal length of lens
V : Vertical field of view
H : Horizontal field of view
D : Distance from Lens to Object
v : vertical multiplier (see table below)
h : horizontal multiplier (see table below)

CAMERA FORMAT1″2/3″1/2″1/3″1/4″
V9.66.64.83.62.7
H12.88.86.44.83.6
CAMERA FORMAT1.2″ (SQUARE)1/1.8″1/2.5″CMOSIS CMV2000CMOSIS CMV4000
V15.25.34.295.9811.26
H15.27.25.7611.2611.26
CAMERA FORMAT4/3″43.3MM1/2.8″ 1080P HD SONY IMX1361/1.2″ SONY IMX1741/1.2″ SONY IMX249
V13.6025.703.097.137.13
H18.1337.255.4411.3411.34
CAMERA FORMAT1/1.8″ SONY IMX2651/1.8″ SONY IMX2522/3″ SONY IMX2642/3″ SONY IMX2501/1.9″ SONY IMX185
V5.295.297.077.074.05
H7.077.078.458.457.2
CAMERA FORMAT1.1″ SONY IMX253 12.3MP
V10.35
H14.13

Substitute these data to formula #1 above:

f = 4.8 x (2500/340) = approx. 35 mm 

Therefore, specify a 35mm lens to achieve a 340mm vertical f.o.v. from 2500mm object distance with a 1/2″ format camera.

Note: f.o.v. (V or H) and object distance (D) can be in any units of measure, provided they are both the same.