Depth of field, or DoF, is one of my favourite subjects and tools. Although DoF can very quickly become a complex subject with many nuances, I’m going to tackle the most basic concepts and more importantly the three main factors you can instantly and easily learn to utilize this valuable concept.
The simple definition of DoF is the range, or depth, of acceptable focus. Another way to put it is DoF is the distance between the closest and farthest points away from you that are still in acceptable focus. If someone says they have a depth of field of 3 feet, they mean that while they are focusing at a single point, there is a total depth/range of 3′ in focus, part of which is in front of the point they are focusing on and the rest behind the focus point. So with the 3′ DoF example, if I was focusing at 10′, the depth of field could be from 9′ to 12′. The actual depth of field, along with how much is in front of and behind the point of focus can vary due to several factors, but for the sake of this post I am going to stick with the three simplist factors, which are lens aperture, lens focal length, and focus distance (distance between you and the point you are focusing on.
I’m now going to distill down to three simple points how each work! How’s that for fast?
Aperture! The larger the aperture (i.e. the smaller the number), the shorter the depth of field. The smaller the aperture (i.e. the larger the number), the longer/deeper the depth of field. So…at the same exact distance of a specific point you are focusing on, an example of DoF via aperture could be say 3′ at f/2.8, 10′ at f/8, but a whopping 100′ at f/16. Easy enough so far?
Lens Focal Length! The longer (higher number of mm) the focal length, the shorter the depth of field. The shorter the focal length (lower number of mm), the deeper the depth of field. So…at the same exact distance of a specific point you are focusing on, an example of DoF via focal length could be 1′ at 200mm, 5′ at 50mm, and a nice 20′ at 24mm. Still with me? Hope so!
Focus Distance! The closer the subject is to you, the shorter/smaller depth of field. The farther away a subject is, the deeper the depth of field. So, with the same exact aperture and focal length, a subject at 5′ away from you might have a depth of field of maybe 6 inches, while the same subject and other settings at 10′ away from you might have a DoF of 3 feet, and a focus point on a subject at 30′ might have a depth of field 10 feet. So far, so simple?
Here’s the short-hand, abbreviated, or rather cliff-notes:
Larger/Bigger Aperture (smaller numbers like f2.8), longer focal lengths (bigger/higher mm #’s), and the closer the subject is to you, ALL make the effective depth of field smaller/shorter.
Smaller Aperture (bigger numbers like f/22), shorter focal lengths (smaller mm #’s on the lens), and the farther the subject is from you, ALL make the effective depth of field longer/bigger.
Another quick way to remember it is:
Small/Short Depth of Field – Large Aperture, Long Focal Length, Close Subject.
Large/Long Depth of Field – Small Aperture, Short Focal Length, Far Subject.
or
Very Small/Short Depth of Field Examples – f/2.8, 200mm, 5′ away.
Very Large/Long Depth of Field Examples – f/16, 14mm, 40′ away.
I hope this helps. It’s a quick way to remember DoF and how to increase or decrease it. Subject distance is easier to remember (longer distance = longer DoF), but aperture and focal length are counterintuitive when you think about the numbers (bigger aperture and bigger focal length = smaller DoF). BUT, always remember that a smaller aperture number is bigger, while a bigger aperture number is actually smaller.
Go have some fun experimenting with these examples and concepts to master them, or even remember them, lol.
See Think Shoot 