ExpoImaging ExpoAperture2 User Manual

expoaperture2

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-of­confusion 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-of­confusion 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 ExpoAperture2 Depth of Field Guide.

George W. Ziegler, Jr.
Morgan Hill, California
March, 2007

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Table of Contents

1. - An Introduction to Depth-Of-Field - 4

1.A. - Focus as a Compositional Tool - 4

1.B - Controlling Depth of Field - 5

1.C - Image Sharpness - 6

1.D - Permissible Circle-of-Confusion - 7

1.E - The Advent of Digital Cameras - 9

1.F – Conclusion: Taking Advantage of Depth-of-Field in Your Photography - 10

2. - Using the ExpoAperture2 Depth-Of-Field Guide - 11

2.A - Distance Dial - 11

2.B - Focus Zone Dial - 11

2.C - Focal Length Dial - 11

2.D - f/stop and Circle-of-Confusion Dial - 12

2.E - Determining The Correct Aperture and Focal Point - 15

2.F - Determining Depth-of-Field for a Given Aperture and Focal Distance - 16

2.G - Determining Hyperfocal Distances - 16

2.H - Setting Apertures for Lenses with Focal Lengths Greater than 135mm - 17

2.I - Close and Macro Photography - 19

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1. - An Introduction to Depth-Of-Field

"There is nothing wrong with your television set. Do not attempt to adjust
the picture… We can reduce the focus to a soft blur, or sharpen it to crystal
clarity..."

The Outer Limits
Opening narration, The Control Voice, 1963-1965

The human eye is marvelously flexible and adaptive. It can adjust its focus so rapidly that
the impression is given that its entire field of view is in sharp focus when only a small
portion is actually in focus at any one time.

A camera lens, in contrast, freezes the focus on the image plane the instant the shutter is
released, preserving forever whatever the lens has rendered in or out of sharp focus. A
viewer of a photograph can immediately discern the different areas that are in sharp focus
and those that are not.

The distance between the nearest and furthest points from the camera at which everything
appears sharp is termed the depth-of-field. In many types of photography, such as
landscape photography, it is desirable to have the entire image as sharp as possible. In this
case the photographer may focus on the hyperfocal distance to obtain the maximum depth­of-field possible. In others, the creative use of depth-of-field can be used to isolate elements
that the photographer may wish to emphasis or deemphasize. This opens the photographer
to the possibility of using selective focus (the ability of lenses to render some objects within
a scene in focus while others appear out of focus), as a compositional tool.

1.A. - Focus as a Compositional Tool

All photographers know that they need to focus their cameras, but only skilled
photographers know how to use selective focus as a compositional tool. Selective focus can
be used compositionally in two ways, to direct attention to the subject, and to eliminate
distractions.

Directing attention – In a photograph, a viewer's eye is irresistibly drawn to the area of sharpest
focus. It follows that a photographer who can control focus can also control the viewer's
attention. Sharp focus implies that the photographer is placing emphasis on an area for a
specific reason. In addition, elements that are in sharp focus are united by their similarity,
which separates them from the other out-of-focus elements. Consequently, the creative use
of focus can help build and support visual relationships.

Eliminating distractions – Often, either the foreground or background elements of a scene
detract from the main subject, directing the viewer's eye away from the photograph's center
of interest. When such areas are purposefully thrown out-of-focus, they are less likely to
distract the viewer. In some cases, a slight softness is all that is necessary. In other cases,
the effect may need to be exaggerated. The photographer needs to make the appropriate
compositional decisions based on the circumstances and his or her intent.

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Hyperfocal distance – In landscape photography in particular, photographers want as much of
the picture in sharp focus as possible. In the 1920s, such notables as Ansel Adams and
Edward Weston, along with several other photographers of the time, started the "Group
f64" which was dedicated to making photographs as sharp as possible throughout. The
group met only a few times and held only three shows, the first at the de Young Memorial
Museum in San Francisco. The term f64 evidentially arose from the smallest aperture setting
marked on large format lenses of the time, associating the use of this aperture with the
sharp, clear pictures prized by the group.

The group’s choice of f64 was symbolic, however, to the extent that the smallest aperture
does not always provide the largest depth-of-field. Instead, focusing your lens on the
hyperfocal distance with the correct aperture, you will maximize depth-of-field in your
image. The hyperfocal distance is the distance setting at a given aperture and focal length
that creates the most depth-of-field. The ExpoAperture Guide provides the ability to
determine hyperfocal distances (see Section 2.G, “Determining Hyperfocal Distances.”)

A photographer who is unaware of how selective focus can be put to use in composing an
image is likely to select any lens, shutter speed and aperture setting that will produce a good
exposure. While this approach may be necessary in some circumstances, skilled
photographers will never take a photograph without first deciding which areas of a scene
they wish to be in sharp focus and which ones should not. Although the extent to which
selective focus is used or is not used in a particular image is a creative decision,
implementation is a technical matter accomplished by controlling a lens' depth-of-field.

1.B - Controlling Depth of Field

The optical characteristics of lenses that permit control over depth-of-field are lens aperture,
lens focal length, and focusing distance. These characteristics are the primary variables in
depth-of-field calculations. It is not necessary to understand depth-of-field formulas in
order to understand the relationships these variables have with one another. This
understanding is enough to allow you to use depth-of-field creatively. The accompanying
ExpoAperture2 Depth-Of-Field Guide will do the calculations for you, allowing you to
concentrate on the aesthetic elements of your image.

Aperture – Every serious photographer understands that the size of the lens diaphragm
(aperture setting) affects depth-of-field. The ability to set the aperture is present in all but
the simplest cameras. Specifically, the smaller the aperture (the larger the f/stop number),
the more depth-of- field obtained, and, conversely, the larger the aperture setting (the
smaller the f- stop number), the less depth-of-field obtained. In most situations, the f/stop
selected for use will noticeably affect the image's appearance in the final photograph.

Focal length – The longer the focal length used, the less depth-of-field obtained, and,
conversely, the shorter the focal length used, the more depth-of- field obtained. Like the
f/stop selection, the selection of a different focal length will affect the image's appearance in
the final print.

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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-of­confusion 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

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aperture setting. For an f/2.8 lens this would be f/5.6 or f/8. Therefore, using a smaller
aperture, like f/16 or f/22 to improve depth-of-field may actually decrease sharpness in your
photos.

The most common reason for lack of sharpness in photographs is camera movement, or
what is commonly referred to as motion blur. You can usually assume that camera
movement reduced sharpness in a photograph when nothing in the image is in focus. If you
had a steady camera and did a reasonably good job of focusing, at least some part of the
photograph should be in focus.

To reduce camera movement, use a tripod. If you are unable to use a tripod, the faster your
shutter speed, the more likely you will be to avoid motion blur. As a rule of thumb, your
shutter speed should be set to at least one over the focal length of the lens you are using. For
example, if you are using 250mm lens, your shutter speed should be set to 1/250 of a
second. When possible, you should also brace yourself against a tree or wall to improve your
steadiness. For really large enlargements of your images, this may not be enough. When
handholding or using a tripod remember to press the shutter slowly, or even better, use a
cable release with your tripod. Use mirror lockup, if available, so the camera doesn't vibrate
when the mirror snaps up in order to take the picture. Following these guidelines should
result in tack sharp images.

If you are using a digital camera with a LCD viewing screen it will be nearly impossible to
tell if your picture is sharp or not by quickly looking at the LCD. Nearly everything on the
LCD appears in focus because of its small size. If you have a zoom feature that you can use
during playback, magnify the image as large as possible and then examine the areas that need
to be in critical focus to confirm they are as sharp as you want them to be.

Finally, the introduction of image editing software like Adobe Photoshop® also changes
whether portions of a photograph, and the photograph as a whole, are more or less sharp to
the viewer. A complete discussion of the merits of post-processing sharpening are beyond
the scope of this manual. However, suffice it to say that, just as with all other aspects of
photography, there are numerous creative and quality benefits to be gained by capturing in
camera an image as close as possible to your intended print.

1.D - Permissible Circle-of-Confusion

In optics, a circle-of-confusion is a slightly out-of-focus point of light. A perfect lens when
focused on a point of light like a distant star would render it sharply on the image plane – as
a point of light. Since a lens can only render one plane in focus at a time, points of light that
are either closer or further away from the lens than our subject would be out-of focus.
Slightly out- of-focus, the points would appear as tiny circles. The permissible circle-of­confusion is the largest that this circle can be and still appear to be sharp to the eye at a
normal viewing distance. Circles smaller than the permissible circle-of-confusion will always
appear sharp.

In order to determine the permissible circle-of-confusion we need to start with the final
product – the enlarged print. You may well ask, "Why start here?" The reason we are