Canon PowerShot S40 Review
ISO (Sensitivity) Adjustment
ISO equivalence on a digital camera is the ability to increase the sensitivity of the CCD to allow for faster shutter speeds and/or better performance in low light. The way this works in a digital camera is by "turning up the volume" on the CCD's signal amplifiers, nothing is without its price however and doing so also amplifies any noise that may be present and often affects colour saturation.
The S40 features four selectable ISO sensitivities of 50, 100, 200 and 400 as well as Auto mode which varies the sensitivity between ISO 50 and 100 depending on available light. The S30 also has an ISO 800 mode.
|Good light (10 EV)||Low light (3 EV)|
|ISO 50, 1/8 sec, F5.6||ISO 50, 4.0 sec, F4.5 *|
|ISO 100, 1/15 sec, F5.6||ISO 100, 2.0 sec, F4.5 *|
|ISO 200, 1/30 sec, F5.6||ISO 200, 1.0 sec, F4.5|
|ISO 400, 1/80 sec, F5.6||ISO 400, 1/2 sec, F4.5|
* These images will have noise reduction applied because the shutter speed was slower than 1.3 seconds, this is unfortunately unavoidable (as you can't turn off noise reduction).
As we saw with the G2 the S40's ISO 50 is clean and smooth, moving up to ISO 100 you can begin to see visible noise, although it's certainly not as much as a 3 megapixel at ISO 100. Up to ISO 200 and 400 you must expect noise, although with a 4 megapixel image downsampled to a reasonable size (such a 1024 x 768) you wouldn't be able to see most of this noise.
The S40 implements the same white balance as we saw on the G2 which is an evolution of the same from the IXUS 300 / A10 / A20. Auto white balance wasn't particularly good under artificial light. The preset white balances did work well with almost perfect grays, good strong colour and no visible hue shift. Manual white balance seemed to work well under any type of light.
|Outdoors, Auto||Outdoors, Sunny or Cloudy||Outdoors, Manual|
|Incandescent, Auto||Incandescent, Incan.||Incandescent, Manual|
|Fluorescent, Auto||Fluorescent, Fluor. or Fluor. H||Fluorescent, Manual|
Aperture Priority Mode
Aperture priority is where you designate the aperture and the camera calculates the best shutter speed, if the exposure is outside of the cameras range (either over or under exposing) the nearest shutter speed will display in red on the LCD screen. Used properly Aperture Priority can be invaluable as it has a direct effect on depth of field (the distance in front and behind the focal point which will be in focus when taking the shot).
The S40 has a good range of apertures at wide angle, but is more limited at telephoto:
- Wide: F2.8, F3.2, F3.5, F4.0, F4.5, F5.0, F5.6, F6.3, F7.1, F8.0
- Tele: F4.9, F5.6, F6.3, F7.1, F8.0
Aperture Priority is an exposure mode is accessed by turning the exposure dial to Av. You can change aperture by pressing the left or right arrow keys on the 4-way controller. A basic example of aperture priority is shown below for more read my digital photography glossary:
| F2.8, 1/30 sec
(Less depth of field)
|F8.0, 1/4 sec
(Most depth of field)
Just like the G1 and G2 the S40's macro ability is poor to say the least. This button shouldn't really carry the macro icon, but rather some representation of 'close up'. Add to this that the S40 doesn't support add-on lens (and thus can't use Canon's 250D macro lens) and you'll be limited to 'close up' shots with the S40.
The best frame coverage was found at the wide angle end of the zoom range and produced a closest coverage of approximately 10 cm (4 inches) across the frame.
Low Light Focus
This test measures the minimum amount of light under which the camera can still focus. The focus target is our lens distortion test chart (shown here on the right), camera is positioned exactly 2 m (6.6 ft) away.
Light levels are gradually dropped until the camera can no longer focus. This is carried out at both wide angle and telephoto zoom positions (as more light reaches the focusing systems with a larger aperture).
This test target is the optimum type of subject for most "contrast detect" AF systems (as it has a vertical line at its center), you should consider the results below the best you could expect to achieve.
|Lens position||Aperture||Lowest light focus|
|Wide angle (35 mm)||F2.8||Complete darkness|
|Telephoto (105 mm)||F4.9||5.0 EV (80 Lux / 7.4 foot-candle)|
Light intensity (Lux) = 2.5 x 2^EV (@ ISO 100), 10.76391 Lux = 1 foot-candle (fc)
No surprise that the S40 can focus in complete darkness, it does have a focus assist lamp. However, what was a surprise was that this lamp isn't powerful enough to work at full telephoto, indeed we need a fairly significant amount of ambient light before we could get a reliable focus lock at full telephoto (F4.9).