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Home >> Learning >> e-Book on Photography Table of Contents Photography e-Book Chapter 18 - Depth of Field Depth of Field or DOF is an important part of a photographer's developing sense of seeing the world. It is probably the one aspect of seeing that you will most often come across in reading about how a photo was taken and what the photographer was trying to achieve, such as, "I was trying to limit the depth of field to isolate the subject", or "I was trying to maintain as much depth of field as possible for this grand landscape." For every image captured on film, there is only one true plane of focus that will be assuredly sharp because a photo is merely a two-dimensional representation of a three-dimensional reality. DOF refers to the zone in front of and behind the true plane of focus and your choice of lens and aperture will dictate how sharply rendered those DOF zones are on film or prints. Depending on your subject matter, you may desire more or less DOF. Generally speaking, landscapes require more DOF and portraits require less DOF. DOF is comprised of two parts, the zone in front of your subject focus and the zone behind your subject focus. Often times DOF is taken as a combination of the two. The DOF behind the subject is larger than the DOF zone in front of the subject. The rule of thumb of focusing one-third of the way into a large landscape scene in order to maintain maximum depth of field is due to considering the shallow front DOF zone. This maximizing of the DOF is related to the Hyperfocal distance. There are a few rules of thumb to understand about DOF,
The immediate way of controlling the DOF is by adjusting the aperture. DOF is so important that experience has revealed to me that the majority of photographers choose to shoot in Aperture Priority mode with their cameras. With a given lens mounted on the camera, the photographer can choose to set the aperture wide open to help blur out the background for a portrait session or the aperture can be stopped down to render the background sharp. The important aspect here is that the photographer maintains control of DOF while still being able to enjoy semi-automation from the camera setting the shutter speed for the aperture chosen. Now of course, the use of certain lenses makes controlling DOF more restrictive, such as with wide-angle lenses. However, the photographer still maintains control because the conscious choice of using a wide-angle lens usually means the photographer desires immense depth of field. It is simply the opposite with telephoto lenses, in which the photographer may want a completely blurred out background to ensure that all the attention is on the subject by the viewers. Related to DOF is Hyperfocal distance and Circle of Confusion, or COC. Hyperfocal distance is the maximum DOF possible for a given aperture setting for a given lens focal length. Another way of defining hyperfocal distance is, the closest distance that will be rendered sharply when the lens is focused at infinity (Photonet Lens FAQ). There is a specific calculation that one can use to determine the hyperfocal distance but since I am not so good with numbers, I decided not to pain myself by including it and trying to explain it. Others have done the job already and sell charts for many popular focal lengths and aperture settings. If you have prime lenses or use manual focus zoom lenses, then it is very likely you have some basic hyperfocal distance information already. Some photographers do not trust DOF charts or what the lens DOF setting tells them for a given aperture. Charts and settings are based upon a known physical calculation and would be right on for perfect lenses but due to vagaries of quality control and sample to sample variation and the fact that the perfect lens cannot be produced, experience has led some to be more conservative with DOF considerations. Instead of setting the focus and DOF for a desired f11, they will use the distance for f16 but still use f11 as their actual aperture setting. Circle of Confusion Circle of Confusion may get a bit esoteric for those starting out in photography and after you learn more about it, you may find it to be nothing more than the delight for the anal-retentive among us. I have never thought or worried about what the COC would be for any of my images in the four years I have been involved in photography. However, since it is related to Depth of Field, I thought I should add some comment about it. COC relates to the scene detail outside of the one true plane of focus in your image. The out of focus points become small circles and if the circle is small enough, it will be considered sharp by the human eye (Photonet Lens FAQ). COC is dependent upon the film format you use and the smaller the format, the smaller the COC must be in order to appear as a sharp point in the image. For 35mm film format, the COC is generally accepted as 0.03mm but some lens companies apparently apply a stricter standard of 0.025mm. This is considered the threshold for the human eye to consider something as being sharp or blurred, hence the term Circle of Confusion. The smaller the film format, the sharper the lenses must be to resolve the details in the scene and withstand enlargement. Medium and large format lenses are not as technically sharp as 35mm lenses but the results with the larger films will still be superior to that from the best and sharpest 35mm lenses. Simple reason why, the larger film images do not have to be enlarged as much as the smaller films for a given print size. This generality can be proven wrong though if a photographer were to use a super high resolution film with 35mm format and uses a lower resolution film with a larger format. Photodo did such a test and the results revealed surprising parity between 35mm format using Kodak TMAX film and large format using the older Kodak Tri-X. COC is obviously related to your enlargement factor. Viewing a 35mm slide with a 4x loupe will probably lead you to believe that most everything is in sharp focus but move up to a 8x loupe which basically provides you with what an 8x10 would look like (8x10 being a 8x enlargement of a 35mm slide), and you will likely change your mind about how sharp the image is. The greater the enlargement, the tighter the COC must be in order to maintain sharpness. The distance of the viewer to the print is also a factor as a person would not normally keep his nose scant inches away from a 20x24 size print but would instead be a few feet back to take in the whole print. The relation to depth of field is that if you are concerned about the COC then it is likely that you are trying to maximize your zone of sharpness in a landscape scene, from very close up to infinity for the purposes of large print sizes. With only one true plane of focus, you need the other scene elements encompassed by your depth of field to be as sharp looking as possible to maintain integrity in your enlargement. Sometimes, no matter how stopped down a lens is and no matter how wide a lens is, we just simply cannot obtain enough depth of field to cover everything from the foreground to the background into sharp focus. Now when I say, no matter how wide a lens, I qualify that within reason because normally, landscapes are not taken with ultra wide lenses such as those available from Voigtlander for their Bessa bodies (and Leica). When you use larger film formats, the wide-angle lenses are actually normal or even moderate telephoto in terms of 35mm format. Example, my Bronica PS 40mm lens is the widest rectilinear lens available for the 6x6 SQ-Ai body. This lens if mounted to a 35mm body, would be a near normal lens but for 6x6, equates to an angle of view approximating 24mm in the 35mm format. Although it is a wide-angle lens, the 40mm does NOT have the DOF as a 24mm focal length in 35mm format. It still has the DOF of a 40mm lens. This is true of any focal length for any film format.
In all three cases, the 150mm lens maintains the same DOF in each format. Just because a medium format 150mm equates to an 85mm lens view in 35mm, does not mean the lens has the same depth of field as an 85mm lens in 35mm format. I hope you understood that because this is a common misconception among beginning photographers. The point is this, the 150mm lens will appear to lose DOF as you move up in film format but it does not actually lose anything. The problem for users of large format is that the shallow depth of field offered by a 150mm lens means greater difficulty in having an expansive DOF from very close foreground to infinity, despite it being a normal view lens. This shallow DOF is exacerbated with large format but thanks to large format cameras having tilt and shift capabilities, the shallow DOF can be ameliorated with the use of the Scheimpflug Principle. Since I am not a large format photographer YET, I would direct you to other online sources of what this technique is about and how it works.
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Correspondence & About this website Copyright © 1998-2008 Edwin Leong |
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Current
autofocus zoom lenses do not have DOF markings anymore
Older
lenses including zoom lenses, have DOF markings as indicated by the
red markers
