A Simple Guide to Understand Aperture and Bokeh (Background Blur)

January 16, 2018

 

Your DSLR camera provides incredible flexibility in how it captures an image.  This flexibility allows for creative decisions that can lead to vastly different representations of the same moment.  One key camera setting which will dramatically effect your photos in multiple ways is aperture.  This article will discuss what aperture is, and the creative effects that can introduced by using different  aperture values.


Your camera is designed to manipulate light through a lens, and then capture that light via a sensor.  One of the key factors that determine the details of the image captured by the sensor is the total amount light impinged upon the sensor.  This can be adjusted by the lens aperture and your camera’s shutter speed setting.  Shutter speed determines how long the shutter is open, and therefore the amount of time the sensor will see the light.   Aperture refers to the size of the hole through which light is allowed to pass.  A larger hole allows more light in, while a smaller hole provides less light to the sensor.  This is very similar to the way the iris in your eye opens and closes to adjust the amount of light that reaches your retina. 

 

  
The aperture of your DSLR's lens is adjustable in either Aperture-Priority or Manual mode.  The maximum available opening or closing of the aperture depends upon the lens used.  It is measured in f-stops with values of 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 and 22.  These values are derived from the lenses focal length divided by the diameter of the opening.

 

Typically, DSLR’s also offers options for 1/3 or 1/2 stop increments as well.  
The f-stop represents a fraction, and a higher value represents a smaller opening. 

 


 

Changing the aperture one stop will half or double the amount of light seen by the sensor.  Therefore, a change of one f-stop requires doubling or halving the shutter speed or ISO to maintain the same brightness or exposure.


In addition to effecting exposure, aperture also affects the depth of field.  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.”  When a single point of focus is used, that area will be sharp, and depending on your focal length, aperture, and distance to subject, some distance in front and some distance behind that subject will be in focus.  The rest of the image will gradually fade into a blur. 

 

 
 

This can provide a great creative tool for certain types of images such as portraits.  Here, the desire is to emphasize the subject, and de-emphasize the background, while still providing some context to the photo, compared to using a plain backdrop.
 

This effect, referred to as bokeh, is accomplished by using larger apertures such as f/1.4 to f/2.8, long focal lengths, and a small distance to subject relative to the background.


While this effect can be highly desirable for portraits, it would often be avoided in landscape photographs where the desire is to have a very large area in focus.  This is accomplished by using smaller apertures, typically above f/8.

 

Creating a shallow depth of field (bokeh effect), is one of the signature looks of high-end DSLR cameras.  It adds a professional and attractive look to your photos.  One key reason to opt for more expensive lenses is often the larger aperture and shallower depth of field they can provide.  Additionally, full frame cameras have a larger sensor which will further decrease the depth of field.

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