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3 - Depth-of-field &

Distant Background Blur

Photographers often conflate depth-of-field with the blur of a distant background.

Remember, depth-of-field involves the zone of acceptable focus near the subject.

Depth-of-field doesn't affect the blur of a distant background.

Telephoto focal lengths blur distant backgrounds more than wide-angle focal lengths.

Below, the camera was focused on the fence post in the center of the frame.

Two different focal lengths were used—18mm and 70mm.

The camera was moved back for the 70mm-photograph to maintain the same field-of-view as the 18mm-photograph.

The magnification (focal length ÷ object distance) of the scene is the same for both photographs.

The background is more out-of-focus in the 70mm photograph.

Focal Length = 18mm

Aperture = f/5.6

Aperture diameter is 3.2mm

Focal Length = 70mm

Aperture = f/5.6

Aperture diameter is 12.5mm

This is due to the physically larger lens opening of the lens when the focal length is at 70mm.

The degree of background blur depends on the physical size of the lens opening.

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

At a 18mm focal length, the lens opening diameter is 3.2mm (18mm ÷ 5.6).

The 3.2mm lens opening allows smaller circles-of-confusion to reach the sensor, which we see as being sharper.

At a 70mm focal length, the lens opening diameter is 12.5mm (70mm ÷ 5.6).

The 12.5mm lens opening allows larger circles-of-confusion to reach the sensor, which we see as being less sharp.

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