The actual sensitivity of each indicated ISO is measured using the same shots as are used to measure ISO noise levels, we simply compare the exposure for each shot to the metered light level (using a calibrated Sekonic L-358), middle gray matched. We estimate the accuracy of these results to be +/- 1/6 EV (the margin of error given in the ISO specifications). In our tests we found that measured ISOs from the Canon EOS 650D match the marked ISOs within 1/6 stop accuracy, meaning ISO 100 indicated = ISO 100 measured.
Noise and Noise Reduction (JPEG)
At its default ('Standard') level of noise reduction, the EOS 650D exhibits a bit more image noise than its peers between ISOs 1600 and 6400. Noise suppression increases dramatically at the camera's two highest ISO sensitivities. And as the samples bear out, image detail falls off significantly beyond ISO 3200.
Looking at the graph it's interesting to note that the 650D also shows higher noise levels than its predecessor, the EOS 600D, throughout nearly the entire range of ISO values. Yet based on the samples here, the 650D maintains a near identical level of image detail while also offering a top ISO of 25,600 (versus 12,800 in the 600D).
RAW noise (ACR 7.1, noise reduction set to zero)
Looking at the graph of raw file data, you can see that at the sensor level, noise performance of the EOS 650D falls roughly in line with its direct competitors, with only marginally higher noise levels than its predecessor, the 600D beginning at ISO 3200. As you can see in the sample images here, the noise performance of the 650D remains fairly uniform up through ISO 1600, with image detail becoming significantly obscured only at ISO 6400 and higher.