The impact of the Lens Focal length on Depth of Field
There are four factors which impact the Depth of Field (DOF). They are:
- Sensor Size
- Lens Focal Length
- F stop
- Distance from the camera to the subject.
This blog will briefly discuss the impact of the lens focal length on the DOF.
In this illustration we have set all of the factors or variables to a constant set of values except for the focal length of the lens so we can see what impact the lens focal length has on the DOF.
- The Camera is focused at 10 feet.
- We used the same set of f stops for each diagram (f/2.0, f/2.8, f/4.0, f/5.6, f/8, f/11, f/16 and f/22).
The shaded area between the Near Focus Limit and the Far Focus Limit is the DOF or area of acceptably sharp focus for each f stop. See lesson DP-111C Focus and DP-112C Depth of Field for a complete discussion of this topic.
The following diagram shows the DOF of four lenses with focal lengths of 18mm, 50mm, 100mm and 200mm. The DOF of each lens is overlaid one on top of another so you can easily see the results. The 18mm lens DOF is the bottom overlay and it’s total DOF consists of the shaded areas for the red, green, blue and yellow areas. The next overlay is the 50mm lens whose DOF consists of the green, blue and yellow shaded areas etc. (Click on the diagram to see a larger version.)
You will note that the 18mm lens has a much greater DOF than does the 200mm lens. This illustrates the principle that wide angle lens or lens with a shorter focal length have inherently greater DOF than lenses with longer focal lengths. You can use this optical characteristic to your advantage in certain photographic situations. For example:
- Use a shorter focal length lens when you want to maximize or gain as much DOF as possible. This might be for a landscape image.
- Use a longer focal length lens when you want a narrower DOF. This might include portraiture etc.
Another interesting observation is how rapidly the 18mm lens goes from its’ most limiting DOF at f/2.8 toward the maximum DOF at f/22. If we compare that to the 200mm lens’ DOF, we note how the DOF is limited across the entire range of f stops and does not change as rapidly throughout the f stop range.
How rapidly the DOF falls off can have some significant implications when you want to create a completely out of focus background. In those cases you want a fairly step vertical curve to get a very rapid DOF drop off. This is the topic of another blog.
As always there are exceptions and personal preference. You don’t always have to go to a longer focal length lens to get a shallow DOF. Some newer 50mm and other shorter focal length lenses now have very small f stop numbers in the range of f/1.0. (Some even go below f/1.0.) They are intended to provide a very limited DOF in a non-telephoto lens and to provide a large aperture for higher shutter speeds in low light situations. The following diagram illustrates the DOF characteristics for one of those types of lenses with a 50mm focal length and a native f stop of f/1.0. We show the DOF for f stops from f/1.0 to f/2.0 in green. The DOF for F stops f/2.8 through f/22 is in red. (Note: F stops down to f/2.8 are considered fast lenses. Lenses with f stops less than f/2.8 are very fast lenses. (Click on the diagram to see a larger version.)
An analysis of this diagram shows that you do get some small reductions in DOF as you move from f/2.8 toward f/1.0. Much of the advantage of this lens at this focal distance is in the larger physical aperture to allow more light into the camera and thus increase potential shutter speeds.