Depth of Field Part II
The sun has yet to rise. I am sitting outside, enjoying the beautiful yet elusive time of day called “civil twilight”. Through my viewfinder I see a background of dark silhouettes, the mountains south of my home, and in the foreground I see grass heavy with dew. I wait for the perfect moment to press the shutter—and enjoy the feeling of anticipation that marks many morning shoots. The urge to snap away is tempered by patience, an attribute that doesn’t come easy but rather is a learned skill of photography: through the years I have learned that less can be more and rather than shoot every time the impulse strikes, I must take the time to visualize my images and wait for them to materialize. This makes the difference between chancing upon good composition and planning it—the difference, of course, is that only the latter can reliably be replicated. So much depends on vision and using the technical features of our camera properly.
Seeing, like exercising patience, is also an essential skill to the craft. I’d even label it a technical skill, and like other such techniques, it must be both learned and practiced. A photographer must spend time behind their lenses to actually “see” how differently their eyes and their camera perceive the world. Sure, Lightroom® and Photoshop® can minimize some of this difference, but the more technically savvy the photographer, the less dependent her or she is on such programs—and the more dependable and creative their work.
The above skills are essential elements of a photographer’s repertoire that also includes depth of field—which I discussed last month and continue below.
To review, the three main variables that determine depth of filed are aperture (the smaller the aperture, such as F22 or F32, the more depth of field); length of lens (the wider, such as 10 mm or 16 mm, the more depth of field); and focus distance (long distance or infinity yield the most depth of field).
Depth of Field Preview Button: Use It or Not?
As the name implies, the depth of field preview button allows one to view the depth of field before shooting. Almost all SLR have these buttons because virtually all lenses are automatic, which always open to their widest aperture (f 2.8 or 3.5) except when one takes a photo. The lens is always open so the image is bright enough for one to see clearly through the viewfinder. As a photo is taken, the lens stops down to the chosen f stop, but while it is stopping down, the mirror goes up, the view finder goes black, and one never gets to see the true depth of field that the sensor or film sees. Therefore, what one actually sees is the minimum depth of field. Let’s say I am shooting a portrait on a fairly bright day, and I have a zoom lens at 70 mm, a nice portrait length. My lens has a fixed aperture of f 2.8, which yields a beautiful soft background. However, not knowing any better and to adjust for the brightness of the day I close down to f 11. I still see the f 2.8 depth of field through the viewfinder, but moving from f 2.8 to f 11 gives me a much greater depth of field and diminishes the softness I see through the viewfinder in favor of a sharper background. This sharpness, in many instances, only distracts the viewer from the image’s subject. How can I see the difference between f 2.8 and f 11? By using the depth-of-field preview button.
Most SLRs have a depth-of-field preview button. When pressed, the scene darkens because the lens stops down to the set f stop. While the lower light makes the scene more difficult to see, our eyes usually adjust quickly so we can see the true depth of field of the photograph.
When should you use the depth-of-field preview button? Always in macro photography, usually in shooting portraits of people or animals, and while shooting landscapes when the foreground is close.
When I teach this feature, I have my students set their normal zoom to 50 mm or longer and then focus on a subject that is within two or three feet from them. Behind the subject should be a background substantially father away and out of focus. Then, while looking through the viewfinder, I have them stop down to a small f stop such as f 11 or f 16 to see the difference—I remind them, of course, that the entire scene will darken when the button is depressed. I then advise the students to press the depth of field preview button and click through the full range of f stops to find the most pleasing aperture for the shot.
Other Tips for Using Depth of Field
- Practice. Often, this means shooting the exact same subject in several different f stops and seeing how the depth of field changes. The easiest way to do this is to have the camera on a tripod, go to the aperture-preferred setting and click through the full range of f stops for a particular lens. For example, if you have an f 2.8 zoom, your full f stops would be 2.8, 4, 5.6, 8, 11, 16 and possibly f 22 or even f 32. Each time you change the aperture, the shutter speed changes in the opposite direction to maintain the exposure. So if nothing is moving in the photo, the only visible change is depth of field.
- Choose Backgrounds & Foregrounds. Landscapes are often composed of foregrounds and backgrounds; for instance, you may have a close-up field of flowers leading up to a distant mountain range. In this instance, even the smallest aperture (example f 22) may not give enough depth of field to make everything in the photo sharp. As such, I’d advise making the foreground area of the image truly sharp. Because the foreground is closer, one expects to see detail; likewise, because the background is farther away, viewers don’t expect its features to be tack sharp. In this scene, I’d manually focus on an individual flower or a small cluster of them in the foreground. As a general rule, my flower of choice would be something a third of the way into the frame. The composition, therefore, mimics how someone would actually experience the scene and is more natural.
- Bracket. I often bracket where I focus. This means focusing one third of the distance into the frame for one photo and then taking a subsequent shot with a closer focus; if I don’t have quite enough depth of field to make my foreground tack sharp in the first shot, I’d likely have it in the second, knowing that in most cases a slightly soft background will not hurt my photograph.
- Use Image stabilization/Vibration Reduction. If one hand-holds their camera and stops down to a smaller f stop to gain depth of field, the shutter speed decreases each time the aperture becomes smaller. This is where image stabilization/vibration reduction lenses are extremely useful, as they allow a photographer to stop down two to four shutter speeds slower than otherwise possible. If I don’t have this feature, an option is to raise the ISO; thankfully, newer DSLRs do not degrade the image as much as older ones.
NOTE: Remember, when seeking sharpness and proper depth of field, nothing beats a tripod, and even though a small lightweight tripod is better than handholding, a heavier one with a sturdy head is ideal. I use a Gitzo tripod with carbon fiber legs with a Kirk Enterprises BH-3 ball head with a Kirk L-bracket so I can shoot both horizontally and vertically right over the top of the tripod. Also, remember not to use the tripod neck—if possible, keep the center post collapsed all the way.
- Avoid Smallest Aperture if Possible. Again, as aperture decreases (f 22, 32, etc.), depth of field increases until the smallest aperture. As we stop down, more of the photo sharpens. There is a problem, though, that I need to alert you to: the smallest apertures somewhat degrade image quality. As such, I avoid them when possible, especially with inexpensive lenses or if I plan on making larger prints of the image in the future. The sharpest f stops on lenses are usually those in the middle, such as at f 5.6, 8, 11—I try not to go to f 22 or 32 unless I really feel the need to obtain a maximum depth of field. Still, I often recompose my subject to avoid these very small apertures.
Learning depth of field takes practice and a lot of time spent analyzing your images. This feature (as well as all the above tips) will make sense the more you shoot and critique your work. Many times, photographers don’t pay enough attention to the sharpness of their images until months or years later when they want to make larger prints but don’t have the quality to do so. Among other reasons, this is why it is so important to invest in this and other technical skills as early as possible in your photographic career.