Understanding Depth of Field: How Aperture Affects DOF

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 and forthcoming series of articles will focus on understanding Depth of Field (DOF). Scroll down for video.

What is Depth of Field?

In a Nutshell

Simply put Depth of Fieldis 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 of DOF

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.

What to Know Before You Watch

The simulated overlays is this video 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.

Depth of Field Visualized

Please comment on what you would like to see displayed in the next video. Topics might include: How DOF changes with focal distance | What is a hyperfocal distance | Best DOF for ___

Specific Equipment and Variables

Camera: Sony A6000

Lens: Canon FD 50mm 1.8 Manual Focus Lens

Focal distance of 10ft calculated by marking distance in focus when lens focal distance read "10" ft.

Distance measurements calculated in post by thin grid overlay using the 10ft mark as 10 feet.

Field of View (FOV) calculated based on crop factor of full frame Canon FD lens on Sony APS-C sensor.

Depth of Field calculated using DOF calculator and aligning field overlay to corresponding feet and inches along grid. Likely margin or error + or - 1".

Animation of overlays done by key framing preset published variables in the form of a final cut pro template Made in Motion 5 using rigs and sliders with multiple snapshots based on DOF calculations.