Journal Number 101
November 2006

NOTES etc

Depth of Field for Digital Cameras

Depth of field for digital cameras? What we are all trying to achieve is good sharp photographs. In flower photography (as in portrait photography) we may want artistically to blur out the background to help the viewer concentrate on the subject, so at best we would like control of depth of field.

Basic fact: 1. At any given focal length depth of field increases as the lens is stopped down
- ie, the f setting increases.

Basic fact 2. At the same f setting and the same subject distance, depth of field is greater for short focal length (wide) lenses than for long focal length lenses.

Basic fact 3. At the same focal length and f setting, depth of field increases with the subject distance - ie it is shallow in macrophotography.

Thus, to get enough depth of field for close ups of little NZ orchid flowers I need a short focal
length lens with a high f setting. My old 35mm film Olympus OM4's 50mm lens stops down to
f 32, and with enough light (flash outside, or long exposure inside away from the breeze)
I can get clear as crystal shots of orchid flowers, all the flower parts acceptably sharp.

My Sony F707 DSLR (digital single lens reflex) has a 38-190mm f2.0-2.4 lens, and on "aperture priority" it allows me up to f8.0. How can you get enough depth of field with that?

Basic fact: 4. The human eye can accept dot sizes of 0.4mm at 40cm as sharp.
This is called the circle of confusion (CoC).

Reduced onto a 35mm film the CoC is 0.03mm. Most digital cameras have sensors smaller than the 35mm film frame. The Sony has 8.8x6.6mm. Reduced to that size the acceptable CoC is .0065mm. (You begin to see why a dust speck on the sensor can ruin your photos).

The CoC size used to define "acceptably sharp" is thus much smaller for digital than film cameras. That would cause less depth of field in small sensor cameras, except for this:

Basic fact: 5: A smaller sensor requires a shorter focal length lens.

For instance a 100mm lens on a 35mm film camera gives the same field of view as a 25mm lens on a digital camera with a sensor the size of the Sony's. The "focal length equivalence ratio" is thus 4 for my Sony.

The rule: the depth of field of a digital camera with focal length equivalence ratio of N, at a given aperture of f, is the same as that of a 35mm camera with a lens of the same angle, closed down to an aperture of N x f.

For my digital Sony, N = 4, f = 8.0: N x f = 32.  It will thus take photos with the same depth of field as my 35mm OM4 with an f setting of 32.

What about the newer DSLRs (digital single lens reflex cameras)?

I looked at the Olympus E-500 with its 17.5-45mm f 3.5-5.6 lens, and N = 2.
The greatest depth of field would be at f = 29 x 2 = 58. Extraordinary.

I looked at the Canon 350D with its standard 18-55mm f 3.5-5.6 lens, and N = 1.6.
Depending on the lens you can stop down to an incredible f = 91!

You would never need more depth of field!