ExpoImaging ExpoAperture2 Manual (Imperial/Standard) - Manual (Page 2)

ExpoAperture 2 Depth-of-Field Guide Manual
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Preface
The original ExpoAperture Depth of Field Guide was introduced over 20 years ago.
Primarily designed to be used with 35mm and medium format film cameras, the original
Guide used a fixed circle-of-confusion to make the necessary depth-of-field calculations.
Although the two formats required different circle-of-confusions, calculations were easily
converted between the two formats through a simple mental calculation. This was possible
because George A. Wallace, the inventor of the guide, selected a circle-of-confusion for
medium format (120) film that was approximately twice that of 35mm film. The original
guide used 35 microns as the circle-of-confusion for 35mm film – the upper limit in most
calculations. Wallace, a student of Ansel Adams, believed that the traditional circle-of-
confusion calculation for 35mm film (30 microns) was too exacting since it assumed that the
final print enlarged from a negative would be viewed at a distance equal to its diagonal
dimension. This would mean that an 8" x 10" print would be viewed at a distance of
approximately thirteen inches. Wallace's opinion was that the final print would be viewed at
a more comfortable distance – twenty inches for an 8" x 10" print. This theory is also
upheld in Alfred A. Blaker's book, Applied Depth of Field. Blaker demonstrates, through
various calculations, that the most comfortable viewing distance of a print or projected
image is twice the long dimension. In Blaker's explanation an 8" x 10" print would also be
viewed at a distance of twenty inches. The point of the foregoing explanation is that the
determination of the value of the circle-of- confusion used in depth-of-field calculations,
although based on a mathematical formula, is somewhat subjective based on the
photographer's preferences and intended use.
Additionally, many changes in photography have occurred in the intervening period from
1980 to now, not the least of which is the transition to digital and its myriad different sensor
sizes. As a result, it is no longer possible to use the original guide with its fixed circle-of-
confusion to perform depth-of-field calculations for all the formats (film and digital) on the
market today because of the need to use different values for each different sensor or format
size. Today's photographers asked us to address these issues and to redesign the guide to
make it more flexible for modern day use and reduce or eliminate the need for mental
calculations.
In the middle of 2006, ExpoImaging began such a redesign. Wallace died in 2001, leaving
many incomplete notes on the design of the original guide, requiring us to reverse engineer
the guide to determine how it worked. Once we discovered the "secret" of his calculations,
it was just a matter of modifying the design of the guide to make calculations based on a
variable circle-of- confusion – one that could be based on a photographer's preferences or
on the sensor size of a camera. The result is the ExpoAperture 2 Depth of Field Guide.
George W. Ziegler, Jr.
Morgan Hill, California
March, 2007