Depth Of Field

How to control Depth of Field in your photos

In the last couple of months, I have received a question multiple times “how do I create a creamy blur background in my photos. And why do almost all portrait photographers prefer f/2.8-200 mm lens ?”

Thanks for the question because the blurry background is the key to a beautiful portrait shot. Technically, a “creamy blur background” is called “shallow“ Depth of Field. On the other hand, in landscape photography, we want nearly everything in focus. technically we called it “Deep” Depth of Field. So, first, we have to understand a few concepts pertaining to Photography.

A focus point, Focal Plane( Plane of Focus), and Focus Distance
Area of Acceptable Focus
Depth of Field (DOF)

Ok, Ok… I know what you are thinking “I am a creative person, when I am reading about photography-related topics and the big words, then the weird numbers start flying around me…” Please calm down. I am going to explain those topics in a very simple and understandable way.

1. Focus Point, Focal Plane ( Plane of Focus ), and Focus Distance:

Nowadays, almost all DSLRs have a focus point indicator in their view-finder to point out where we are focusing. When we focus on a subject, we use these focus indicators to focus on a single point of our subject. The point on the subject on which we are focusing is called “Focus Point”. But in reality, we are not actually focusing on a single point, rather we are focusing on a 2 Dimensional imaginary plane, where that Focus Point exists. This imaginary 2 Dimensional plane is called the “Focal Plane or Plane of Focus”. The Focal Plane is the area in an image within which an object or objects are in sharp focus.

Focus Distance is the distance between the camera and the subject you have focussed on.

Focus Point, Focal Plane & Focus Distance

an important point to understand is that the Plane of Focus is perpendicular to the shooting direction.

Focal Plane Angle

I have found one interesting discussion on “The shape of the Focal Plane”.If you would like to know more in-depth, please follow the link below.

2. Area of Acceptable Focus:

To understand this concept, we have to understand How cameras capture images vs How the human brain interprets them.

Let’s first understand How the camera behaves. The camera can only focus sharply on Focal Plane. As we move away in front of and behind the subject on which the lens is focused, the image will become blurry. But the transition from sharp to unsharp is gradual. The more we move away from Focal Plane the more blurry the result will be.

Let’s understand the concept from the below reference. I have exaggerated the blurriness in order to make it more obvious to understand.

Gradual Defocus

Now, let’s understand How brian interprets images. Theoretically, the Focal Plane is the only sharpest plane of focus in an image. But in reality, it is not the only part of an image that appears to be in focus. When we look away from the Focal Plane, the points of light gradually become less sharp. But our brain cannot immediately detect this tiny loss of sharpness. To us, the point still looks focused. This area which is adjacent to the Focal Plane in both directions ( in front of and behind ) appears to be focused and is called the “Area of Acceptable Focus”. Beyond this area, the image will become blurry. Scientifically, the Area of Acceptable Focus is based on something called the Circle of Confusion (CoC) which is beyond the scope of this article. But, I have explained the Circle of Confusion (CoC) in another article. Please visit the CoC link once you complete all the topics of Depth of Field…

Let’s understand the concept below reference.

Is “Area of Acceptable” Focus equally divided ?

“Area of Acceptable Focus” is not equally divided on both sides of the Focal Plane. It is typically distributed unequally, with about 1/3 (34%) of the total field of focus lying in front of the Focal Plane, and 2/3 (64%) of the total field of focus lying behind the Focal Plane.

But as the focal length increases, it becomes more equal.

3. Depth of Field (DOF):

Now we have understood all the necessary concepts to understand “Depth of Field”. The “Area of Acceptable Focus” that I have discussed earlier is actually called “Depth of Field”.

So, in a nutshell. A camera can only focus at a single point, but there will be an area that stretches in front of and behind this focus point that still appears sharp. Depth of field refers to that range of distance that appears acceptably sharp. It can be described as either ‘Shallow’ (where only a narrow zone appears sharp) or ‘Deep’ (where more of the picture appears sharp).

Now the million-dollar question will be ” I have now fair understanding of what Depth of Field is, but how can I control it in my photography ?”

Ok. Ok.. relax. Take a break and grab a cup of coffee then start again. Or if you have already digested those topics then without further ado let’s go to page no 2.

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