Friday, July 20, 2012

Controlling Depth of Field

Do you like having a shallow depth of field?  As shallow (ahem) as it sounds, I confess, I still do.  That's why all but a couple of the lenses I have had have a maximum aperture of f/2.8 or wider.

Meanwhile, take a look at these shots (all were taken with a Nikon D3).  Care to estimate the apertures I used?  [Don't cheat with the EXIF... :) ]

Sample #1:


Sample #2:


How about these second set of samples?

Sample #3:


Sample #4:


Hit the jump for the answers.


Sample #1 appears to have a shallow depth of field.  However the aperture I used was f/6.3.
Sample #2 appears to have a deep depth of field.  Yet, the aperture was at f/1.4.

In the second pair of samples, the apertures were taken a full stop apart, yet the images appear to have a similar depth of field.
Sample #3 was at f/4.0
Sample #4 was at f/2.8.

My point with these examples is that aperture is not always the most significant factor for controlling depth of field.  Depth of field is also influenced by focal length (the actual focal length, not the equivalent focal length), distance to the subject, and the circle of confusion (which is partly based on the sensor size and partly based on the viewing size).

Take a look at these comparisons (assume full frame camera for now).

Let's start with a baseline:
Focal length: 105mm
Distance: 10 ft.
Aperture: f/5.6
The resulting depth of field is 0.91 feet

Let's see how quickly DOF changes if we alter focal length (let's say by a factor of 1.4):
Focal length: 145mm
Distance: 10 ft.
Aperture: f/5.6
Depth of field: 0.47 feet

Focal length: 75mm
Distance: 10 ft.
Aperture: f/5.6
Depth of field: 1.81 feet

Another comparison, this time altering the distance to the subject (also by a factor of 1.4):
Focal length: 105mm
Distance: 7.14 ft.
Aperture: f/5.6
Depth of field: 0.46 feet

Focal length: 105mm
Distance: 14 ft.
Aperture: f/5.6
Depth of field: 1.8 feet

Finally, let's try changing the aperture by 1 stop:
Focal length: 105mm
Distance: 10 ft.
Aperture: f/4.0
Depth of field: 0.64 feet

Focal length: 105mm
Distance: 10 ft.
Aperture: f/8.0
Depth of field: 1.29 feet

In this theoretical example, there was arguably a more significant change to the depth of field when the focal length or the subject distance was changed compared to a change in aperture.

Don't get me wrong.  I'm not saying that aperture is not important.  All I'm saying is, it's a juggling act.  After all, the focal length and distance aren't just going to affect the depth of field - they will also of course affect the composition and perspective.   Similarly, you can't just rely on changing the aperture.  Sometimes, it's just not possible to use a wide aperture, as when there is bright ambient and you are using a monobloc flash with your camera already at its sync speed.

Indeed, those were the circumstances when I took the headshot above.  The subject was lit with an SB-800 on a beauty dish as key, with an AlienBee B1600 with 60-inch shoot through umbrella as fill, plus a white reflector on the side.  The B1600 doesn't have high speed sync so my shutter was limited to 1/250.  Although we were partially shaded, it was still relatively bright, and the widest my aperture could go at base ISO was f/6.3.  The client wanted a shallow depth of field -- was it a problem?  Not really.  By shooting at a relatively close distance (around 5 feet) and a somewhat long focal length (I used from 105mm to 150mm), I was still able to get a reasonably shallow depth of field.

BTW, these are the lenses I used here:
Sample #1: Nikkor 70-200 VR I
Sample #2: Sigma 50 1.4
Sample #3 and #4: Tamron 28-105 2.8

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