Since there has been a lot of interest in this camera after previous posts, I wanted to mention that DxO published their rating of the Nikon D3100 sensor.
I’ve used DxOMark a lot in passing discussions, and a friend (and recent Nikon D3100 owner) last month mentioned, “I don’t know how to read this.” Whoops!
I had better explain what these values mean.
The Overall Score is an aggregate based on the other scores normalized against some baseline value. It really serves no purpose other than bragging rights. The only important thing to know here is that this (and all DxO scores) are a measurement of the sensor and RAW files—so in-camera JPEG processing, and camera and lens performance are not a factor in any of these.
The Portrait score is the number of bits of color the sensor can distinguish at the base ISO under ideal conditions. This means, for example than the Nikon D3100 can distinguish 6 million colors (222.5). As a reference point, the human eye distinguishes somewhere under 10 million and your monitor renders 16.2 million.
The Landscape score is the number is f-stops of dynamic range the sensor can distinguish at base ISO. Dynamic range is the total tonal range from deepest near-blacks in shadow to the highest brights in highlights (without blowing them). This means, for example, that the D3100 can distinguish 11 f-stops while the Nikon D300s and Nikon D3s give you an extra f-stop. Remember, exposure bracketing can extend this range, and that not all ranges are equal (the shadows have less bits than the highlights, but the highlights clip) as mentioned in the article linked. So, this measures the absolute range, but the effective range is going to be less.
The Sports score is the “real” ISO of the camera, and here’s how you read it: the noise level of the ISO score are the same across cameras. This means that ISO 919 on the Nikon D3100, ISO 787 on the Nikon D300s, and ISO 3253 on the Nikon D3s have equivalent noise. In other words, if you never shoot higher than ISO 800 on the Nikon D3100, then you should be just as happy with the noise performance of ISO 3200 on the Nikon D3.
The last score shows very extreme differences between makes and models. And you may be disappointed with the Nikon D3100’s performance, but that’s because we’re comparing it to a $4000 body. Instead, let’s compare it to the high-rated “pro” level pocket digicam: the Canon G11. Hmm, that ISO 919 is an anemic ISO 161 on this camera—one which people rave about its low light performance. Standards for pocket cameras are different!
Now back to the Overall Score. While the ISO performance has gone up relative to the last of the previous generation designs, the Color Depth and Dynamic Range has gone down. How is that possible?
The reason is that megapixel has gone up while the sensor size hasn’t changed. What happens when megapixel goes up is the pixel pitch increases. This may mean a drop in ISO performance (and why big sensors like the D3 smoke the D3100 which smokes the G12), but there are various tricks Nikon is doing to get around it. What can’t be avoided, however, is that the electron well that collects the photo counts also gets smaller. With a smaller well, the accuracy of the counting goes down and so does the dynamic range and color-depth. Which depends on the engineering tradeoff in the sensor design.
This also explains why Nikon cameras, with their more modest megapixel, score higher than their Canon counterparts. The DxO score doesn’t reflect the megapixel and higher pixel counts are penalized with lower DxOMark scores.
Think of it this way. When purchasing a camera, people focus too much on the megapixel, the DxOMark complements that number with its own. The combination gives you an idea of what sort of sensor is in the camera you are purchasing.