One of the most important decisions a photographer makes when creating a photograph is how much of the image to have in focus. This is a creative decision because whatever you decide will limit what the viewer will focus on in your photograph.
Most of us are aware of the relationship between aperture and depth of field, that is, the larger your aperture, the shallower your depth of field meaning less of your image will be in focus. The smaller your aperture the greater the depth of field resulting in an image that is much more in focus. What you may not know is that your choice of lens also affects how much is in focus. Here’s where a handy little app called Simple Depth of Field is very useful.
As its name suggest, it is simple. After you download the app, select your camera from the list. In this screenshot I’ve selected my Olympus OM-D E-M5.
Next choose your focal length, aperture and focus distance. The app will then show you the depth of field i.e. what portion will be in focus. In this screenshot below, I’ve selected my 25mm lens (a 50mm equivalent) and an aperture of f8 with a focus distance of 5 feet. This indicates that if I focus at 5 feet from where I am standing everything in my view from about 3 feet 11 inches to 7 feet will be in focus. Pretty cool, isn’t it?
Now here’s where things get really interesting. In the next two screenshots I’ve selected my 17mm lens (a 35mm equivalent) and my 25mm lens (a 50mm equivalent) and set them at the exact same aperture (f1.8) and focal distance (3 feet) to determine the depth of field. Notice how the depth of field for the 17mm lens is more than double that of the 25mm lens at the same settings.
If you don’t have an iPhone, you can use this online depth of field calculator DOFMaster instead. It is not as up-to-date as the app but many cameras are listed there.
This information is very useful when you are out making pictures as you need to make a lot of creative decisions in a very small amount of time. If you have a favourite lens like a 50mm lens, you can figure out ahead of time what settings to use. Chances are you will have more keepers when you are armed with this information.