ExpoImaging ExpoAperture2 Manual (Imperial/Standard) - Manual (Page 8)

ExpoAperture 2 Depth-of-Field Guide Manual
V 1.0
8
starting with a discussion of the circle-of-confusion present in the final print is because it is
the most subjective of the variables and depends on factors outside of the camera, in
particular the photographer's visualization.
In determining the circle-of-confusion, prints are considered to be viewed at a distance equal
to their diagonal dimension. In actuality, prints are viewed from distances further away. An
8" x 10" print has a diagonal of 12.8 inches. Unless you are scrutinizing the print very
closely an actual viewing distance of 14 or 15 inches is more practicable. You can observe
this type of viewing behavior in museums where a person stands back from a painting to
take in the whole view and occasionally moves in closer to look a specific element and then
moves back to the original view position.
There are several ways to determine the permissible circle-of-confusion. One way is based
on normal human visual acuity. The resolution of the human eye in a person with normal
visual acuity is generally assumed to be 5 lines per millimeter when viewed at a distance of 25
centimeters. Twenty-five centimeters is roughly 10 inches, the diagonal dimension of a 6" x
8" print. Therefore along the 25 centimeter dimension the viewer could discern 1,250 lines
or points (5 x 25 x 10). The circle-of-confusion would be 0.200mm (250mm divided by
1,250.) Given this information the circle-of-confusion in an 8" x 10" print would be
approximately 0.250mm since the 8" x 10" print is larger by a factor of approximately 25%.
Another method assumes that the circle-of-confusion is equal to 1/100th of an inch in an 8"
x 10" print. 1/100th of an inch is equal to 0.254mm. Since these methods are so close, and
the second may have derived from the first one, we will use 1/100th as the permissible
circle-of-confusion in further discussions.
The traditional value for the circle-of-confusion for 35mm film has been 30 microns. The
30 microns is based on using a 1/100th of an inch circle-of-confusion in an 8" x 10" print.
The 30 microns is calculated by dividing the 0.254mm circle-of-confusion desired in the final
print by the number of times the negative needs to be enlarged to make the print. In this
calculation we are assuming that the negative will be enlarged only as many times as
necessary to fill the 8" x 10" print format or 8.47 times (the 203mm print dimension [or 8"
dimension] divided by the 24mm film dimension). The required Circle-of-Confusion
necessary in the negative is 0.030mm (0.254 divided by 8.47) or 30 microns.
These calculations reveal two critical factors in determining the circle-of-confusion to use
when making an image: 1) the viewing distance of the final print or projected image, since
the further away the image is viewed, the larger the permissible circle-of-confusion can be;
and 2) the number of times the image source needs to be enlarged to make the print or
project the image.
Adjustments to the calculations need to be made for cameras that capture images on film or
sensors that differ in size to 35mm film. In digital cameras, which generally use sensors
smaller than traditional 35mm film, the image needs to be enlarged more times. For
example an image taken with a Nikon DX sized sensor has to be enlarged 1.5 times more
(the crop factor) than 35mm film to make the same size image therefore you need to use a
20 micron (30 divided by 1.5) circle-of-confusion in your calculations.