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). Note that these tests are based on the sRGB JPEG output of the cameras, in accordance with ISO 12232:2006, the standard used by camera manufacturers. In our tests we found that measured ISOs from the D3300 at least a 1/6 stop over sensitive, meaning ISO 100 indicated is closer to ISO 125 measured.
Noise and Noise Reduction (JPEG)
ISO range noise comparison
The D3300 keeps noise and fine detail in a good balance until ISO 1600. Above that, visible noise begins to increase rapidly and detail declines. ISO 12800 with NR On produces a very noisy JPEG, but real world results are good enough to be downsized and usable for small prints and web. With NR Off, fine detail looks salvageable up to about ISO 6400.
ACR Raw noise (ACR 8.4 noise reduction set to zero)
Up to and including ISO 1600, Raw ACR noise takes on a fine appearance and detail looks good. At ISO 3200 color noise becomes more blotchy and overwhelming. There's another significant jump in noise level from ISO 6400 to ISO 12800, but at this point it looks no better or worse than the cameras it's compared to. And, since this is a pixel-level test, the higher pixel count means the image should be less noisy, scaled to a common output size.