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2 - Depth-of-field & Focal Length

Depth-of-field depends on three factors.

1) Lens opening (aperture, f/stop)

2) Distance from the camera to the subject

3) Sensor size

Focal length is sometimes considered to be another factor.

However, focal length doesn't affect depth-of-field.

A wide-angle lens doesn't have more depth-of-field than a telephoto lens.

Photographs taken at different focal lengths have about the same amount of depth-of-field, if the field-of-view is the same for both photographs.

Field-of-view is the same means that the image in the viewfinder is the same for both focal lengths.

The term magnification (focal length ÷ object distance) is also used instead of field-of-view.

The magnification is the same for the wide-angle photograph and for the telephoto photograph.

So, when the field-of-view (magnification) is the same, we're comparing apples to apples.

The depth-of-field is identical with both focal lengths.

Depth-of-field isn't affected by focal length.

Visual Proof

Let's say you're photographing a still life at a focal length of 18mm.

You're very close to the still life—20 inches.

Then, you change the focal length to 70mm.

You have to move away from the still life in order to keep the the field-of-view the same.

The camera is now 66 inches from the still life.

The perspective is very different in the two photographs.

This is due to moving the camera to maintain the field-of-view.

However, the depth-of-field, the zone of acceptable focus in front and behind the subject, is about the same for both versions of the still life.

The camera was focused on the glass block for both photographs below.

To avoid possibly confusing the affect of a change in resolution with depth-of-field, I didn't reduce or enlarge the photographs.

Note how the amount of blur of the brass faucet threads, and the lettering on the plumber's putty, are the same in both photographs.

The depth-of-field is the same in both photographs, because the field-of-view is the same.

q

Focal Length = 18mm

Aperture = f/5.6

Distance = 20"

q

Focal Length = 70mm

Aperture = f/5.6

Distance = 66"

Distant Background Blur

You may have noticed that the distant background is more blurred in the 70mm-version.

This increased blur of the distant background is not due to depth-of-field.

It's due to the larger physical size of the lens opening at a 70mm focal length, 12.5mm, compared to the size of the lens opening at a 18mm focal length, 3.2mm.

The lens opening is f/5.6 at both focal lengths, but the physical size of the lens openings are different.

Go to the next section, Depth-of-field & Distant Background Blur.

Mathematical Confirmation

Let's use a depth-of-field calculator, DOFMaster Online Depth of Field Calculator, to look at the numbers of the above example.

The depth-of-field is similar, despite the difference in focal length.

Focal Length: 18mm 70mm
Lens Opening: f/5.6 f/5.6
Distance to the Subject: 20" 66"
Total Depth-of-field: 7.05" 4.9"
In Front of the Subject: 17.1" 63.6"
Behind the Subject: 24.1" 68.5"

The depth-of-field figures don't match because the field-of-view wasn't exactly the same.

Next, we'll see how depth-of-field doesn't affect the amount of blur of distant backgrounds.