Sigma DP1 Selective Focus

Better selective focus? Better Depth of field control?

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Sigma-DP1-Selective-Focus

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?

No and yes! When both the LX2 and DP1 were focused on a flower arrangement 11.8 inches from the camera (the DP1's close focusing limit) and the LX2 was set to its widest focal length, the DP1 delivers much shallower depth of field than the LX2. (see photo A, closeup B and closeup C). However, at a typical portrait distance of six feet from the camera, the DOF difference between the cameras (LX2 set to f/2.8, the DP1 set to f/4) is far less obvious and details in distant backgrounds appear very similar.

The depth of field advantage (and therefore control) for the DP1 is thus limited to objects fairly close to the camera (such as a head shot taken three feet from the camera) up to its close focusing limit of 11.8 inches.

However, after that, the close focusing capability of cameras such as the LX2 (set to its widest focal length) deliver much shallower depth of field (see photo G). Finally, with cameras such as the Nikon P50 or Panasonic DMC-LX2 you can also zoom in to crop your subject, thereby increasing the focal length and decreasing depth of field. However, since aperture also changes you loose the brightness advantage and have to resort to a slower shutter speed, plus a smaller aperture increases depth of field but doesn't fully cancel the zoom advantage. The LX2 also has optical image stabilization so you can set a slower shutter speed to offset the loss of light without significantly increasing camera blur.

Bottom line? In our opinion, the shallower depth of field provided by the longer focal length lens of the DP1 (needed to accommodate the larger sensor) is limited in scope, and offset by the DP1's f/4 aperture and uninspiring close focus distance. However, we noticed an advantage when shooting headshot-style portraits, but couldn't see a visible difference in DOF when shooting group shots or subjects more than 6 feet from the camera. Now, if the DP1 had a closer-focusing zoom lens with image stabilization, it would be an entirely different story.

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