ISO / Sensitivity accuracyIn a new addition to our reviews we are now measuring the actual sensitivity of each indicated ISO sensitivity. This is achieved 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 Sekonic L-358), middle gray matched. We estimate the accuracy of these results to be +/- 1/6 EV. Unlike the EOS 350D (Rebel XT) the EOS 400D (Rebel XTi) proved to be slightly less sensitive (by a third of a stop) which means that its actual sensitivity is as per the indicated sensitivity.
UPDATE 18/Dec/06: In our original review a mistake was made in the measurement of ISO sensitivity, this has now been corrected and the releveant pages of this review updated. ISO Sensitivity / Noise levelsISO equivalence on a digital camera is the ability to increase the sensitivity of the sensor. The works by turning up the "volume" (gain) on the sensor's signal amplifiers (remember the sensor is an analogue device). By amplifying the signal you also amplify the noise which becomes more visible at higher ISO's. Many modern cameras also employ noise reduction and / or sharpness reduction at higher sensitivities.To measure noise levels we take a sequence of images of a GretagMacBeth ColorChecker chart (controlled artificial daylight lighting). The exposure is matched to the ISO (ie. ISO 200, 1/200 sec for consistency of exposure between cameras). The image sequence is run through our own proprietary noise measurement tool (version 1.4 in this review). Click here for more information. (Note that noise values indicated on the graphs here can not be compared to those in other reviews). Room temperature is approximately 22°C (~72°F), simulated daylight lighting. Canon EOS 400D (Rebel XTi) vs. Nikon D80 vs. Sony Alpha DSLR-A100
As you can see there is no discernable difference between the EOS 400D, D80 and DSLR-A100 up to ISO 400 (except for slightly better sharpness from the EOS 400D image). The EOS 400D continues this sharp detail all the way up to ISO 1600 but does have noticeably more chroma noise than the D80 which employs better chroma noise reduction. At higher sensitivities the EOS 400D delivers more detail but with slightly more chroma noise, the D80 less detail but a cleaner more 'film like' appearance to noise grain, the DSLR-A100 has the noisiest image although has good detail at ISO 800. Luminance noise graphThe graph below is a little complicated but what it tells us is that noise levels between all these cameras (and the EOS 350D thrown in for comparison) are similar up to ISO 400. At ISO 800 the D80's noise reduction suddenly drops noise levels although they return to a similar level as the EOS 400D at ISO 1600.
Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity on the vertical axis. Luminance noise graph (zoomed, only 10 MP cameras)The graph below is based on the same data as above except it only shows ten megapixel cameras and the standard deviation range (vertical axis) has been reduced to a maximum of six. A difference in standard deviation less than one would be quite difficult to distinguish in a normal everyday shot.
Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity on the vertical axis. Chroma (color) noise graphIn addition to measuring luminance noise we now also measure chroma (color) noise. The D80's stronger chroma noise reduction keeps levels lower at ISO 800 and 1600, however the 400D is still relatively low and at least predictable.
Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of color on the vertical axis. |
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