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 Nikon D3200 match the marked ISOs within 1/6 stop accuracy, meaning ISO 100 indicated = ISO 100 measured.
Noise and Noise Reduction (JPEG)
If you look at the noise graphs above you can see that, at default settings (NR Off), the Nikon D3200's measured noise levels are higher than some of the competition in its class, albeit pretty close to its predecessor D3100, pretty much across the entire ISO range. Noise levels increase slowly but steadily up to ISO 3200, then skyrocket at the two highest sensitivity settings. A look at the samples confirms the measured results. Image noise becomes visible at ISO 200 but stays within acceptable limits until ISO 3200. At ISO 6400 and the Hi1 setting (ISO 12800) both chroma and luminance noise levels become pretty intrusive while at the same time noise reduction is blurring most fine detail away.
There is practically no benefit to switching noise reduction on at low ISOs, a difference between the 'Off' and 'On' settings only becomes visible at ISO 1600 and higher. That said, even the 'Off' setting doesn't mean that no noise reduction at all is applied. Instead there appears to be a baseline level of mainly chroma noise reduction that cannot be disabled. The noise reduction setting controls the amount of additional noise reduction on top of this baseline level.
At higher ISOs the NR 'On' setting is the better option as it produces a visible cleaner image without a significant additional impact on fine detail. Overall up to ISO 3200 the Nikon D3200's noise levels are fairly typical for the entry-level bracket of the market albeit at the higher end of the spectrum. At the two highest settings image quality deteriorates quickly and ISO 6400 and 12800 images should be reserved for use at small output sizes only.
RAW noise (ACR 7.1, noise reduction set to zero)
With all noise reduction switched off in raw conversion the Nikon D3200's raw noise levels are on a very similar level to the competition in the entry-level bracket and its predecessor D3100. Converting high ISO raw files and applying a customized noise reduction mix can get you better results than the out-of-camera JPEGs, as we have demonstrated in the image quality sections of this review.