The article is translated from the french version. Only the results are shown below. The protocol is explained in the Ch. Buil pages.
Keywords : Canon Rebel XS, CMOS dimensions, PRNU, readout noise, gain, linearity, dark current, RGB coefficients
1. Foreword
The test has been performed at a temperature of about 24°C (75°F). The photos are stored in RAW format and the 200x200 px central area green plan has been studied with the Iris software.
2. Sensor geometry
It is a Canon proprietary CMOS sensor. The photosites are 5.71 µm in width/height. The RAW images are 3888x2592 pixels in size. The sensor effective area is 22.2x14.8 mm. The Rebel XS shares the same sensors as the older Rebel.XTi, together with a more recent Digic 3 instead of a Digic 2.
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CMOS Rebel XS |
Digic III chipset |
3. Pixel Response Non Uniformity (PRNU)
A vertical banding and a dark area in the bottom right corner are clearly visible in this contrast exagerated picture. The 3 white bands are about 3% brighter than the darker bands. This is not a good performance. However this phenomenon can be easily removed by substracting a bias master frame.
4. Gain
| ISO | g : Gain (e-/ADU) |
G : inverse gain (ADU/e-) |
ratio/previous ISO |
| 100 | 9,31 | 0,107 | - |
| 200 | 5,86 | 0,171 | 1,59 |
| 400 | 2,83 | 0,353 | 2,07 |
| 800 | 1,41 | 0,710 | 2,01 |
| 1600 | 0,72 | 1,384 | 1,95 |
Unity gain is obtained at 1170 ISO (the "unity gain" is when 1 electron is converted in 1 ADU).
5. Noise measurement
5.1. Readout Noise (RON)
| ISO | RON (e-) |
RON (ADU) |
| 100 | 17,12 | 1,84 |
| 200 | 12,26 | 2,09 |
| 400 | 7,41 | 2,62 |
| 800 | 5,56 | 3,95 |
| 1600 | 4,71 | 6,51 |
Readout noise at unity gain is 5.13 e-.
5.2. Optimal ISO sensitivity
The optimal sensitivity that balances the dynamic vs the gain is 670 ISO, and the closest setting is 800 ISO. At unity gain, the associated ISO is 1170 ISO, the closest setting is again 800 ISO.
However one should reduce the ISO setting to 400 ISO (or even less) in case of light polluted areas, so that the dynamic of the picture is increased.
For deep sky pictures where the signal of the faintest stars is the key and no light terrestrial light is polluting the picture, the highest gain is a priority and one can set the DSLR to 1600 ISO.
5.3. Signal to Noise ratio (S/N)
The S/N is calculated from Ch. Buil's explanations, at 400 ISO for a signal of 300 ADU, ignoring the dark current.
S/N = 28.3
The Rebel XS is showing slightly better results than the Rebel XTi and is just midway between the Rebel XT (EOS 350D) and the EOS 20D.
5.4. Thermal sensitivity
The curve below is showing the noise of some 5 minutes pictures taken at several temperature (each dot). The temperature is the one returned in the Exif of the RAW pictures (thanks to the Liveview that stores the sensor's temperature). The Canon engineers have succeeded in reducing the thermal noise in the normal shooting temperature range, i.e. until about 30°C.
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41°C unprocessed (noise 40.4 ADU) |
41°C, offset substracted |
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30°C unprocessed (noise 13.5 ADU) |
30°C, offset substracted |
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20°C unprocessed (noise 7,2 ADU) |
20°C, offset substracted |
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10°C unprocessed (noise 5,2 ADU) |
10°C, offset substracted |
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1°C unprocessed (noise 3,7 ADU) |
1°C, offset substracted |
The contrast has been amplified by a x10 binning. Between 0 and 15 °C, the offset pattern is dominating over the dark current. Between 15 and 25 °C both patterns are mixed. Above 25 °C, the dark current pattern is dominating.
6. RGB Coefficients
R=2.11 V=1.00 B=1.39
7. Sensor linearity
The linearity is good up to 3800 ADU.
The Rebel XS experiences a strange behaviour when the exposure time is equal to or higher than 10 seconds. The saturation level is 4037 ADU for exposures less than 10 s but drops to 3652 ADU for greater exposures. Why ?
8. Full well capacity and dynamic
The full well capacity is the maximum number of electrons each pixel can hold at saturation. It is based on the maximal number of ADU (in theory 2^12) multiplied by the gain at the lowest ISO. However, Canon adds a fixed bias to the picture and, as one can see in the previous section, the maximum value is limited to 3652 ADU for long exposures. The practical FWC is therefore lower than the theoretical FWC :
- Theoretical FWC : (2^12-1)x9.31 = 38 124 e-
- Practical FWC : (3652-256)x9.31 = 31 617 e-
The dynamic is calculated for each ISO with the associated practical FWC :
| ISO | FWC (e-) |
Dynamic | Dynamic (f:stop) |
| 100 | 31 617 | 1 847 | 10.8 |
| 200 | 19 901 | 1 623 | 10.7 |
| 400 | 9 611 | 1 297 | 10.3 |
| 800 | 4 788 | 861 | 9.7 |
| 1600 | 2 445 | 519 | 9.0 |
