ExpoImaging ExpoAperture - Manual (Page 17)

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3. Setting the Aperture for a Telephoto Lens
Shooting with a telephoto lens, defined for the purposes of
the ExpoAperture Depth-of-Field Guide as an actual focal
length greater than 135mm, requires a simple calculation to
determine the required aperture setting for the desired depth-
of-field.
You must simply divide the actual focal length by one num-
ber, and the minimum and maximum distances by the square
of that number to bring them back to within the ‘standard’
range on the ExpoAperture Depth-of-Field Guide.
The purpose is to look for the smallest divisible number that
will result in a focal length utilized by the Depth-of-Field
Guide. For example, if shooting with 300mm focal length,
then divide the focal length by 2 and the distances by 4.
3a. Using a Focal Length Between 135mm
and 300mm.
In order to find the aperture setting for a given distance range
using a lens with an actual focal length of between 135mm
and 300mm, simply divide the focal length by 2 and the dis-
tance range (minimum and maximum) by 4 and then proceed
using the new numbers in the “standard” manner. See the
following example.