ExpoImaging ExpoAperture2 Manual (Imperial/Standard) - Manual (Page 6)

ExpoAperture 2 Depth-of-Field Guide Manual
V 1.0
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Focusing distance – Some lenses have markings on their barrels that indicate the distance at
which the lens is focused. Some fixed focal length lenses may also have depth-of-field
indicator lines that show a range of distances determined by the lens' manufacturer that will
be rendered in sharp focus when the lens is set at a particular f/stop. Almost all variable
focus length lenses (zoom lenses) do not have depth-of-field markings. If your lens has
depth-of-field markings, you will see that as the focus distance is brought closer to the lens,
the depth-of-field at any particular aperture setting diminishes. On the other hand, as the
focus distance is set further away from the lens, the depth-of field increases. Therefore,
moving the camera closer or further away from the plane of critical focus (or focus point)
can control the depth-of-field.
Therefore, the primary ways to control the depth-of-field are:
To increase the depth-of-field:
1) use a smaller aperture; or
2) use a shorter focal length; or
3) move further away from the plane of critical focus.
To decrease the depth-of-field:
1) use a larger aperture; or
2) use a longer focal length; or
3) move closer to the plane of critical focus.
Obviously, you can use any one of these methods or a combination of several of them to
achieve your desired result.
1.C - Image Sharpness
Depth-of-field and image sharpness are not the same thing. Depth-of-field is the range of
sharp focus produced by the combination of lens aperture, lens focal length and focusing
distance. These are theoretical values derived from optical formulas that assume a
theoretically ‘perfect lens’ with one element that is free from all aberrations and distortions,
and that focus precisely. In other words, depth-of-field calculations assume that the lens is
capable of delivering the degree of sharpness corresponding to the chosen circle-of-
confusion desired in the final print. [Note: The determination circle-of-confusion to be used
will be discussed later in Section 2.D.]
Real world lenses differ from the theoretically perfect lens used in depth-of-field calculations
in at least four significant ways. First of all, most lenses have aberrations, which are more
evident when the lens is set to large aperture openings. Spherical aberrations prevent the
lens from converging the incoming light rays on a single point, resulting in focusing errors.
Chromatic aberrations focus light of different wavelengths (colors) at slightly different
angles. Secondly, the lens must be able to focus precisely, i.e., there cannot be any defects in
the lens' focusing mechanism. Thirdly, real world lenses almost always have more than the
perfect lens’ one element. Finally, at very small aperture settings the sharpness of the image
is influenced by diffraction as light rays pass by the diaphragm. It is generally accepted that a
lens performs at its best when it is stopped down two to three full stops from its largest