White Balance

The A640 has six white balance presets (daylight, cloudy, tungsten, fluorescent, fluorescent H and underwater) in addition to the default auto white balance. There is also a 'custom' white balance setting, which allows you to point the camera at a white or gray object and set the white balance manually. The custom white balance setting is remembered even if you turn the camera off. In normal outdoor shooting the auto white balance works perfectly (as confirmed by our studio tests). Indoors it's a bit more hit and miss, as we've seen with most Canon PowerShots, fluorescent lighting doesn't cause much of a problem, whereas incandescent (tungsten) lighting causes a fairly strong orange color cast - something Canon assures me is intentional (to preserve the atmosphere of the scene). Best to stick to the preset (or one-push custom WB) or the tungsten preset if you want more neutral colors.

Incandescent - Auto WB
Red 10.2%, Blue -15.5%
Incandescent - Incandescent preset WB
Red 1.7%, Blue -2.1%
Fluorescent - Auto WB
Red 2.0%, Blue -15.9%
Fluorescent - Fluorescent preset WB
Red 1.8%, Blue -5.4%

Flash Performance

The A640's built-in flash appears to be identical in specification and performance as the A620 before it. It has a quoted working range of 0.45m - 4.2m (1.5 - 11.5 ft) at the wide end of the zoom and 0.45m - 3.0m (1.5 - 9.8 ft) at the tele end. It also works down to about 25cm (11.8 inches) in macro mode (in all cases assuming the ISO is set to auto). In our real-world tests the flash did a perfectly good job within its effective range, exposing perfectly in a wide range of situations and with virtually no color cast. Recycle time - though still not class-leading - isn't a major problem unless your batteries are running low.

Skin tone
Excellent color and exposure
Color chart
Slightly warm tone

Macro Focus

Macro performance is identical to the A620 (not surprising since it's the same lens). Is common to most compact digital cameras the A640's macro mode is most effective at the wide end of the zoom, where you can get as close as 1cm - very impressive. At the long end of the zoom the performance is less impressive - 25cm subject distance - but still pretty useful. There is inevitably some distortion when shooting very close up at the wide end, but it is not too strong, and certainly less so than many of its competitors.

Wide macro - 21 x 16 mm coverage
166 px/mm (4221 px/in)
Distortion: Average
Corner softness: Low to average
Equiv. focal length: 35 mm
Tele macro - 76 x 57 mm coverage
48 px/mm (1215 px/in)
Distortion: Very low
Corner softness: Low to average
Equiv. focal length: 140 mm

Barrel and Pincushion Distortion

Barrel distortion is - at 1.0% - fairly low for a camera in this class, and certainly doesn't mar real world scenic shots. There is a tiny (0.2%) amount of measurable pincushion distortion at the telephoto end of the zoom.

Barrel distortion - 1.0% at Wide angle
Equiv. focal length: 35 mm
Pincushion distortion - 0.2% at Telephoto
Equiv. focal length: 140 mm

Specific Image Quality Issues

Although there's been a small increase in resolution and mid-ISO performance is slightly better, the A640 doesn't represent a major leap forward over the A620, certainly not one you'd notice in a standard-sized print. That's no bad thing, as the A620 was one of the better cameras in its class.

So then, no big complaints here - it's not groundbreaking quality but the A640 produces clean, detailed results in a wide variety of shooting situations. The images are a touch on the soft side when viewed at 100% on-screen, but they respond well to a little unsharp masking, and I'd rather that than images that are over-processed (especially over-sharpened). Colors are natural, and white balance generally very accurate (unless you're shooting under tungsten lighting). ISO 200 and 400 look slightly noisier than the A620 (though they are measurably similar), but there is a little less of the muddying of low contrast detail that camera exhibited at ISO 200 and 400, and ISO 200 is surprisingly usable.

There is very little evidence of purple fringing or CA in any of our real-world shots (and where there is it is only at the edge of areas of gross overexposure), and the only major concern is the occasional highlight clipping in very bright, contrasty conditions - something we see with virtually all high pixel count compacts.

Highlight clipping

As is common with small sensor, high pixel count cameras, the A640 struggles to capture the full range of brightnesses in scenes with a large dynamic range (high contrast, very bright days). Like the A620 before it, the A640's problem seems to be a combination of the usual limited dynamic range of such sensors and a fairly steep tone curve, which can lead to quite harshly clipped highlights (the overexposure we saw with the A620 is still there, but is common).

You can fairly easily counteract this with a little negative AE compensation (turning the contrast down helps too), and the playback histogram / clipping warning is useful, but it would be better if it just didn't happen in the first place! To be fair the problem isn't widespread (and seems to be less of an issue than it was with the A620), but it does spoil the A640's otherwise impeccable 'point and shoot' performance. Note that the bottom example also shows mild red fringing around the clipped areas.

100% crop 72 mm equiv., F4.0
100% crop 35 mm equiv., F4.0