Selective Focus: A Tale of Two Formats


There are a number of reasons and advantages for choosing to shoot with wider-aperture lenses. Included are the ability to capture sharp, low-light imagery at slower shutter speeds, quicker autofocus and exposure response times—which in turn reduce shutter lag times—and the option to capture your subject in a narrow, selective band of focus. All of these attributes hold true, regardless of the size of the imaging sensor in your camera, with one exception: the ability to capture your subjects with narrow, selective bands of focus.

When touting the benefits of shooting video with HDSLRs containing larger imaging sensors, i.e. full frame, APS-C and Four Thirds format cameras, capturing shallow depth of field (a.k.a. DOF or selective focus) is at the top of the list. What’s not made clear is how the degree of focus selectivity (how narrow a band of focus you can achieve) is largely determined by your choice of camera format, and that the larger the sensor, the easier it is to isolate your subject from the foreground and/or background.

Left: Canon EOS 5D Mk II with 28mm @ f/1.8     Right: Ricoh GR II with 6mm @ f/1.9 (28mm equivalent)  

In a nutshell, the DOF of an image captured at f/2 (or f/1.4, f/1.8, etc) with (as an example) a lens that has an angle of view of 74° on a full-frame camera, is significantly narrower, compared to an image captured at the same aperture with a lens that has an angle-of-view of 74° on a point-and-shoot camera, most noticeably when photographing subjects at closer distances.  

The reasoning here is that the focal length of the shooting lens required to capture an image with a 74° angle of view is determined by the size of the camera sensor. For a full-frame camera you need a 28mm lens. Similarly, an APS-C format DSLR requires a 17 or 18mm lens (depending on who’s compact camera you’re using), and if you’re a Four Thirds convert, you’ll need a 14mm lens in order to capture an image with the same 74° angle of view.

The imaging sensors used in point-and-shoot camera are smaller yet and as such require lenses in the 5- to 6mm focal range, which have focusing and depth-of-field characteristics far different than longer focal range lenses. The net result is that point-and-shoot images captured at the widest possible apertures can never display the onion-skin slivers of sharpness you can capture with larger-format cameras with comparably matched lenses without first resorting to post-capture “creative filters."

Left: Canon EOS 5D Mk II with 28mm @ f/1.8     Right: Ricoh GR II with 6mm @ f/1.9 (28mm equivalent)  

All Photographs © Allan Weitz 2011

To illustrate this point, we set out with a Canon 5D Mk II coupled to a Canon EF 28mm f/1.8 lens and a Ricoh GR Digital, which features a 6mm f/1.9 lens that takes in the same 74° field of view as the full-frame Canon 5D Mk II and 28mm/f1.8 EF lens. Each of these photographs was taken in Aperture Priority mode, with the lenses of each camera set to their respective widest apertures of f/1.8 and f/1.9, which while not identical, have depth-of-field discrepancies far too small to invalidate the story line.  As for the differences between the respective depths of field of each of the accompanying illustrations, they speak for themselves.

Note: Cameras and lenses with similar specs are available from other manufacturers. The Ricoh GR II and Canon EOS Mk II and Canon EF 28mm f/1.8 lens were used strictly for illustrative purposes and similar results can be achieved using comparable cameras and lens combinations.