Choosing the right aperture
Text and photographs © Nigel Dennis
 Due to the difficulty in
approaching our subjects, telephoto lenses of between 400mm and 600mm are regarded as
pretty much standard fare for bird photography. However, tele lenses have what at first
may appear a great drawback in that their depth of field is extremely shallow. A 600mm F4
lens when used at its widest aperture to focus on a small bird filling the frame will have
an extremely narrow plane of focus. In fact, unless the bird is exactly flat on to the
camera, it will be impossible to get both the head and tail of the bird sharp.
Depth of field improves as a progressively smaller aperture is used but, as less light is
allowed to pass through the lens, the shutter has to remain open longer to maintain
correct exposure. For each additional aperture F stop, shutter speed is halved. In other
words, the 600mm lens used at its widest aperture of F4 would give a shutter speed of
about 1500th second with 100ASA film in bright sunlight. Changing to F5.6 would increase
depth of field slightly but would halve the shutter speed to 750th second.
Correspondingly, F8 would give a speed of about 350th second; F11, 180th second and F16,
90th second. Now, at F16, there will be plenty of depth of field to get a bird in sharp
focus from head to tail but the 90th second shutter speed will result in a blurred image
unless the bird is sitting absolutely still.
The trade-off between aperture (depth of field) and shutter speed (ability to freeze
movement) is one of the fundamental compromises of photography. Unfortunately, other than
using an extremely fast film which will give excessive grain, it is impossible to have the
best of both worlds with both good depth of field and a fast shutter speed. The decision
to opt for either depth of field or fast shutter speed will depend on the kind of results
you wish to achieve with your photography.
For bird portraits with the subject sitting still, a smaller aperture of say F11 would
work well to give sufficient depth of field to get a sharp image head to tail. Because the
subject is static, the correspondingly low shutter speed is not a problem. Remember though
that tele lenses need to be kept absolutely steady with low shutter speeds so use a sturdy
tripod and cable release to avoid blurred, shaky pictures. I generally focus on the eye of
the bird for portraits. When the lens is stopped down, the zone of sharp focus extends one
third in front and two thirds behind the point of focus.
To freeze movement, it is of course necessary to use a fast shutter speed. This results in
a wide aperture and shallow depth of field. Fortunately modern autofocus is so fast and
accurate that a high proportion of pictures are generally in focus even with so little
leeway in terms of depth of field. Often when shooting an action sequence with wide
aperture/high shutter speed, it is simply impossible to achieve sufficient depth of field
to get the whole bird in focus. In this instance I try to keep the autofocus sensing area
in the viewfinder on the head of the bird. Even though the tail may be out of focus, the
picture will generally still work. When shooting action, I use aperture priority mode on
the camera set to the widest aperture to automatically maximise shutter speed.
At the start of this feature I said that shallow depth of field was an apparent
shortcoming with telephoto lenses. It is possible to make this limitation of the
photographic medium work for you in making a bird look like it is about to jump out of the
frame. By photographing in situations where the background is a considerable distance
behind the subject, it registers as a mere mist of colour not unlike a watercolour wash.
The result is a clean uncluttered image with the subject clearly prominent.
Text and photographs © Nigel Dennis
 Nigel
Dennis is the author of several spectacular nature photography books on
African wildlife. You can purchase these titles available from Amazon com by clicking on
the images or links below. To see some of Nigel's stunning images visit the Nigel Dennis Wildlife Photography web site.
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The
Kalahari – Survival in a Thirstland Wilderness
Published 1997. Struik New Holland Publishers. ISBN 1 86872 019 5. Hardcover 29cms x
26cms. 168 pages. The Kalahari Gemsbok Park is one of our favourite photographic
destinations. Wendy and I spent fourteen months there over a two year period when
collecting pictures for this book. My aim was to portray the diversity of life in the
Kalahari throughout the seasons. Includes coverage of rare and seldom seen species, such
as the prehistoric looking pangolin. Excellent well researched text by Michael Knight and
Peter Joyce. |
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National
Parks and Other Wild Places of Southern Africa
Published September 1999. Struik New Holland Publishers. ISBN 1 86872 212 0. Hardcover
30cm x 24cm. 176. Co – photographed with Roger de la Harpe. Our aim was to produce
not only a strong visual portfolio, but also a book packed with useful information for the
traveller. The reader friendly narrative, by Brian Johnston Barker, highlights the
subcontinent's finest ecotourism destinations. Maps and fact-filled boxes compliment the
text. |
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The
Ultimate Wildlife of Southern Africa
Published September 1998. Sunbird Publishing (Cape Town). ISBN 0 62022 614 5. Hardcover
37cms x 29cms. 160 pages. Text by Brian Johnson Barker. This large format coffee table
book showcases a selection of my favourite pictures taken over the last decade. The
selection was made from tens of thousands of images in my photo library. I paired this
down to about 800, and the publisher and designer then had a tough job deciding what to
include in the book! Shows many rare and seldom photographed species, as well as the more
familiar big game. |
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The
Kruger National Park – Wonders of an African Eden
Published 1995. Struik New Holland. ISBN 1 85368 593 3. Hardcover 30cms x 27cms. 176
pages. Also the result of a two year photographic project, this book has proved very
popular and is soon due to go into a fifth reprint. The photographs show the animals,
birds, reptiles, flora, trees and varied habitats of South Africa premier National Park.
Comprehensive text by Bob Scholes gives a wonderful insight into the ecology of the
region. Don't visit the Kruger Park without a copy of this book! |
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