The key to mastering photography is not to follow the rules but to understand them. Understanding the photographic principles that define photography is a barrier that must be broken to truly unlock your full potential as a photographer.
The problems that plague beginners are the same problems faced by the pros, that is controlling your image with the exposure triangle; shutter speed, aperture, and ISO. This article will focus on understanding depth of field (DOF).
In a Nutshell
Simply put “depth of field” is the area of your image that is in focus. Dissecting the phrase a bit we can say that depth of field is the degree of depth within the field of view that is has acceptable focus.
This should bring a bit more understanding to the common term and desired photographic technique of ‘shallow depth of field’. A shallow DOF would mean that the depth of the field that is in focus is very minimal or shallow, therefore everything in front of and behind that range is out focus. This out of focus light outside the range of your DOF appears blurry or soft, often referred to as “bokeh”.
A Deeper Understanding
The depth of field is determined by the aperture of the lens being used. How it is determined exactly is by a principle called the “circle of confusion.” Let’s revisit what a lens is at its most basic level:
..an object or device that focuses or otherwise modifies the direction of movement of light, sound, electrons, etc.
Scientifically speaking what we perceive as focus occurs when light hitting a given point, diffuses, refracts (through a lens), and realigns within the variable circle of confusion. When I say realignment, I’m referring to the alignment of the light rays on your cameras sensor, film, retina, or other image capturing device.
Light rays that hit the sensor of your camera from a given point which are greater in area to the applicable circle of confusion appear out of focus. Light rays converging equal to or less that the applicable circle of confusion appear in focus.
In conclusion depth of field is the amount of depth in the field of your image where all the light rays diffuse, refract, and converge on your cameras sensor at an area equal to or less than the applicable circle of confusion.
Depth of Field Visualized
The simulated overlays in the following video is calculated based of the exact variables used in the experiment. Any changes in sensor size, focal distance or focal length, would drastically change the outcome of this experiment. This video is a visual to give you a jumpstart in understanding depth of field.