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 (i.e. ISO 200, 1/200 sec for consistency of exposure between cameras). The image sequence is run through our own proprietary noise measurement tool (version 1.6 in this review). Click here for more information. Room temperature is approximately 22°C (~72°F), simulated daylight lighting.
Nikon D3000 vs. Canon EOS 1000D vs. Sony Alpha 230
- Nikon D3000: Nikkor 50 mm F1.4 G lens, Manual Exposure, Manual WB,
Default Parameters (Standard), High ISO NR Default (on, 'low'), JPEG Large / Fine
- Canon EOS 1000D: Canon 50 mm F1.4 lens, Aperture Priority, Manual WB,
Default Parameters (Standard PS), High ISO NR Default (off), JPEG Large / Fine
- Sony Alpha 230: Sony 50 mm F1.4 lens, Manual Exposure, Manual WB,
Default Parameters, High ISO NR default (on), JPEG Large / Fine
|Nikon D3000||Sony Alpha 230||Canon EOS 1000D|
Although high ISO noise reduction is automatic in the Nikon D3000 at default settings, it is clear from these images that it has a relatively subtle effect. Assuming (as seems very likely) that the Nikon D3000 and Sony Alpha 230 share the same sensor, the differences visible here between the two cameras can be explained by the different approaches that Nikon and Sony have taken to noise reduction. The Nikon D3000 gets rid of chroma noise very effectively, but leaves luminance more or less alone. The Alpha 230 on the other hand attempts to remove both, which gives smoother continuous tones, but leads to the masking of detail that is most obvious here at ISO 1600 and 3200.
The Canon does very well here, with low noise and little evidence of noise reduction affecting detail right up to ISO 1600. As previously noted, at default settings the EOS 1000D turns out sharper images than the other two cameras too, which improves the definition of fine detail in these shots. It is worth noting too that at default settings, JPEGs from the Nikon D3000 are noticeably lower in contrast than those from the EOS 1000D and Alpha 230, which gives a slightly deceptive impression of reduced detail resolution.
The graphs below confirm what we have seen in the sample crops above - Nikon has taken a reasonably 'hands off' approach to the default level of noise reduction in the D3000 compared to the higher-level D5000, and the sudden increase in noise above ISO 1600 illustrates exactly why ISO 3200 is an extension setting. The Sony Alpha 230's noise reduction is much more aggressive, and really kicks in at ISO 1600 - hence the sudden drop in measured noise at this setting. Looking at the noise levels in these graphs, the Canon EOS 1000D could almost certainly operate at ISO 3200, but Canon has been surprisingly conservative and has capped its maximum ISO sensitivity at 1600.
High ISO noise reduction
The D3000's high ISO noise reduction is automatic at ISO 800 and above, but by switching noise reduction to 'on' in the shooting menu, the amount of noise reduction applied to images is increased. The effects can be seen clearly in the images and graph below. With noise reduction at its higher level, the grittiness characteristic of the D3000's default high ISO output is greatly reduced, but with it, detail.
|ACR (no noise reduction)||JPEG (NR 'off')||JPEG (NR 'on')|
This graph shows noise as standard deviation of luminosity at all of the D3000's available high ISO noise reduction settings (all two of them). The further the bar extends to the right, the more noise is present in the images. It is clear from this graph that all three types of noise - black, gray and chroma - are significantly reduced when high ISO noise reduction is operating to its higher level. Remember though, that even with noise reduction turned 'off', the D3000 still applies a certain amount of noise reduction to images shot at ISO 800 and above.
Here, we have converted raw files from the Nikon D3000, Canon EOS 1000D and Sony Alpha 230 in exactly the same way, in Adobe Camera RAW, with sharpening at default settings, and noise reduction turned off (to 0). Naturally, these images look somewhat different when processed using the bundled raw converters, but by processing the files in the same program in the same way, we achieve the closest thing to a 'level playing field' that is practical (whilst still being meaningful).
With ACR's noise reduction turned off, we get a better idea of what the sensors in these cameras are actually capable of. The Nikon D3000 and Sony Alpha 230 are pretty close in terms of detail capture and visible noise levels until ISO 800, when the Sony's in-camera noise reduction (which is switched on by default in both raw and JPEG modes) kicks in. The Nikon D3000 delivers the noisiest files of the three cameras when processed using our standardized method, but up to ISO 1600, a lot of detail can be recovered by careful post-capture sharpening and noise reduction. Yet again, despite its fairly old (in digital terms) sensor, the Canon EOS 1000D is the star of this selection, and gives crisp, relatively 'clean' results across its entire ISO span.
|Nikon D3000||Sony Alpha 230||Canon EOS 1000D|
Raw Noise graphs
Using a common raw converter tells us a lot more about the actual characteristics of these cameras' sensors when it comes to noise. Notice how measured noise levels from all four cameras plotted on this graph have shot up compared to their JPEG output, but all give very similar performance up to ISO 800.
The Sony Alpha 230 appears to give the least noise at ISO 1600 and 3200, but as noted previously, this is a consequence of in-camera noise reduction on its raw output at default settings. The clue is in the sharp 'kink' downwards at ISO 1600 compared to the other cameras. The Nikon D5000 gives the lowest levels of measured noise in raw mode, and the D3000 the highest. As always though, the graphs don't tell the whole story, and in real-world shooting, with standard post-processing routines, image quality from all of the cameras plotted here is perfectly acceptable up to ISO 1600.