DxOMark Sensor scores
Three use cases (with one score for each) — Portrait, Landscape, and Sports — report on different aspects of sensor performance. Each use case score is associated with one defined image quality metric as defined below:
Portrait photography: Color Depth
Flash studio photography involves a controlled and usually maximal amount of light. Even when shooting with hand-held cameras, studio photographers rarely move from the lowest ISO setting. What matters most when shooting products or portraits is to aim for the richest color rendition.
The best image quality metric that correlates with color depth is color sensitivity. Color sensitivity indicates to what degree of subtlety color nuances can be distinguished from one another, often meaning a hit or a miss on a pantone palette. Maximum color sensitivity reports, in bits, the number of colors that the sensor is able to distinguish.
The higher the color sensitivity, the more color nuances that can be distinguished. As with dynamic range, color sensitivity is greatest when ISO speed is minimal, and falls rapidly with rising ISO settings. DxO Labs has focused on measuring only maximum color sensitivity.
A color sensitivity of 22bits is excellent, and differences below 1 bit are barely noticeable.
Landscape photography: maximum Dynamic Range
Landscape photographers often carefully compose their images and choose the optimal time to shoot. This type of photography commonly involves mounting the camera on a tripod and using the lowest possible ISO setting to minimize noise, thus maximizing image quality.
Unless there is motion, relatively long shutter speeds are not an issue with a tripod. What is paramount is dynamic range, especially because photographers will often aim for detail in high-contrast settings, juxtaposing bright sky with shadowy foliage, mountains, etc. Ideally, the dynamic range of the camera should be greater than the dynamic range of the scene, otherwise details in shadows are lost or highlights are burned.
Dynamic range falls rapidly with higher ISO settings, as any analog or digital amplification performed will increase the noise in the darker areas, making it harder to distinguish between fine levels of contrast.
Maximum dynamic range is the greatest possible amplitude between light and dark details a given sensor can record, and is expressed in EVs (exposure values) or f-stops, with each increase of 1 EV (or one stop) corresponding to twice the amount of light.
Dynamic range corresponds to the ratio between the highest brightness a camera can capture (saturation) and the lowest brightness it can capture (typically when noise becomes more important than the signal, i.e., a signal-to-noise ratio below 0 dB).
A value of 12 EV is excellent, with differences below 0.5 EV usually not noticeable.
This scale is open, as incoming light is not a bounded quantity.
Sports & action photography: Low-Light ISO
Unlike the two previous scenarios in which light is either generous (studio) or stability is assured (landscape), photojournalists and action photographers often struggle with low available light and high motion. Achieving usable image quality is often difficult when pushing ISO.
When shooting a moving scene such as a sports event, action photographers’ primary objective is to freeze the motion, giving priority to short exposure time. To compensate for the lack of exposure, they have to increase the ISO setting, which means the SNR will decrease. How far can they go while keeping decent quality? Our low-light ISO metric will tell them.
The SNR indicates how much noise is present in an image compared to the actual information (signal). The higher the SNR value, the better the image looks, because details aren't drowned by noise. SNR strength is given in dB, which is a logarithmic scale: an increase of 6 dB corresponds to doubling the SNR, which equates to half the noise for the same signal.
An SNR value of 30dB means excellent image quality. Thus low-light ISO is the highest ISO setting for a camera that allows it to achieve an SNR of 30dB while keeping a good dynamic range of 9 EVs and a color depth of 18bits.
A difference in low-light ISO of 25% represents 1/3 EV and is only slightly noticeable.
As cameras improve, low-light ISO will continuously increase, making this scale open.