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. Like many recent digital SLRs all four in this comparison proved to be right on the spot with their indicated sensitivity, that is to say an indicated sensitivity of say ISO 200 was exactly that (unlike some older digital SLRs which were slightly more or less sensitive than indicated).
ISO Sensitivity / Noise levelsISO equivalence on a digital camera is the ability to increase the sensitivity of the sensor. This 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. Sony DSLR-A700 vs. Canon EOS 40D vs. Nikon D300 vs. Olympus E-3
From ISO 100 to 400 there is virtually no difference from a noise point of view between any of these cameras, none exhibit noticeable any noise. At ISO 800 the DSLR-A700 begins to exhibit some chroma (blotchy color) noise, the cleanest of the remaining three appears to be the D300 followed by the EOS 40D and then E-3. At ISO 1600 the DSLR-A700's noise reduction appears to become stronger and while noise is still noticeable it's less pronounced, although this at the expense of image detail which is beginning to soften. By this point the E-3 is beginning to look quite grainy and has also lost some image detail to noise reduction, the EOS 40D has some chroma noise and the D300 remains fairly clean and detailed (thanks to Nikon's emphasis on chroma noise reduction). At ISO 3200 the DSLR-A700 image has become quite soft due to heavy noise reduction. The latest firmware (V4) actually increases the high ISO noise reduction to remove the chroma noise we originally saw at 1600 and above. Again the cleanest image at this sensitivity comes from the D300 with the best detail coming from the EOS 40D (although distracting chroma noise is visible), the E-3 is struggling at this point. Surprisingly at ISO 6400 the D300's image could be considered 'usable' although very noisy, the same could not really be said for the DSLR-A700. Overall the DSLR-A700's performance trails both the Nikon D300 and Canon EOS 40D although does seem better than the Olympus E-3. What's clear is that Sony are using a fair amount of luminance noise reduction (too much) when we'd prefer it to concentrate on chroma NR and leave some 'texture' in the shots. * It's worth noting that these results are with the cameras in their default modes, the EOS 40D for example does have an optional stronger chroma noise reduction option which delivers images with almost no chroma noise. Noise graphs
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