Canon PowerShot A640

Color (9.23)
The Canon PowerShot A640 has a Digic II image processor that is included on previous A-series models; this processor typically fares well in terms of color reproduction. Results usually vary with different lenses and sensors and such. We tested the A640 by securing it to a tripod and photographing an industry standard color chart in optimal studio lighting.

The color chart, manufactured by GretagMacbeth, is pictured below but has been modified by Imatest imaging software to show the difference between the original colors of the chart and those same colors produced by the Canon A640. The PowerShot’s colors are depicted as the outer portion of each tile; the inner square corrects the ideal colors for luminance. The vertical rectangle in the center of each tile shows the original color of the chart; this is what colors should look like.

Below is another rendition of the digital camera’s colors. The GretagMacbeth’s colors are shown as squares, and the Canon PowerShot A640’s colors are shown as circles. The line connecting the two shapes depicts the degree of error.

The Canon PowerShot A640 shows fairly realistic colors with a relatively low mean color error of 6.5. Colors were over-saturated by an average of 7.3 percent, which is lower than most compact digital cameras’ colors. The A640’s overall score of 9.23 is impressive for a compact digital camera and shows that it has the capability to reproduce pictures very closely to what is seen with the eye.

Still Life Scene
Below is a shot of our still life scene, captured with the Canon PowerShot A640.

Click on the image above to view the full resolution image.

Resolution (4.96)
The Canon PowerShot A640 touts the most resolution of any A-series camera with 10.1 megapixels on its 1/1.8-inch image sensor. The sensor is the same size as its predecessor, the A620, that had a 1/1.8-inch CCD but only 7.1 megapixels on it. We tested the PowerShot A640 by using it to photograph an industry standard resolution chart in optimal studio lighting.

Click on the chart above to view the full size image

The photo above is the sharpest shot we could glean from the A640. We photographed the resolution chart using different focal lengths and apertures to ensure we got the absolute sharpest shot. The image above was taken using an aperture of f/3.5 and a focal length of 21.7mm. Even at its sharpest, the corners of the frame are blurry.

Imatest imaging software determined the sharpest shot and output sharpness results in terms of line widths per picture height (lw/ph). This measurement describes how many alternating black and white lines of equal thickness could fit in the camera’s frame without blurring. The Canon PowerShot A640 resolved 1930 lw/ph horizontally with 9.6 percent oversharpening. It resolved 2163 lw/ph vertically with 5 percent oversharpening.

The Canon A640 performed decently; it captured more details in pictures than its predecessor. The 10.1-megapixel A640 earned a 4.96 overall resolution score.

Noise – Auto ISO (2.62)
As with other compact digital cameras, the Canon A640 is equipped with an automatic ISO setting. We tested its ability to meter an optimally lit scene and measured the resultant noise. The metering system worked fine, as it chose an ISO 100 setting. Unfortunately, there is way too much noise at that relatively low sensitivity setting. The Canon PowerShot A640 got a lackluster 2.62 overall auto ISO noise score, which is a new low below the A620’s 5.58 and the A520’s 6.76.

Noise – Manual ISO (3.79)
The A640 adds a little more sensitivity than its predecessor offered, but both cameras have the same Digic II image processor. We tested the manual ISO settings from 80-800 and plotted the noise on the following chart with the noise on the vertical axis and the settings on the horizontal axis.

Sadly, there is a lot of noise even at the lower sensitivity settings, so the steady curve doesn’t help at all. The 3.79 overall manual ISO noise score is only slightly lower than the A620’s 4.03 score.

Low Light (7.0)
So far, all of our testing has been done with bright studio lights. Because most users won’t have bright lighting around all the time, we tested the Canon A640 in lower lighting conditions of 60, 30, 15, and 5 lux. The first two tests of 60 and 30 lux are more common shooting situations equivalent to photographing a dinner party at a restaurant or the kids in the living room after dusk. We secured the A640 to a tripod, manually set the white balance, and turned off the flash for these images.

Low Light Tests
60 Lux	30 Lux
15 Lux	5 Lux

Colors remain fairly accurate the darker the lights go, but the color saturation fades from 8.8 percent oversaturated at 60 lux to 4 percent undersaturated at 5 lux. The noise increases as the shutter remains open; this happens on all compact digital cameras. Below is a chart showing just how much noise resides at each shutter speed. The noise is on the vertical axis and the exposure time is on the horizontal axis.

The shutter remains open from 1-15 seconds, and the noise definitely increases but it is not as bad as on other models. The Canon PowerShot A640’s ability to keep subjects illuminated in low light and its decent handling of noise makes it a decent all-around compact digital camera.

Speed/Timing
Startup to First Shot (6.5)
Some of the best pictures are spontaneous – something happens suddenly, and a good, or lucky, photographer gets a great shot. The length of time a camera takes to turn on and be ready to shoot can have a big impact on how lucky a photographer is apt to get. The Canon A640 took 3.5 seconds to get off its first shot after being turned on, in our tests. That’s longer than most cameras we test. Many compact cameras score in the neighborhood of 2 seconds, and DSLRs are usually under a second.

Shot to Shot (9.32)
The A640 shoots an average of 1.45 frames per second in burst mode, when set to largest and best quality images. We did not run it long enough to fill a 2 GB memory card (which we were using for testing), but our experience suggested that its buffer keeps up with that shooting speed – it ought to keep shooting until the memory card is full, the batteries run out, or the photographer’s finger gets tired.

Shutter-to-shot (7.74)
The biggest complaint we hear about compact cameras is that they have long shutter delays – the lag between when the user presses the button and when the shutter clicks is too long. The Canon A640 is not a stellar performer in this respect – in our tests, the A640’s median delay was 0.63 seconds. That’s longer than many competing cameras. A good score for compact cameras is about 0.3 seconds. Users should learn to prefocus when possible, which can shorten the delay.

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