Canon PowerShot S40 Review: 15. Compared to...: Digital Photography Review

Compared to... (contd.)

Colour Chart Comparison

Now we'll use the data in our review database to compare the S40's colour and resolution to three other four megapixel digital cameras: the Olympus C-40Z, Canon PowerShot G2 and Sony DSC-S85.

Colour charts are shot in daylight, Auto White Balance, EV compensation +0.3 (all cameras), measured light ~10 EV. This test is also dependent on the accuracy of the camera's auto white balance, the colour charts are shot in daylight but some camera's white balance is better than others. All cameras are given 20 seconds to "settle" before the shot is taken.

Canon PowerShot S40 Olympus C-40Z

Canon PowerShot G2 Sony DSC-S85

As we'd expect virtually identical performance as the G2. The G2's shot has a slight cyan cast, this could be put down to very slightly different ambient light when we took the shot, it's not that significant.

In the table below we're only measuring colour. The RGB values were measured from a VGA reduced image (to average colours, remove noise and eliminate JPEG artifacts) using the Eyedropper tool in Photoshop with a 5 by 5 Average Sample Size.

Patch	Canon PowerShot S40	Olympus C-40Z	Canon PowerShot G2	Sony DSC-S85
Black	30,30,30	30,31,29	45,43,42	17,17,19
Black
Middle Gray	103,102,103	100,99,99	107,106,110	76,81,79
Middle Gray
White	215,210,211	214,216,211	208,210,208	188,189,189
White
Magenta	199,41,113	208,51,116	196,37,116	170,12,124
Magenta
Red	196,33,42	196,45,41	195,30,35	176,0,40
Red
Yellow	211,192,38	206,207,50	205,195,20	181,180,14
Yellow
Green	33,140,67	8,143,63	43,152,68	24,131,27
Green
Cyan	26,160,223	17,155,224	23,169,234	23,158,175
Cyan
Blue	33,44,116	49,48,111	61,56,133	23,31,112
Blue

Resolution Chart Comparison

Shots here are of the PIMA/ISO 12233 standard resolution test chart (more of which are available in our comparison database). This resolution chart allows us to measure the actual performance of the lens and sensor system. It measures the ability of the camera to resolve lines at gradually higher resolutions and enables us to provide a definitive value for comparison purposes. Values on the chart are 1/100^th lines per picture height. So a value of 8 equates to 800 lines per picture height.

Studio light, cameras set to auto, all settings factory default. Exposure compensation +0.7 EV for all cameras.

Horizontal resolution Vertical resolution 5 degree diagonal res.

Canon PowerShot S40

Olympus C-40Z

Canon PowerShot G2

Sony DSC-S85

Measurable findings (three measurements taken for each camera):

Camera	Measurement	Absolute Res.	Extinction Res.
Canon PowerShot S40	Horiz LPH	1250	*1400
	Vert LPH	1100	1400
	5^o Diagonal LPH	1000	n/a
Olympus C-40Z	Horiz LPH	*1200	*1350
	Vert LPH	1100	1400
	5^o Diagonal LPH	1000	n/a
Canon PowerShot G2	Horiz LPH	1250	*1450
	Vert LPH	1200	*1500
	5^o Diagonal LPH	1000	n/a
Sony DSC-S85	Horiz LPH	1250	*1650
	Vert LPH	1200	1650
	5^o Diagonal LPH	1000	n/a

* Some artifacts / moiré visible

Definition of terms:

LPH	Lines per Picture Height (to allow for different aspect ratios the measurement is the same for horizontal and vertical)
5^o Diagonal	Lines set at 5^o diagonal
Absolute Resolution	Still defined detail (below Nyquist frequency*)
Extinction Resolution	Detail beyond camera's definition (becomes a solid gray alias)
n/a	Not Available (above the capability of the test chart)
n/v	Not Visible (not visible on test results)

* Nyquist frequency defined as the highest spatial frequency where the CCD can still faithfully record
image detail. Beyond the Nyquist frequency aliasing occurs.

The S40 did remarkably well, especially considering its tiny lens. Resolution was virtually identical to the G2, perhaps a little less in the vertical direction. Where the G2's lens will perform better is corner sharpness (the S40 can be a little soft in the corners) and its relative speed (F2.0 - F2.5 vs. F2.8 - F4.9). Also putting up a good fight is Olympus's C-40Z, although it's a shame about all that moiré.

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