Do You Need A Full-Frame D-SLR?

For the better part of the last three years, one of the biggest buzz topics for pro photographers going digital was the full-frame camera—an SLR with an image sensor that’s physically the same size as a frame of 35mm film. Along with the implication of higher resolution, these cameras have the added benefit of not requiring users to apply a magnification factor to their lenses in order to determine the apparent focal length. In the past few months, however, several advancements call into question the superiority of the full-frame sensor.

We touched on this in “Clash Of The Titans,” Digital Photo Pro, May/June 2005, but with so many professionals contemplating the pros and cons of a new full-frame image sensor, the topic merits a more in-depth discussion.

Background

When we’re talking about fullframe D-SLRs, we’re talking about the image sensor. While commonly referred to as the heart of a digital camera, the image sensor is one of many components that contribute to making the final image. The sensor is by no means the only element that needs to be considered, however. As the heart of a camera, the sensor is the single most well-known component and the one that photographers are most apt to discuss when kicking around the features and specs of a given camera.

Since digital SLRs became more mainstream, two types of image sensors have formed the great polarizing debate among photographers: full-frame sensors and sub-full-frame sensors. While these aren’t industry-standard terms necessarily, they’re descriptive. When we say “full frame,” we’re referring to the relative size of the sensor vis-à-vis a frame of 35mm film. A full-frame sensor is the same physical size as a frame of 35mm film.

Sub-full-frame sensors are somewhat more difficult to pin down in terms of their precise size. Rather than list a specific measurement, we tend to discuss the size of these sensors in terms of their magnification factor. This is done for purely pragmatic reasons. The actual sensor size is less important to a photographer than the effect that sensor will have upon the way the photographer uses the camera. A 1.5x sub-full-frame image sensor, therefore, is one whose physical size creates a 1.5x magnification factor for the focal length of a particular lens. Most sub-full-frame sensors are in the 1.5x to 1.7x range. One exception is the Olympus 4/3rds sensor in the E-1, which has a magnification factor of 2.0.

The Magnification Factor Myth

If you’ve been following digital camera technology for a while, you’ve no doubt come across the phenomenon of the magnification factor. You might have seen it expressed or explained in any number of ways. Typically, you’ll see comments like “...camera X has a magnification factor of 1.5x, so your 100mm lens will really be a 150mm lens on this model.” This is preposterous, naturally. The focal length of a lens doesn’t change simply because there’s a different sensor behind it. The laws of physics don’t care a whit about the sensor size. A 100mm lens still focuses at infinity when it’s 100mm from the image plane (okay, this assumes we’re not talking about a lens with a telephoto design, which can be made to focus closer to the image plane).