The C850's overall performance is surprisingly good for a budget camera. It starts up in a little over two seconds and shuts down again almost as quickly. In single shot mode at the highest quality setting it can shoot at a sustained 1.5 seconds per shot, which is pretty good for a cheap camera, however when shooting at higher ISO settings it takes progressively longer, showing a "Busy…" message for several seconds while it processes the image. Continuous mode is also quite impressive, managing to sustain well over a frame a second. However the camera doesn't focus between shots, so it's lees useful for moving subjects.
One concern with AA-powered cameras is battery life. The C850 was supplied with a set of standard Panasonic industrial alkaline batteries, as are many AA-powered cameras. Normally these will last for the entire testing period, but the camera's battery level monitor indicated that they were exhausted after only a few dozen shots. I tested the batteries with a separate meter and found that they were still OK, but as far as the camera was concerned they were drained. This may just be a reaction to that type of battery, because when I switched to NiMH rechargeables the camera worked perfectly well for another 50 or so shots with the charge meter reading full.
The C850's major weak point is its AF system, and it really is very poor indeed. With a well-lit high-contrast target it is fine, and focuses quickly and accurately. However I found that even a slight lack of texture was enough to confound it, even in good light. Its low light performance is exceptionally poor, failing to focus at all in a room lit brightly enough for reading, and since it lack and AF assist lamp
Unfortunately the final image quality isn't too good either. Exposure metering is somewhat erratic, and the multi-zone metering seems to heavily favour the centre zone. Colour rendition is also poor, with highly saturated reds and especially yellows turning out as featureless blobs of colour. Image noise is also a problem, with some noise effects visible even at the lowest ISO setting, and getting progressively worse. Automatic white balance under artificial light was also very inaccurate, with strong colour casts in both tungsten and fluorescent illumination. This is a pity, because the lens actually performs very well, with good edge-to-edge sharpness and minimal distortion or chromatic aberration.
Some BenQ cameras are outstanding value for money, but sadly the DC C850 isn't one of them. It starts off well with the promise of superior optics, good build quality and decent performance at a rock-bottom price, but it is let down by a very poor AF system and inferior image processing. It may be very cheap, but for just a few pounds more you can get a much better camera, including some other BenQ models.