Depth Of Field…Not Simply Determined By Aperture
Understanding depth of field (DOF) is one of the keys to great photographs. Having too shallow a DOF can result in important subjects being out of focus. A very deep DOF may result in background distractions — areas that you intended to be out of focus may be sharp and detract from your subject.
For a long time I thought that the only thing to know was “large aperture = shallow DOF”. However, in addition to aperture, DOF depends on many other factors like the focal length of the lens, the size of your camera’s sensor, and the distance to the subject you are focusing on.
I have no intention to discuss the equations and physics which govern DOF here. If you find that interesting then search for articles on a site such as photo.net. What I will do is give a few examples of what to expect with certain settings. All the calculations come from dofmaster.com. I use their iPhone app — quite handy. Google “DOF calculator” or search the iPhone app store to find other programs if you’d like.
Let’s assume you are using a Canon 50D (my current camera) with the trusty 50mm f/1.8 lens. If your subject is 5 feet away (that’s your “focus distance”) and you’re shooting at f/2.8, whatever is between 4.85 feet and 5.16 feet will be in focus (roughly 4 inches). In a portrait situation this implies that if the eye is in focus, the tip of nose will barely be in focus (depends on the size of the nose 🙂 ). The back of the ear may be out of focus a bit. If you stepped back to a distance of 20 feet (granted this will be an entirely different composition) you now will be in focus from 17.7 feet to 23 feet — total DOF of over 5 feet. Note that this depth is obtained using the same lens and aperture as the portrait above but the DOF is different due to the focus distance.
Here’s a shot taken with a 50mm lens, f/2.0, approximate distance was 6 feet which gives a total DOF of only a few inches. The near eye was my focus point. Note how the rear eye and cheek are already a bit out of focus (I like it).
Here’s a shot taken with the same 50mm lens at f/4.5 (reasonably large aperture) but with a large focus distance. Everything is in focus from the front of the car to the sign behind it.
Let’s look at an example using a smaller aperture. Most of us would consider f/7.1 to be a reasonably small aperture and expect this to give “good” DOF. If you are focusing on a single person at a distance of 10 feet, they will be nice and sharp. However, your total DOF is only about 3 feet. If you were shooting into a crowd at this distance you would only be focused at a depth equivalent to a few people.
In the above image of Elijah, a 28mm focal length at f/7.1 gives exactly the effect I intended. The hand is in focus and prominent due to the wide angle, his face is blurred but still very much recognizable and part of the picture, and the distant background is quite blurred. Someday I’ll photoshop the kiddie pool out of the background…
So — pay attention to DOF and know how it will affect your images.
There’s a LOT more to know. I’ve ignored the fact that DOF is constant in a *plane* rather than simply a radius from the lens. I’ve not explained hyperfocal distance. Research and learn the basics at least. You may avoid the disappointment of finding out your small aperture didn’t give you much DOF or that your large aperture didn’t blur the background like you thought it would do.
© 2009 Michael Tuuk