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Sigma SD9

Sigma's digital SLR uses Foveon's "X3" sensor technology to deliver more detail per pixel!

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Page 5:Optics

Review First Posted: 11/09/2002

Optics

The SD9 is an interchangeable-lens SLR (the body ships by itself, with no lens included), with the lens attached via an SA-type, bayonet lens mount on the front panel. Maximum aperture, focal length, and focusing distance range will thus vary with the particular lens being used. A small release button below the bottom of the mount releases the lens, allowing it to turn and slide out of the mount. A Depth of Field Preview button (next to the top left side of the lens mount, when viewing the camera from the front) stops the lens down to the designated aperture setting, so that you can preview the depth of field you'll have in the shot through the viewfinder.

Because most current digital SLRs have a sensor that's smaller than a 35mm film frame, they have what's referred to as a "focal length multiplier" relative to 35mm cameras. This is the factor by which the camera reduces the field of view (effectively magnifying the image) with a given lens, relative to how that lens would function on a 35mm camera. The SD9 has a focal length multiplier of 1.7x, which means that a "normal" 50mm lens will show the same field of view as an 85mm lens on a 35mm camera. This makes it difficult to do true wide angle photography with the SD9. By way of example, the normally "extra wide" 20-40mm zoom lens Sigma shipped along with my eval camera is the equivalent of a 34-68mm zoom on a 35mm film body. Focal length multiplier is a fact of life with all but a few of the digital SLRs currently on the market, but 1.7x is a higher ratio than any I'm aware of on other models. (Most run between 1.5 and 1.6x.) Not a crippling defect by a long shot, but a factor to consider if you like wide angle shots.

The SD9 offers Single and Continuous autofocus modes (manual focus is activated via a switch on the lens, if available). Single AF adjusts focus only when the Shutter button is halfway pressed, using a TTL phase difference detection system. Conversely, Continuous AF mode constantly adjusts the focus, without waiting for you to press the Shutter button. Continuous AF uses an AF Predict function, which "predicts" where the subject will move to next, based on its current pattern of movement. Combined with the Sports Finder viewfinder display, the SD9's Continuous AF system makes tracking moving subjects a little easier.

I don't have any objective test for autofocus speed, so I hesitate to say too much on the subject here, not wanting to color my reviews with purely subjective impressions. That said though, I found the SD9's AF system rather uncertain at times, as it would sometimes "hunt" quite a bit for the correct focus setting. It also on several occasions got "stuck" when I moved from a very close subject to one far away, or vice versa. In these situations, the focus mechanism seemed to traverse a fair distance in the right direction, but then suddenly reverse itself, and end up locking against the opposite focus limit. Several times I needed to "help" the camera by manually turning the focus ring until I got it somewhere close to the right setting. Low light focusing was also a bit limited, as the camera would only focus down to light levels of about 1/2 foot-candle (about 5.5 lux), and then only with just the right subject. (A nice, sharp, high-contrast edge.) Reliable focusing is limited to 1 foot-candle and above, a level roughly corresponding to normal city streetlighting at night. Overall, the impression I have of the SD9's AF system isn't a favorable one.

The SD9 has a "dust cover" right behind the lens mount, protecting the mirror/shutter/sensor chamber. The idea is to keep dust from reaching the sensor, an issue for most digital SLRs. Dust on the sensor is a much more severe problem with digital SLRs than it is on film-based cameras, since the film is more or less continuously cleaned as it passes through the light seal on the cannister. With a digital SLR though, any dust that makes its way into the camera body and ends up on the sensor is there permanently, or at least until you clean the sensor, a delicate operation that's best avoided if at all possible. Sigma's approach seems like a good one, for two reasons. First, cleaning the dust cover is a relatively easy and risk-free proposition. Second, any dust that settles on the dust cover won't be imaged, because it's too far up the optical path. (In other words, it won't cast sharp shadows on the sensor, and so won't affect the images.

A shot of clear, blue sky, at f/22, to make the dust specks visible.. A cropped example of the dust on the sensor surface. (Click either image to view the full-sized image.)

 

There are two downsides to the dust cover approach though. First, it introduces two additional optical surfaces into the light path, potentially affecting sharpness, aberration, flare, etc. The second issue is only a potential downside, but based on my eval unit, could be a very real one: If dust does manage to get into the camera body, you've got an even bigger problem that if the dust cover wasn't there, because you'll have to remove it to get the dust out. But how would the dust get in there in the first place? - Probably in manufacturing: My test camera had several prominent dust specks on the mirror that were obviously there when it left the factory, since it had come directly to me from Sigma in Japan. Dust on the mirror is one thing, but dust on the sensor is another, and my unit unfortunately had that problem as well. - In spades. I didn't notice the sensor dust at first, since most of my shots were being taken at larger aperture settings - f/6.7 or larger. I saw large, very faint ghostly blotches on the images, very slightly darker than the surrounding area, and only visible if I was looking for them. Later though, I stopped down the lens (the Sigma 20-40mm f/2.8 zoom) to f/22, aimed it at a clear patch of sky, set the focus to a relatively macro distance (to guarantee that nothing in the scene would be sharply imaged on the sensor), and snapped the shot shown above. - The sensor is really dirty! Sigma can easily correct this in future production by implementing better cleanliness standards in their assembly area and instituting better QC procedures to insure that no dirty units go out, but I'd advise early purchasers to conduct this test for themselves, so they can have the problem addressed under warantee.

Confused by Apertures and Depth of Field? - Do you know how to use "Front Focus" or "Back Focus" to get *all* your subject in focus? Visit our free Photo Lessons area and click on the lessons "Focusing Up Close" and "Selective Focusing Outside!"

Reader Comments! --> Visit our discussion forum for the Sigma SD9!



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