depth of field comparisons for portraiture

see also:

• depth of field
• examples of lenses for portraits and their background blurring capabilities
• SLRs compared
• portrait lenses
• outdoor portraits
• Sony A7II full frame with Canon EF 50mm f/1.8ii vs Olympus OM-D with PanaLeica D 25mm f/1.4 lens – real world comparison images
• Sony A7II full frame with Canon EF 85mm f/1.8 vs Olympus OM-D with Olympus 75mm f/1.8 lens – real world comparison images
• Sony A7II full frame with Canon EF 85mm f/1.8 vs Olympus OM-D with Olympus 45mm f/1.8 lens – real world comparison images
• my blog post - FP/HSS flash outdoor portraits
• my blog post - tricking your camera into higher X-sync for outdoor portraits

Here we are aiming to attain a very shallow depth of field so that just the important features of the face are sharp and the remaining composition is out of focus. Given we are dealing with head and shoulders portraiture, for simplicity, I will assume a telephoto of 100mm in 35mm equiv. terms at 2m from the subject. The figures show calculated DOF range.

*The fill-in flash in sunlight calculations assume exposure is set to over-expose sunlit areas by 0.5 stop eg. ISO 100, f/16, 1/70th sec.

#GN for fill-in flash are for ISO 100 in metres to achieve the exposure at 2m.

Thus, it shows that where daylight flash fill-in is not needed, then the narrowest depth of field is with cameras that have a large sensor (eg. medium format or full frame 35mm) or at least a f/2.0 100mm lens available as with the Olympus E series.

When shooting portraits with fill-in flash in daylight without using filters, the picture changes a little with consideration required for the fastest possible flash sync, the lowest possible ISO, here the medium format leaf shutter cameras again win, but the Olympus E series with super FP mode and a 50mm f/2.0 lens is able to match them, with the Sony DSC-R1 not far behind. To achieve a comparable result with the Canon 5D you would have its special 50ISO mode, while the Nikon D200 and Canon 350D/20D/30D along with older 35mm film cameras with their slow x-syncs lag a considerable way behind. When the Olympus cameras are used without a super FP mode flash, then, given their smaller sensor size not surprisingly come last.

Note that the Olympus FL-50 flash in super FP mode at 1/2000th sec gives a maximum effective GN of about 7 in m at ISO100, which limits is use for group shots or subjects if they are more than 3.5m away.

Note also that the lowest true ISO reading on the Canon 350D and Olympus E330 is really 125 not the indicated 100.

Using a 135mm lens and stepping back to 2.7m does not change your DOF range significantly, but of course, if you stayed at 2m to get a tighter headshot, your DOF range would get narrower but the relative differences between each of the above scenarios would not change appreciably.

The Canon 1D Mark III has x-sync up to 1/300th sec, but if you use a PC sync cable and take the flash off the hot shoe and place both flash and camera in manual mode, you can use faster shutter speeds with 1/400th sec posing no problems at all, but a narrow band at the bottom of the frame appears at 1/500th sec (this can be avoided by using the Pocket Wizard hypersync functionality and you regain full TTL capability).

The Olympus E510 max TTL x-sync is 1/180th sec (you can use higher shutter speeds in super FP sync but at lower flash output). By using a PC sync cable attached to a hotshoe adapter, you can do manual flash at up to 1/320th sec without issues. At 1/400th sec there is a narrow band at the bottom of the frame as with the Canon 1DMIII at 1/500th sec.

The Panasonic GH-1 unfortunately forces the fastest shutter speed to be 1/160th sec when it detects a flash is turned on and attached via a PC sync cable.

• depth of field is only one aspect, there are two others that are important to the portrait photographer:
  • how much is the background blurred:
    • however, depth of field is NOT directly related to degree of background blur. Depth of field equations tell you over what range of distances objects will appear to be acceptably sharp (or at least not unacceptably unsharp). It tells you nothing about how much blur there will be of objects well outside the depth of field. That's governed by different physical parameters and determined using totally different equations:
      • background blur of infinity point source = focal length x image magnification / f-stop
    • Merklinger uses the concept of disk-of-confusion to help determine what will be in focus (see page 32 of his pdf):
      • the disk-of-confusion is the hypothetical diameter of the cone of light from the film projected onto an object
      • disk-of-confusion at distance of the object = 
        • (focus distance - lens to object distance) x focal length / (focus distance x f ratio)
        • THUS, at the same subject and background distance, a 100mm lens should have about the same degree of background blurring at f/2.8 as a 50mm lens would have at f/1.4 but of course, the subject will be twice as large as well.
        • IF you adjust your subject magnification to be constant by moving in closer or further from subject depending on lens focal length, but keep distance of subject to background constant and aperture constant, then the degree of blurring remains constant irrespective of lens focal length.
      • if you wish to make letters on a sign in the background unreadable:
        • the disk of confusion of the sign must be equal to or larger than the letter height
        • NB. it will be readable if disk-of-confusion is less than 1/5th of letter height
        • NB. between 1/5th of letter height and letter height, readability will depend on style of letter, shape of lens diaphragm (bokeh effect), orientation of diaphragm shape to letter, contrast of letter, and characteristics of lens itself.
        • unfortunately we may run into the problem that at the f ratio needed to blur the background letters will result in insufficient depth of field for our primary subject. Changing focal length will not help this, as to keep the same subject size, we need to alter our camera-subject distance and thus while the actual relative size of the letters in the background may change due to the change in perspective, the degree of readability is only dependent on f ratio.
      • let's have a look at some examples of his disks of confusion:

• how nicely is the background blurred (ie. bokeh):
  • this is largely dependent on physical characteristics of the iris with an almost circular aperture usually giving the best bokeh - most of the Olympus ZD lenses and the newer L series Canon lenses and Nikon lenses have circular apertures and the optical design of the lens itself.