How to Blur Your Background Using Depth of Field and Really Make Your Photos Pop!
“Depth of Field” is a phrase that is often used in photography articles, magazines, and manuals. It’s one of those confusing uses of the Photography language that often form barriers of understanding which separate the pros from the novices. I don’t particularly care for that practice. Be it computers, mortgage documents, or photography, I think we need to try to improve the process of communication, not add to the confusion. So, let’s call it “Depth of Focus” or DOF for short. That is really what we are talking about.
DOF is Your Range of Clear Focus from the Front to the Back of a Scene.
What is DOF?
DOF is a simple concept, but one that confuses many people. It refers to the range of clear focus from front to back in a scene. Although we live with DOF all the time, we never need to give it much thought until we try to take a photograph. Our eyes, like a camera, are limited in the range they can focus. But our eyes are connected to that supercomputer we call a brain, and so they can adjust very quickly. If you are reading this on your computer, your eyes are focused at about two or three feet. When someone enters from across the room, you look up and instantly refocus on them. At that point, the computer screen is blurred or out of focus. When you return your gaze to the computer, your eyes again refocus and the friend across the room loses sharpness. So, for any given distance, what you see clearly is limited by where you place your eyes to focus. This is Depth of Focus. It’s true for eyes, camera lenses, and even pinhole cameras without lenses. We’ll talk about these later.
You Have Control Over How Deep the Range of Clarity Appears.
The important idea is that there is a limited range of clarity when we try to take the three dimensional world and represent it onto a two dimensional photo. If near objects are in focus, distant ones may be fuzzy, and vise versa. Notice I said “may” be fuzzy. I didn’t say “will” be fuzzy. That’s because we have control over how deep the range of clarity appears. You’ve seen lots of landscapes that were tack sharp from the flowers in the foreground to the mountains twenty miles away. You have also seen close-up photos of people with a background only a few feet away so blurry that it's unrecognizable. How can there be both a deep DOF in the landscape and a very shallow DOF with the people? The primary reason is aperture.
How Does Aperture Affect DOF?
Most cameras have an adjustable iris, just like the one in your eye, that opens and closes to adjust the light passing through your camera lens. The size of the hole the iris makes determines how much of your scene is sharp or not so sharp. This hole is the Aperture ,and controls how much light from your scene passes through your lens to the camera's sensor where your photograph is recorded. A large hole lets in more light, a small hole lets in less light, and there are multiple options in-between each changing your DOF.
Now here’s the really cool part. When the iris is wide open, the Depth of Focus is narrow. This shot was taken with an aperture of f-2.
What's your Focus Using DOF to make your photos pop image F2
When the iris is closed to only a pinhole, the DOF is deep. This shot was taken with an aperture of f-16.
What's your Focus Using DOF to make your photos pop image F16
So that gives us, as photographers, lots of control over the look of the photo that is taken. Open the iris to get a narrow DOF and the girl stands out in front of a fuzzy background. Close the iris and stretch the DOF from the girl to infinity for a breathtaking vista. There is, however a catch.
When the iris is changed, the amount of light coming through the lens also changes. We need to adjust for this by changing the time that the shutter is open to allow the sensor to be exposed. If we use a small aperture to gain a deep DOF, the light passing toward the shutter and sensor is greatly reduced. So, to compensate we need to keep the shutter open longer. OK, that’s easy. But wait… If the shutter is open too long, moving subjects, like cars or people will blur in the photo. So to get the deep DOF I need for my great landscape, I have to live with blurry birds flying overhead. Well, not quite. There is some help available. Most cameras allow the sensitivity of the sensor that records the image to be changed. If I can’t change the aperture, and that makes the required shutter speed too slow, I can increase the ISO to make the sensor require less light to make a correct exposure. Then I can increase the shutter speed to freeze the birds. But, as with many things in life, there are side effects. Too high an ISO setting can introduce “noise” or a stippled look to smoother parts of the photo, like the sky. This is why pros increase the ISO only as a last resort. And that gets us to why the professional photographers will spend thousands of dollars for cameras that don’t seem to have more features than the pocket models we can buy for a couple of hundred dollars.
There are other factors that can help to control DOF, exposure, and motion. Lenses that open much wider to let in more light will allow for faster shutter speeds. But those extra wide apertures cause distortions in the photos. So high grade glass which is precision shaped is needed to eliminate them. That’s expensive. If you frame 8x10 prints of your photos as gifts for friends, the simple lens in your pocket camera is fine. But if you need to create wall size posters or you have to crop a small section from an image and expand it, an expensive lens is a necessity. And there are some situations when only the big, expensive cameras will get the shot. I once photographed a space shuttle launch at night from twelve miles away. The image filled the frame. I printed it onto posters that were 13 x 19 inches. That required a high quality telephoto lens and a very heavy tripod. No pocket camera has that ability.
Another factor that affects DOF is the focal length of the lens. Sorry for using a technical term. Focal length is just a way to designate the angle that a lens sees. Telephoto lenses see narrow angles and therefore make it seem as if you are closer to your subject. Very handy when shooting temperamental wildlife. Wide angle lenses, as their name implies do just the opposite and let you fit all the family into the frame for the Thanksgiving portrait. But these different lenses have different DOF qualities. Long, telephoto lenses produce narrower Depth of Focus, The wider the lens, the more the DOF increases.
Depth of Focus is also affected by the size of your camera. If a very small lens is used to squeeze into a compact pocket camera, the DOF will be deeper than from a lens with the same angle on a larger DSLR camera. All that means is, don’t expect to get quite as much background blur when shooting close-ups in the rose garden using a pocket camera. Distance is also a factor. The closer you are to your subject, the narrower the DOF. That’s why extreme close-up photos of lady bugs on a flower’s petal will always blur most of the flower. These extreme close-ups are called macro photography, and it is a specialty unto itself.
So let’s finish with some handy ways to use these new ideas in the field. If you are using a pocket camera, expect that most of your photos will have a deep Depth of Focus. But if you take close-up shots, use the Aperture Priority setting ( Av, Ap, or A ) to lock in the DOF that you want. Deep DOF requires high aperture numbers like 8 or 11. For narrow DOF, use the widest aperture that your camera will allow, such as 2.8, 3.5 or 4. If you get a warning that the shutter speed is to slow to hand-hold the camera ( a shaky hand symbol ), increase the ISO until the warning goes away. In any case, use a tripod or find something to support the camera for much better results.
If you have a DSLR, your options are greater. You can affect the DOF in landscapes by adjusting the aperture as explained above. Remember that the longer the lens, the narrower the DOF and vice versa. So if you want very deep DOF, use a wide angle lens and move closer. If you want to blur the background, foreground, or both try a telephoto lens and move back to frame the same scene. The bit about using a tripod or firm support holds even more so for you. Unlike the simple pocket cameras, DSLRs have more moving parts that must be steadied for sharp images.
Oh… by the way. Remember that I said I would talk more about pinhole cameras that don’t have lenses. Well here is a bit of trivia that can save your day. When photography first started, cameras were simply a box with a pinhole punched into one end and a film plate attached to the other end. The size of the pinhole determined the aperture. And just like today’s cameras with adjustable irises, the same rules of optics apply. So a small pinhole produces a deeper Depth of Focus than a larger one. Here’s the part about saving your day. Take a needle and with pliers, hold it in a flame until it heats up a bit. Then use the needle to punch a small hole into an expired credit card or something similar. Keep this card in your wallet or purse. If you ever forget your reading glasses and can’t quite decipher the print on that train timetable or medicine bottle, hold the card up to your eye. You will be amazed at how the narrow DOF of the pinhole will improve your ability to read the fine print.
