The Right Track
A Guide To Depth Of Field

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Wide angle lenses produce greater depth of field than telephotos only when used from the same distance at the same aperture. However, apparent perspective differs, making the choice of focal length--and shooting distance--important creative considerations. (Tamron AF 20-40mm zoom; at f/16; B+W polarizer; Velvia.)
Photos © 1999, Peter K. Burian, All Rights Reserved

Aside from the intricacies of exposure and light metering, photo enthusiasts generally find depth of field the most difficult concept to master. That's understandable particularly since this is a hypothetical factor based on subjective judgment. Granted, some do develop an intimate appreciation of the technical aspects, while others memorize them for night school courses when required to do so, but many ignore them almost entirely or absorb only the bare necessities. I must admit that depth of field seemed like an abstract theory at first, when lectures on "circles of confusion" pretty well described my reaction to the entire issue.

Eventually, I did grow to appreciate its practical value for problem-solving and for achieving specific effects for creative purposes. The scientifically minded can find plenty of data on depth of field in previous issues or in one of Harold Merklinger's excellent books frequently advertised in "Shutterbug Shopper." However, if you do not fall into that category, consider the following tips for controlling depth of field. I'll present these in purely practical terms, in a step by step manner, with new information added in each section. My apologies for any over simplification to technically inclined photographers and college instructors everywhere.

·Understand The Basics. In layman's terms, Depth Of Field (DOF) can be defined as the "zone of acceptable sharpness" that extends behind and ahead of the focused point.

In truth only the focused plane in any photo is truly sharp. However, objects in front of that plane and behind it, may also appear adequately sharp to the eye of the viewer. The extent of sharpness (or only slight blurring) differs based on several criteria, as we'll see in further sections. Textbooks discuss the "circles of least confusion" when explaining DOF, but that is a technical matter not necessary for practical application.

Example: You set focus on the eyes of a Bengal tiger sitting in the shade at a safari park. The eyes will be rendered as tack sharp on the negative. However, its snout--and the bark of the tree behind the animal--will also appear in acceptable focus in the final print. The degree of blurring increases gradually as you move from the focused point: the eyes. Objects in the near foreground and distant background become progressively less sharp the farther they are from the cat's head.

When higher shutter speeds are required to prevent image blur in low light, you'll need to set a wide aperture unless using a tripod. This makes stopping down for expanded depth of field impractical. In that case, refer to the other techniques described to achieve acceptable results. San Simeon State Monument. (Contax G1 with 28mm lens; f/2.8 at 1/15 sec; Fuji Super G 800.)

·Select The "Right" Aperture. Aperture size is the primary factor that influences depth of field. In a nutshell, apertures of small diameter (denoted by large f/numbers) such as at f/16 or f/22 produce an extensive range of sharp focus. Foreground and background elements can appear reasonably sharp. Conversely, wide apertures (small f/numbers) such as at f/2.8 or f/4 produce shallow DOF. Much less of the foreground and background appears acceptably sharp.

A decisive choice of aperture (f/stop) is an essential ingredient in "making a photograph" instead of simply "taking a picture." Even when using a Program mode, you should shift to the most appropriate f/stop/shutter speed combination when feasible. Take care however, as long exposures will create blur from subject movement or from camera shake if shooting handheld. Hence, you may need to compromise: select a wider aperture than would be ideal in order to maintain an adequately fast shutter speed. (Or use one of the other techniques for increasing DOF without stopping down to very small apertures.)

Example: You are taking a picture of a scene framed by a stone archway in England. You set focus on the fountain in the near midground and trip the shutter. In fact, you take six pictures, the first at f/4, the next at f/5.6, and so on down to f/22. Afterward you examine all of the 8x10 prints.

In the first at f/4, neither the archway nor the background castle appears sharp. By f/11, they seem "almost sharp," but not quite up to your standards. By f/16 the arch, fountain, and background castle all appear adequately sharp, and your friends consider this the best picture. The image made at f/22 is blurred by camera shake due to the long shutter speed because you did not use a tripod.

In creative nature photography, "vignetting" the primary subject can be an effective technique. Here, I achieved exactly the intended effect with the following: a telephoto lens, moving in extremely close to foreground subjects and a wide f/2.8 aperture. (Canon EF 70-200 f/2.8L USM at f/2.8; Sensia 100.)

·Switch To A Longer Or Shorter Focal Length. Anyone who has used a 28mm and a 300mm focal length will have noticed that wide angle pictures usually have extensive depth of field. Conversely, those made with telephotos generally have a very narrow DOF. This typically leads to a simple conclusion, which is also published in some magazine articles: "The longer the focal length, the less depth of field at any given aperture setting. If you want greater depth of field, use a shorter focal length."

Myth Breaking Example: The above definitely seems to be an accurate assessment of DOF in practice. Actually, it is a half-truth, but a persuasive one. It is an optical fact that DOF does not change with different focal lengths. Don't believe it? Then try a controlled test with a 35mm and a 200mm lens. First, take a picture of a friend standing next to a long, flowering hedge with the telephoto. Then switch to the wide angle lens and move in very close to the subject--in order to keep her as large in the viewfinder as in the previous shot.

Examine the resulting slides with a 10x loupe and you'll notice that the zone of acceptably sharp focus (check the blossoms) is nearly identical. Granted, perspective differs: the rendition of lines and shapes are definitely not similar nor is the apparent distance between objects at various distances. Wide angle lenses produce "expanded perspective," increasing the apparent distance between objects. This can create an optical illusion which makes the casual viewer believe that DOF differs in the photos.

In a 4x6 print, both buildings appear in focus thanks to extensive DOF; by 8x12 the one farthest from the lens is less sharp, but still highly acceptable. Move up to a 16x20 however, and the lack of critical sharpness becomes quite apparent. Epcot Center, WDW, Florida. (Canon EF 20-35 zoom at 20mm; f/11; Fuji Super G 200.)

·Vary The Focal Length From The Same Position. To be fair, telephotos do reduce DOF, while wide angle lenses expand it--but only when you shoot from exactly the same distance to the subject. It is actually image size which affects DOF, regardless of the lens. Whether you fill the frame with a telephoto from a distance or simply walk closer to the subject with a shorter lens, DOF will be shallower.

Example: Sit down on the grass across the street from an historic mansion and take a picture of the entrance area with a 28mm lens at f/5.6. Stay in the same spot, but switch to a 100-300mm zoom, taking three more frames: at 100mm, 200mm, and 300mm, all at f/5.6. Examine the prints and you will note that the door appears larger and larger in each successive photo, and the zone of acceptably sharp focus is noticeably less with each longer focal length.

So, a telephoto really does produce less DOF than a wide angle lens? Yes, but only when used from the identical shooting distance. It is the increasing subject size (at the greater magnification) which reduces the zone of acceptably sharp focus.

·Change The Camera To Subject Distance. As I have already implied, DOF is affected by the camera-to-subject distance. When you focus on something which is very close to the lens, you'll get very little depth of field. As you focus to greater distances--and this varies on focal length--DOF becomes more extensive.
Hint: When shooting extreme close-ups as in macro nature photography, remember that DOF will be measured in fractions of an inch. Position the camera so the back (film plane) is parallel to the subject plane--perhaps the wings of a Monarch butterfly. Now you need not stop down to a minuscule aperture such as f/32 to keep it all within the depth of field. The higher shutter speed available at a wider aperture will cause less risk of motion blur from camera or subject movement. As a bonus, the optics will provide higher resolution, as most lenses do at intermediate f/stops.

Example: You are working with a 300mm lens with a minimum focusing distance of 8'. Let's say you set an aperture of f/8. You notice a small blooming cactus in the Sonoran desert and snap a few shots from the car, while it is still far away--at infinity according to the distance scale on the lens. You then walk closer until the subject is only 8' away and take a second picture.

In the first photo, the cacti in the foreground are reasonably sharp, at least recognizable; those that are behind your subject all seem quite sharply rendered. In the close-up photo however, only your favorite cactus is sharp. Those in the background are soft shapes of green without any distinct lines and the foreground cacti are blurred away by the shallow depth of field. In both cases, the farther they are from the focused point, the less sharp they appear.

Remember: As you move closer or farther from any subject, two other factors change as well: the size of the subject in the frame and the perspective. The rendition of shapes and lines as well as the apparent distance between objects will appear quite different at 100' than at 8', for example. (This is visible in the viewfinder.)

·Set The "Right" Point Of Focus. As a rule of thumb, DOF is distributed as follows at common shooting distances: it extends about 1/3 in front of the point of focus and about 2/3 behind it. Want lots of depth of field? Then set focus at a point roughly 1/3 of the way into a vast scene.

Especially when working with an autofocus SLR, it's tempting to allow the camera to set the point of focus. While "shooting from the hip" may be necessary to capture a fleeting moment of action, it is important to control the exact point of focus when possible. That's simple even with an autofocus camera--use focus lock while recomposing, usually activated by slight pressure on the shutter release button.

Example: You are at a race track and notice three high performance cars parked nearby: a black Porsche in the foreground, a red BMW farther back, and a white Lotus in the background. You spot a celebrity leaning against the nearest vehicle, his blue eyes clearly visible. You must now make a decision. Do you focus on the BMW in the near midground to maximize DOF, keeping all three cars reasonably sharp? Or do you carefully focus on the person in the foreground so you'll have a razor sharp photo of him to show your friends?

When a situation includes a well-defined object as the primary subject, you will usually set focus for the most important point: the eyes in a portrait, the lettering on the side of a yacht, or the petroglyphs on a cave wall. DOF becomes a secondary consideration, although you do retain some control as described in the other sections.

·Focus At The Hyperfocal Distance. There is a specific point of focus which will produce maximum depth of field. Set focus at exactly this "hyperfocal" distance: the nearest point which will still keep a subject at infinity in adequately sharp focus. Now, DOF will extend from half the focused distance all the way to infinity. Naturally this differs depending on the aperture (f/stop) selected and the focal length of the lens. However, the technique does allow you to increase DOF without stopping down to minuscule apertures which would create blur due to camera shake or subject movement.

There is a formula for calculating the hyperfocal distance to maximize DOF, but it is rather complicated. You can find published charts listing the hyperfocal distance for easy reference. The laminated card (for 35mm camera owners) by Steve Traudt ($5.95, call (303) 245-6700) is but one example. With a 50mm lens, for instance, the hyperfocal distance is approximately as follows: 42' at f/8; 30' at f/11; 20' at f/16; and 15' at f/22.
Example: You are taking a picture of Times Square from a boulevard in the center of the street with a 50mm lens. You want to render the entire area sharply, but cannot stop down beyond f/11. (A smaller aperture would require a long 1/30 sec shutter speed, which would blur the moving cars.) Checking the chart, you set focus at 30', overriding the AF system which wants to focus on the blonde in the foreground.

On the viewing screen, the image looks strange as much of it seems out of focus. You take the picture anyway and find that everything from 15' away to infinity is acceptably sharp in the resulting photo--within the depth of field.

Photos taken with telephotos often exhibit shallow depth of field, leading some to believe that this is an inherent optical property of long focus lenses. In fact that's a fallacy as explained in the text, a concept well worth understanding. (Tokina AF 400mm f/5.6 AT-X; at f/5.6; Elite 100.)

·Focus On Infinity Only Rarely. Considering the information in the previous sections, why would you ever focus on infinity? Well, I very rarely do so with land or cityscapes, because: A) any foreground elements would be blurred (distracting) and such secondary subjects are often important--adding a three-dimensional effect to a two-dimensional photo, and B) most of the DOF would fall behind infinity, providing no real value, "wasting" depth of field.

However, infinity focus can be useful in sports, motor racing, and wildlife photography where the subject is located at a great distance. In these situations, you can live with some partially blurred elements in the frame--if any are included--because your primary goal is to produce an absolutely razor sharp subject.

·Preview Depth Of Field. Some--but not all--SLR (Single Lens Reflex) cameras from 35mm to medium format include a depth of field preview button. Also called a "stop down" control, it stops the lens diaphragm down to the actual "working" aperture selected. This is required for visually estimating depth of field, because otherwise, you always view the scene at maximum aperture with modern cameras.

Press the DOF Preview button and you can view the scene at f/8, f/11, or f/22--with an ability to estimate which parts of the scene will be very sharp, moderately sharp, and completely blurred away. Naturally, the focusing screen will darken, becoming almost black at the smallest apertures, unless the lighting is extremely bright. Before deciding that this control is "useless," remember that your eyes will adjust to the lower light level with time.

Stop down gradually, to f/4 then to f/5.6, and so on, moving to smaller apertures slowly as your pupils open wider, improving low-light vision. Granted, at f/22 on a heavily overcast day, DOF Preview may indeed be of little value: the screen will be so dark that it is impossible to estimate the zone of acceptable sharpness. In nature close-up photography, shine a flashlight on the subject as a focusing aid. In more common circumstances, at least with apertures no smaller than f/11, such precautions should not be necessary.

Remember however that any visual assessment of depth of field is an estimate and not a scientific technique. For example, a foreground swan which looked sharp on the tiny viewing screen may appear quite blurred in a 16x20 print. Nonetheless, particularly in high magnification photography, (with macro or telephoto lenses) DOF Preview is a valuable tool. Especially when the zone of acceptable sharpness is very shallow, it is important to asses DOF at several apertures and points of focus--first on the pistil and then on the stamen, for example. This technique will help to create technically excellent images.

Example: When shooting with an 80-200mm f/2.8 zoom at 200mm, you set f/8 for correct exposure when taking an environmental portrait of a blacksmith shoeing a horse at a pioneer village. You're shooting quickly to capture the instant when the hammer impacts the nail. When your prints come back from the processor, the outline of a green Land Rover roof in the background is readily identifiable, while a partially blurred baseball cap intrudes in the foreground. Of course, neither was visible while you were viewing the scene at f/2.8, but both are serious distractions in the image made at f/8.

To avoid disappointments of this type, especially with telephoto focal lengths, use the depth of field Preview control. This is always useful--unless shooting at the lens's maximum aperture, the only time that DOF is accurately depicted without Preview.

·Check The DOF Scale On The Lens. Frankly, some photographers admit to serious difficulties in assessing DOF visually. Others simply do not find this technique adequately reliable or accurate. If you fall into either category, learn to read the DOF scale on lenses which include such markings. These days, many do not, while those found on some zooms are inadequate or poorly designed: they tend to be difficult (or impossible) to decipher for any given focal length/aperture combination.

Nonetheless, if your lens includes suitable markings, set focus first and decide on an aperture which will probably provide adequate depth of field. Now glance at the two f/numbers (on the scale) for the selected aperture. These will show the distance range (near and far) that will be in acceptably sharp focus in the final image. If necessary, change the focused distance. For an added measure of DOF, stop down to an f/number one larger than the one you will likely require.

Example: You are lying on a sunny Cape Cod beach watching your daughter who is 10' away building sand castles near the surf. Out on the water a catamaran with a multicolored sail is approaching, its course paralleling the shoreline. You focus on the sand castle and set your 50mm lens to f/16 for plenty of depth of field. The DOF scale signifies that everything from about 7' to 21' will be sharply rendered in the photo at f/16.
Unfortunately, the boat is at least 100' away, so it will be rendered as partly blurred. No problem. You simply adjust the focusing ring until the "16" mark (designating f/16) to the left of center on the etched scale lines up with the infinity symbol. Now glance over at the "16" mark to the right. You'll notice it is at the 8' point on the distance scale. Focus at 8' and stop down to f/22 for an extra measure of safety. In the final photo, the youngster will appear very sharp, while the catamaran should be rendered adequately sharp as well--unless a long (motion blurring) shutter speed was used.

Try this exercise with your own lens in hand until it becomes second nature. Then it will be useful in any situation with any lens which is suitably marked, allowing for precise control of depth of field.

·Consider Other DOF Factors. In addition to the information provided so far, additional specifics about DOF are useful in certain photographic situations. First, depth of field depends on a subjective judgment. You may accept a certain degree of blurring as "still sharp," while a more critical viewer may consider it "fuzzy."
Serious macro photographers should know that in high magnification close-up work depth of field is quite evenly distributed. It is roughly equal behind and ahead of the focused plane.

The degree of sharpness we perceive also depends on projection distance with slides, and on the size of a print and the distance from which you view it. The zone of acceptable sharpness diminishes as larger prints are made. A foreground object which appears to be in focus in a 4x6 snapshot may look blurred in a 16x20 print or even in a 8x10 if viewed from very close-up.

All of the distances mentioned in this article relate to photography with 35mm equipment. In larger film formats, the concepts are identical but depth of field is actually less at the same aperture and identical subject distance. That's because larger formats require longer focal lengths to fill the frame from the same shooting position. For example, a 105mm lens in a 35mm system equals a 200mm lens on a 6x6cm (21/4" square) medium format camera and a focal length of 360mm if working with 4x5 equipment. The longer lenses required to produce the same image size (at the same subject distance) produce a narrower zone of acceptably sharp focus.

This is one reason why some 4x5 landscape photos are made at f/32 and even at f/64. However, the tilt/shift abilities of a view camera can also be used to increase DOF at wider apertures. This feature allows the film plane to remain parallel to the subject because the lens can be moved off-axis. A similar technique is possible with certain Perspective Control and Tilt/Shift lenses in 35mm equipment as well.

In addition to hyperfocal charts, you will find full DOF charts in some published material, including some lens Owner's Manuals. These allow the photographer to quickly predetermine the depth of field at any aperture with specific focal lengths, at each of several focused distances.

Conclusion. When combining all of the information provided, you can intelligently decide on how to achieve the exact depth of field desirable in any specific situation. That may be shallow in a portrait--to blur away clutter or extraneous elements which would compete for viewer attention. Or it may be extensive as in a classic landscape--maintaining sharpness in the foreground fence, midground horses, and the distant snow-covered hills. Experiment, make notes, and review the photos afterward to confirm you were on the right track.

Then read books which provide a more advanced coverage of the technical factors. Become intimately familiar with "circles of least confusion," the differences in perspective at various camera-to-subject distances, and the other "complicated stuff." The extra effort will be worthwhile, if only for one reason: the ability to control depth of field separates the snapshooter from the advanced photographer.

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