Depth of Field: The 1/3 - 2/3 Myth

1/3 - 2/3 Myth

When I first started in photography I tried read lots of books and magazines and I attended camera club meetings and seminars. Often a speaker would present a talk about photography and I tried to gleam a fair amount of nuggets of information to try and improve my photography. One thing I realized as I studied the material more was that even some of the most experienced photographers don’t fully understand the material that they are presenting.  

A shallow depth of field isolates a spring flower.

As a result misinformation is passed along and before you know it, there are a whole bunch of people who begin to think the misinformation is gospel.  The best way to describe this is that there are myths in the photo industry and it’s best we understand the correct information before we mess up our images because of a myth. 

Today let’s talk about the Depth of Field myth that I call the 1/3 2/3 Myth.

With Macro focusing distances, the depth of field becomes very shallow.

Depth of Field - as defined in the Canon book “Lens Work III”, tenth edition - is the “area in front of and behind a focused subject in which the photographed image appears sharp.” Other sources define it as “acceptably sharp”.  Depth of Field is something that many folks have a hard time getting their head around so let’s keep this discussion to simple terms. In a later post we’ll really dissect Depth of Field to understand more of the finer nuances of the subject.

We all know that when you focus on something, the area of acceptably sharp focus is both in front and behind the subject upon which you focused.  The 1/3 - 2/3 Myth incorrectly states that of the total depth of field, 1/3 will be in front of the subject and 2/3 will be behind the subject. 

Well, at some point that is true. But as you adjust the focus the total depth of field and the ratio between that portion in focus in front of, and behind, the subject drastically changes. 

Take for example an 80mm lens with an aperture set at f/8.  If you focus at 30 feet, then 1/3 of the total depth of field is in front of your subject and 2/3 is behind.  Just what our myth says. But what if you focus on a subject that is only 10 feet away.  Your total depth of field then is only 2.3 feet and 43% is in front of the subject and 57% is behind.

Depth of Field calculations for an 80mm lens set at f/8

Now focus at something that’s 60 feet away. You’ll have a total depth of field 153.5 feet.  Only 16% will be in front of your subject and 84% will be behind your subject. In fact, just before your far focus distance jumps to infinity (∞) less than 2% of your total depth of field is in front of your subject. 

As you can see, 1/3 - 2/3 is a myth and not at all supported by the facts. So how does something like this get started and is there any benefit to this “rule of thumb”.  

As in any complex topic, teachers are always striving for ways to simplify the subject to make learning easier. I’m sure that this was a well-intentioned over simplification of a topic that can get rather difficult. When you look at the chart above, there is something that jumps out - the total percentage of depth of field in front of your subject is always less than that behind the subject. And, at common distances for shooting many subjects, the percentage of depth of field in front is fairly close to 1/3.  

So, for this lens focal length and shooting in that mid range (20 - 40 feet), 1/3 - 2/3 may be an acceptable rule of thumb.  But for applications that require critical focus, it’s best to calculate precise values. A great source for focus information and depth of field calculation is available on the website which is produced by Don Flemming. On his site you can also find downloads for DOFMaster programs for iPhone and Android.