Crop Sensors & Depth of Field

 

Smaller sensors narrow lenses angle of view. Plenty has been written elsewhere on crop factors and sensor sizes, but what is underappreciated is the impact of DOF (depth of field).

 

As sensors get smaller, the focal length that will provide an equivalent field of view to a 50mm lens on 35mm film will shrink. Shrinking along with the focal length is the ability to seperate the subject from the background.

Portraits really stink if the background is also in focus (or close to it). At shorter focal lengths, DOF is greater. Add on top of that, many point and shoot cameras have a f/3.5-5.6 lens, meaning that you will have, at most, a maximum aperature of f/4  at a "normal" focal length. Sharpness doesn't immediately fall off beyond the DOF range either. For example, the background of the Canon at 8 ft will look distractingly in focus.

This is why the Fuji is acceptable. While they have a crop sensor, their 35/1.4 lens makes up for it.  Sony's widest angle lens for their NEX-5 is f/2.8 and that is for an equivalent 35mm lens. Otherwise, you get their slow kit zoom. While Olympus offers similarly slow lenses for the E-P3, Voigtlander has come out with a 25mm f/0.95. However that lens is $1200.

 

Camera / Format	Sensor Dimensions (mm)	Crop Factor	1/2 ft. DOF at 6 feet	DOF at 8 feet, f/4 with 50mm equiv.
6x9 Medium Format	84 x 56	0.43x	105mm, f/4	0.95 ft
Full Frame, 35mm	36 x 24	1.0x	50mm, f/2	1.86 ft
APS-H (Canon, Leica M8)	27 x 18	1.3	38mm, f/1.8	1.94 ft
APS-C (Fuji, Sony, Nikon)	23.6 x 15.8	1.5x	35mm, f/1.4	2.57 ft
Sigma Foveon	20.7 x 13.8	1.74x	29mm, f/1.1	3.45 ft
Micro 4/3rds	17.3 x 13	2.0x	25mm, f/1	3.92 ft
Nikon VI	13.2 x 8.8	2.8x	n/a	5.9 ft
Fuji X10	8.8 x 6.6	3.9x	n/a	...
Leica D-Lux 4	8.07 x 5.56	4.4x	n/a	8.5 ft
Canon Elph	6.17 x 4.55	5.6x	n/a	15.7 ft
iPhone	3.56 x 2.68	9.7	n/a	Infinite

HTML Tables

 

1. To determine crop factor, divide the diagonal of the 35mm frame by the diagonal of the sensor