More About Depth of Field

DoF Summary

Revision

 My previous posting about Depth of Field only considered aperture size and is summarised in this image:
For less DoF you need a large aperture/small fNo
For more DoF you need a small aperture/large fNo

There are other factors that influence Depth of Field which I now want to consider.

Other Factors that Affect Depth of Field

Focal Length

The following 2 photos of Sonny were taken with a large aperture (f5.6/f8.0) with my telephoto lens set at different focal lengths with the camera roughly the same distance away from the subject.
f8.0 122 mm


 









f5.6 280mm

As we expect having set a large aperture, neither background is very sharp.  Nevertheless you can just make out the strands of the "elephant grass" in the zoomed out image (122 mm) while the background is completely blurred in the close up (280mm) image.

Increasing lens focal length decreases depth of field.

Relative Distance From the Subject to the Camera

In the previous 2 images of Sonny the camera was roughly the same distance away from him so we could compare the impact of the lens focal length.  
The relative distance of the subject from both camera and the background influences your depth of field.  Placing your camera close to the subject with the subject a long way from the background will blur the background (small DoF)
If you think about it, you are probably already aware of this from photos you have taken:
  • Landscape photos (where the camera is focussed on a distant object) often show high depth of field from the distant horizon to quite close to the camera.
  • Portrait photos (where the camera is focussed on a near object) often show small depth of field hence the emphasis on getting the eyes perfectly sharp in your portrait images.  When taking photos of dogs, for example, you need to be wary of the nose being in focus and not the eyes.
Getting further away from your subject increases Depth of Field

Camera Sensor Size

The 4th factor, camera sensor size, is something that you have little control over, except by changing your camera. The bigger the sensor, the shallower the depth of field will be in your images when you compare image size.  This is because larger sensors require you to get closer to your subject or use a longer focal length in order to fill the frame with the subject.  As we have discussed above, these actions decrease the DoF.
This is why it is difficult to create a shallow depth of field optically using a mobile phone and why those phone cameras have to degrade an image in post production in order to produce a shallow depth of field effect.
This is why landscape photographers often prefer APS sensors to full frame sensors as it reduces the focal length they need (and hence size and weight of the lens)
Increasing the Camera Sensor Size Deceases Depth of Field

Summary

How to Decrease Depth of Field

  • Widen your aperture (smaller f-No)
  • Lengthen your focal length (zoom in)
  • Move closer to the subject
  • Use a camera with a larger sensor (i.e. not a mobile phone)

How to Increase Depth of Field

  • Narrow your aperture (larger f-No)
  • Shorten your focal length (zoom out)
  • Move away from your subject
  • Use a camera with a smaller sensor

Additional Reading


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