Table of Contents

depth of field (DOF)

see also:

introduction

DOF equation

degree of background or foreground blurring

comparing lenses keeping subject magnification constant

some basic rules of DOF

  1. for a given subject magnification and sensor size, the ONLY determinant of DOF is lens aperture and NOT lens focal length
    • using different lens focal lengths on the same sensor requires different camera to subject distance to maintain the subject magnification, this is why lens focal length does not affect DOF in this scenario.
    • for backgrounds very close to the subject (much closer than the camera is to the subject), the degree of background blurring is mainly dependent upon f-stop not focal length. For distant backgrounds, blurring is proportional to focal length / f-stop.
  2. stopping down the lens 2 stops gives twice the DOF, while opening it up 2 stops gives 50% less DOF
  3. stopping down the lens 7 stops gives 10x the DOF
  4. to compare DOF of a lens-camera combination to 35mm full frame, multiply the aperture and the focal length by the crop factor of the sensor.
    • ie. a 50mm f/2.0 lens on a 2x crop Micro Four Thirds system gives similar DOF and angle of view and perspective as a 100mm f/4.0 lens on a 35mm full frame camera
  5. for portraits one should usually aim to get everything within the DOF from ears to tip of nose, and thus a 100mm f/4 lens on a 35mm full frame camera (or 45mm f/1.8 on Micro Four Thirds) is ideal for this while giving reasonable background blurring at a subject distance of ~2m.
    • the ideal focal length in 35mm full frame terms for portraits is generally 80-120mm range. Shorter focal lengths force one to get close to the subject which causes distortion of facial features. Longer focal lengths force one to get further away and lose contact with the subject.

some examples for portraits

A normal f/5.6 kit lens on a cropped sensor camera (in this case, equivalent to 84mm f/11) generally is not able to blur the background well for portraits:

www.ayton.id.au_gary_portraiture_portraitlens_olyzd14-42mmkit_42mmf5.6.jpg

A lens equivalent to 100mm f/4 on a full frame camera such as the Olympus ZD 50mm f/2.0 2x crop lens is much better:

This one is taken at f/4 (ie. equivalent to 100mm f/8 on full frame):

www.ayton.id.au_gary_portraiture_portraitlens_olyzd50mmmacro_f4.jpg

And this lens wide open at f/2.0 (ie. equivalent to 100mm f/4 on full frame) becomes a very nice portrait lens:

www.ayton.id.au_gary_portraiture_portraitlens_olyzd50mmmacro_f2.jpg

While a lens equivalent to 170mm f/2.8 such as either a Canon 135mm f/2.0 on a 1.3x crop camera (as shown here) or a Rokinon 85mm f/1.4 lens on a 2x crop camera will give much shallower DOF and allow more background blurring:

www.ayton.id.au_gary_portraiture_portraitlens_canonef135mm_f2.jpg

NOTE: in the above images, subject magnification has been maintained constant, but the change in effective focal length while not altering DOF (this was achieved by altering effective aperture), has meant the camera must be further from the subject, and the background becomes more compressed (“perspective” is altered) as the effective focal length is increased.