The Depth-of-Field Preview Button
uite amazingly, the general consensus on the depth-of-field preview button is that it is a useless tool. I couldn’t disagree more! Also known as the DOF button, the depth-of-field preview button is one of the most useful tools found on many SLRs and DSLRs, yet you wouldn’t know it based on the huge amount of confusion it causes for many amateur photographers. But trust me, if your camera has a DOF button and you actually learn how, where, and when to use it, you’ll soon be wondering how you ever lived without it. (If your camera doesn’t have a depth-of-field preview button, no worries; there’s a way to achieve the same benefit, although it might feel a bit arcane, and I’ll get to this in a minute.)
The depth-of-field preview button gets its name from the function it performs. When you press the DOF button and look through your viewfinder, you are able to preview your depth of field. You can use this feature to check whether or not you have chosen the right aperture. Because of this, I refer to it not as the DOF button but as the DIM-TRAC button—as in, Did I Make The Right Aperture Choice? Additionally, I like this nickname because when you press the “DIM-TRAC” button, the image in your viewfinder does go quite dim (I’ll explain this in a minute) and the feature does allow you to track how much or how little depth of field you currently have.
So, let’s take a look at these three images to see how the DOF, or DIM-TRAC, button actually works. And keep in mind that this feature is not just for macro work; whether you’re shooting up close or with a telephoto or wide-angle lens, the DIM-TRAC button is the tool to embrace!
Top: f/22, no DIM-TRAC
middle: f/22, DIM-TRAC engaged
bottom: results at f/16
Let’s assume for a moment that you and I are shooting side by side in a flower garden, and we are both uncertain if our aperture should be f/8, f/11, or f/16. We know we want some additional depth of field but are not sure how much. So we begin with f/16, and I suggest that we press our DOF, or DIM-TRAC, buttons. Not surprisingly, the first thing that happens is the image in the viewfinder gets a lot dimmer. (Hence, my nickname, DIM-TRAC.)
Why does this happen? Let’s just review how the camera works for a minute. Every SLR and DSLR offers wide-open viewing, which simply means that no matter which aperture you select, you are always viewing things as though your lens were set at a wide-open aperture. (Remember, large apertures are the smallest f-stop numbers.) Regardless of your chosen aperture, your camera will not stop down to that aperture until you press the shutter release to make the exposure. At that point, two very important things happen: (1) The mirror inside the camera body flips up and out of the way to allow the exposure to be made, causing a momentary “blackout” in the viewfinder, and at this point, the lens stops down to your chosen aperture; and (2) when the light passes through the now-stopped-down aperture, sharpness (depth of field) increases. The amount of added depth of field is directly proportional to the aperture choice: the smaller the aperture, the greater the depth of field.
So, returning to that flower we’re trying to shoot, the reason the viewfinder gets dark when you press the DOF button is that you have engaged the same lever that stops down the lens when the actual picture is being made (opposite, middle). Therefore, the diminished light coming through the lens is the result of less light coming through your chosen smaller aperture. (When you don’t press the DOF button, you are viewing through a wide-open aperture and, thus, seeing a brighter image in the viewfinder.)
From now on, however, you shouldn’t care one iota about the diminished light when pressing the DOF button. Instead, start paying attention to something far more important: the increase in front-to-back sharpness. Trust me, a change in depth of field has taken place, and you will see it, even if it takes a few seconds for your eyes to adjust to the dimmer light. It is these areas that you must study closely so that you can determine whether or not you want this added sharpness and detail. If the sharpness is what you want, then release the preview button and take the picture. If it’s not, then with the preview button still engaged, start opening the lens up from f/16 (or whatever your aperture may be), working toward f/11 (for this particular example). As you do this, you’ll notice those defined areas begin to diminish. If at f/11 those areas are still too defined, open up the lens even more; go toward f/8, heck, even f/5.6 if you feel it helps. Once you arrive at an aperture that renders the effect you like, adjust your shutter speed until the camera’s meter indicates a correct exposure and shoot.
In effect, the depth-of-field preview button on your camera is much like the tuning knob on a radio. You “tune in” your image, eliminating any background “noise and static” before you make the final exposure.
80–200mm lens, Canon 500D close-up filter, f/11
Taking this shot of a lone dandelion in the Czech town of Ceksy Kremluv definitely called for DOF button use. With the right lens, I felt I could shoot a close-up of the dandelion and give it a sense of place by including the distant church in the composition. Lying on my stomach, with the camera and lens at ground level, I framed it all up at 55mm. However, when I initially viewed the scene, the church was not nearly distinct enough (above right). With DOF preview engaged, I started by adjusting the aperture to f/16 (too sharp) and then to f/11 (still too sharp) before finally settling on f/9, at which point I felt the church was playing the supporting role it was meant to play—defined but by no means tack sharp.
Right: Nikkor 17–55mm lens at 55mm, f/2.8; left: Nikkor 17–55mm lens at 55mm, f/9
POOR MAN’S DEPTH-OF-FIELD PREVIEW BUTTON
Those of you without a depth-of-field preview button might feel that you’re missing out on a valuable tool. You are, but there are ways around it that are much less expensive than buying a whole new camera system just for the DOF button.
Before shooting a single-theme image (one in which you are featuring a lone subject against a blurry background), pretend to remove your lens from the camera body while still looking through the viewfinder. By pretend, I mean turn the lens one-quarter of a turn or so, as if you are about to remove it. When you do this as you look through your viewfinder, you’ll see the actual depth of field that your chosen aperture will render. (The viewfinder will also become darker, just as it does when you press an actual DOF preview button.) And voilà, you’ve just discovered what I like to call the poor man’s depth-of-field preview button.
At this point, once you’ve turned the lens, take note of how much stuff comes into play in front of and behind your subject. If you like what you see, click the lens back into its On position and fire away. If not, adjust the aperture and repeat the above technique, “removing” the lens and taking a look.
Most images that display effective isolation of the subject are made with a combination of the following: a telephoto zoom lens at focal lengths between 200mm and 300mm with a 50mm or 36mm extension or an added close-up filter, such as the Canon 500D. Most of the time for shots like this, the working distance between the photographer and the subject is around 10 to 12 inches. Assuming that you’ve chosen a point of view that includes a complementary background (i.e., a background about 15 to 30 inches behind the subject) and that you wish to render the background as out-of-focus tines of color, it’s a fairly sage bet that the right aperture is somewhere between f/5.6 and f/11.
If your camera offers 1/3-stop brackets, you could shoot a total of seven correct exposures, and certainly one, if not two, of those will end up being the one you’ll like. Since there’s no film cost for digital shooters, this method of achieving the best exposure may make the most sense. So for example, if you’re getting a reading of f/5.6 at 1/320 sec., you would then shoot one each of the following exposures: f/6.3 at 1/250 sec., f/7.1 at 1/160 sec.. f/8 at 1/125 sec., f/9 at 1/100 sec., f/10 at 1/80 sec., and f/11 at 1/60. Keep in mind that all these exposures are the same in quantitative value but different in terms of their respective depths of field. And that is ultimately what you want to look for with shots like these: depth of field that complements the focused subject.
So that there is no confusion, note that the technique I just described is not bracketing, since each and every exposure is the same quantitative value. Bracketing is when you shoot a number of different exposures. For example, if f/5.6 at 1/500 sec. is a correct exposure, then to bracket, you shoot f/5.6 at 1/250 sec. for a +1 exposure and f/5.6 at 1/1000 sec. for a -1 exposure. These three exposures all have different quantitative values.