Better selective focus? Better Depth of field control?
In our test report, we confirmed Sigma's claims that the larger "DSLR-sized" sensor on the DP1, along with the larger pixels it contains, helped this camera achieve the highest image quality rating we've given a compact digital camera all the way up to ISO 800. Several digital compacts (nearly all in the 10MP or higher class) can capture higher resolution and detail, and a few tested as well for color accuracy, but all of these have higher noise levels that become obnoxious above ISO 400. So kudos go to the DP1 on the image-quality front.
As we also pointed out in our test report, several other factors contribute or detract from the DP1's image quality, including the accuracy (and speed) of the focusing system, the metering system, and the quality of the lens. However, Sigma makes another claim for the DP1 that impacts image quality indirectly, and there wasn't room in the report to delve deeper into this claim. According to Sigma: "The DP1 uses a wide-angle lens with a focal depth equivalent to 28mm on a 35mm film camera. But because of its large image sensor, the depth of field is shallow. So, shooting at close range or with a wide-open aperture, you can use the kind of natural out-of-focus effects you'd expect of an SLR."
We know what Sigma is trying to say, although using terms such as "focal depth" (instead of field of view) or "natural out-of-focus effects you'd expect from an SLR" are counter-intuitive. Sigma is actually claiming that the DP1 delivers shallower depth of field, when you want it, than typical digital compacts. And that control of depth of field has largely been the advantage that SLRs offered in the past. Sigma also makes claims that the DP1's bokeh (an artistic term for the shape of out-of-focus background lights and details) is also more pleasing as a result of its depth-of-field advantage.
To test these claims, we decided to narrow our focus (pun intended) and explain some of the terms involved. Depth of field is determined by a combination of lens focal length, aperture, and subject distance. Increase focal length while maintaining the same subject distance and aperture, and your depth of field will shrink (perspective will also change -- another topic entirely). Conversely, choose a smaller aperture at a given focal length and subject distance, and the depth of field increases (while light transmission to the sensor decreases).
Sigma makes its claims for the DP1 based on the fact that is uses a 16.6mm focal length lens to achieve its 28mm equivalent focal length, while a typical compact digital camera, such as the Nikon Coolpix P50 ($230 street), requires a 4.7mm focal length lens to achieve a 28mm field of view. The longer focal length is required on the DP1 due to the size and shape of its Foveon sensor. The Nikon P50 and most compact digital cameras use sensors with a 3:4 aspect ratio compared to the 2:3 aspect ratio on the DP1's Foveon X3 sensor. If the Nikon used a 2:3 aspect ratio sensor, the focal length required for a 28mm equiv would be closer to 6mm. On the other hand, the Panasonic Lumix DMC-LX2 mentioned in our test report features a 4X zoom, 6.3-25.2mm f/2.8-4.9 Leica lens, with a 28-112mm equivalent. It also features a sensor with a 16:9 aspect ratio and a lens switch that allows you to set it to 2:3 or 3:4 aspect ratio. So when set to 2:3 aspect ratio, the camera captures closer to a 35mm field of view.
Given the rules for depth of field, it's obvious that the much longer focal length on the Sigma DP1 would have a shallower depth of field at a given aperture and subject distance than a typical compact. However, the two cameras just mentioned, as well as several other compacts, feature an f/2.8 wide aperture compared to the DP1's f/4, and wider apertures produce shallower depth of field. Most digital compacts can focus much closer (the Nikon P50 and the LX2 both focus as close as 2 inches compared to the DP1's 11.8 inches.) and can zoom in as well. So in addition to the full stop of light advantage from an f/2.8 lens, the close focusing capability of other digital compacts, and their longer focal lengths (at full zoom) conspire against the DP1's possible DOF advantage. Thus raising the question: Does a f/2.8 lens with closer focusing capability deliver shallower depth of field than the f/4 lens on the DP1, despite its greater focal length?