I'm not interested in the DxOMark overall score
per se, as the weighting of each individual score is DxO's idea, and others may place a difference emphasis of what performance parameter is more important to them. However two of the sensor scores I find very useful:
Portrait Score, based on optimum Color Depth. I can't relate this to real world situations.
Landscape Score, based on maximum Dynamic Range. A high technical DR is indicative of either a high FWC or a low total read noise figure (the RMS combination of 3 non-correlated noise sources: Sensor RN; PGA noise; ADC noise), or both. Since the max DR (used for this score) is at the true base ISO, a high DR score usually indicates a very low-noise ADC implementation. (Sensor RN is more significant at mid-to-high ISOs.) A very low-noise ADC is important when boosting underexposed images (e.g. flash failures) and when boosting deep shadows. So a high Landscape score (e.g. 13-15 EV/stops) indicates good useful performance in these areas.
Sports Score, based on Low-Light ISO. It is the highest ISO at which the imaging system meets all 3 of the following criteria:
SNR of at least 30dB
DR of at least 9 EV/stops
Colour depth of at least 18bits.
Meeting all 3 criteria should produce a good quality image (technically, at least). Usually it's the SNR performance that is the dominant factor in determining the score. A bigger sensor will have a higher score because it captures more photons and has less shot/photonic noise in comparison to the signal. Shot noise is an intrinsic property of light (due to the quantatised nature of photons) and the signal-to-shot-noise ratio is proportional to the square root of the number of photons captured. A higher score in the same-sized sensor comparison (i.e. the same format e.g. APS-C) indicates a better fill-factor/better micro-lens performance/better Quantum Efficiency). As fill-factors have improved, due to better micro-lenses, QE has started to exceed 50% in some cameras. (In old micro-lens-less sensors, QE was less than 25%.) It will be technically impossible to get more than another stop of high ISO performance improvement, (can't exceed QE=100%), but a high Sports score indicates that a sensor is efficiently capturing and converting more incident photons hitting it and will have have a bettersignal-to-shot/photonic noise ratio. Practically, it means you can use a higher ISO for the same amount of visible noise. And, for similar QEs, but when comparing across formats, the ratio of Sport/Low Light scores should be reasonably close to the ratio of sensor areas:
Camera | Format | Sensel Pitch | ISO/Score | Score Ratio | Sensor Area Ratio |
D800 | FF | 4.7µm | 2853 | 2.34 | 2.34 |
K-3 | APS-C | 3.9µm | 1216 | 1 | 1 |
OM-DE-M5 | MFT | 3.7µm | 826 | 0.61 | 0.68 |
Dan.