What is depth-of-field?
Depth- of-field can be described as the part of the photograph in front and behind the subject that also appears to be in focus. I use the term “appears”, because only one part of the picture can be truly in focus, but in some photographs the out-of-focus portion is almost as sharp as the rest of the picture.
We like to use a shallow depth-of-field when we want to concentrate the viewer’s attention on the subject. Shallow depth-of-field makes the subject pop out of the blurred background and the background seems more uncluttered when we can’t distinguish detail.
Aperture Controls Depth-of-Field
Many people rightly believe that depth-of-field is controlled by setting the aperture of the lens. A large aperture, such as f/1.4 will create a very shallow depth-of-field, while a small aperture, such as f/16, will create a deep depth-of-field.
Some photographers have a hard time equating the f-numbers with the size of the aperture. It’s easy if you remember that a small f-number equals small depth-of-field, a large f-number equals a large depth-of-field.
This is where many people’s knowledge ends and why they get into trouble with out-of-focus people in their group portraits. Unfortunately, the apparent depth-of-field in the picture is far more variable than you might think and can change drastically based on a few minor adjustments.
Focus distance controls Depth-of-Field
The distance you are from the subject you are focusing on has a big impact on the depth of field.
The closer you are to the subject, the shallower the depth-of-field.
With a 70 mm lens set at an aperture of f/3.5 and focused on a subject 10 feet away, the depth-of-field is 1.3 feet. If we double the distance to the subject by moving back to 20 feet, the depth-of-field becomes 5.3 feet. At 40 ft the depth-of-field is 22.5 feet and at 80 feet the depth-of-field is around 122 feet.
We can see that the depth-of-field quickly becomes very deep as we back away from a subject. Likewise, as we move closer to the subject, depth-of-field becomes critical. Macro and micro photographers, who work very close to miniature subjects, have to use very small apertures (i.e. large f-numbers) just to get all the petals of a flower in focus. If you were photographing a family group from 10 feet away with the settings described above, you might have Baby nicely in focus at the front while Dad at the back of the group looks blurred. The answer would be to step back and/or stop down the lens (i.e. choose a bigger f-number.)
Jeff photographed in my kitchen. Available light at f/1.4 50mm.
The cluttered background falls into a soft out-of-focus effect.
Lens focal length affects apparent depth-of-field
Most DSLR cameras come with kit lenses and many of these are 18-55mm zooms. When you zoom out to a wide angle setting you are using the lens at 18 mm, which is a wide angle. When you zoom all the way in to magnify the subject as much as possible, you are using the lens at 55mm. Unfortunately, when you zoom in or out to frame your subject the way you wish, you are also affecting the apparent depth-of-field. (Not really, this is why I used the word apparent again, but bear with me and let’s keep it simple.)
- The longer the focal length the shallower the depth-of-field.
The depth-of-field becomes shallower the more you zoom into a subject and it becomes deeper the more you zoom out from it. Shallower depth-of-fields are best achieved with lenses bigger than 50mm. Forget trying to get a shallow depth-of-field with your lens zoomed out to 18mm, because at just over 10 feet from the subject, that f/3.5 aperture will bring everything in the picture in focus that lies from just over 5 feet away to infinity.
So what do I do?
It all sounds so complicated, I know. Where we might have thought there was just one thing, aperture, controlling depth-of-field we now find there are three variables all working against each other! Such is the life of those who choose to capture images with lenses. Actually, the type and make of camera you have affects depth-of-field too, but I haven’t gone into that.
Most of the time, knowing the exact depth-of-field isn’t too critical. As long as you are not too close to your subject with a large lens and wide aperture, you’ll get your subject in focus. The problem is often that you can’t seem to get the background to fall out of focus.
Something like a 70 mm f/2.8 lens will get you a nice out-of-focus background for portraits if you have the money to spend on it. If you are working with your Canon 18-55 mm kit lens, then you have to work harder.
Working with a kit lens
Most kit lenses are slow, meaning that at the maximum zoomed in setting of 55mm the widest aperture you can set is f/5.6.
To get an out of focus background with such a lens, it helps if you choose an evenly lit location in the shade and do not have your subject close to the background. In fact, the more distant the background, the better.
- Select aperture priority mode (Av or A) and set the aperture to f/5.6. (If you can’t do this, stop reading and book yourself on one of my workshops!)
- Zoom all the way in to 55mm.
- Step in to the subject as close as you can while framing the composition you want.
- If you see the shutter speed is slower than 1/60 sec, it is getting too dark to hand hold and you will need to turn up the ISO.
- Fire when ready.
The best way to get a feel for how depth-of-field works is to find a scene and practice with different f stops, distance from the subject and amount of zoom.
The image above was taken at f/4.0. The background flowerpot is out of focus and the window is even more blurred.
The image above was taken at f/16. Now, both flowerpots and the window are in focus.
If you wish to experiment with depth-of-field, there are many calculators on the internet or applications for smart phones. I use a a free one called Field Tools that can be searched for in the iTunes store.