DxOMark provides five lens metric scores that are numerical values computed from various test results. These values constitute an easy-to-understand summary of lens performance based on five key metrics (sharpness, distortion, vignetting, transmission, chromatic aberration).
The DxOMark resolution score shows sharpness performance of a lens-camera combination averaged over its entire focal length and aperture ranges.
The resolution score is computed as follows:
For each focal length and each f-number, we first compute sharpness and then weight it throughout the field, tolerating less sharpness in the corners than in the center. This gives one number for each focal and aperture combination.
Then, for each focal length, we select the maximal value of sharpness over the range of available apertures. We average this value over the whole range of focal length to obtain the DxOMark resolution score that we report (in P-MPix).
Note that for a wide-range zoom, there are huge differences between the resolutions for different focal lengths.
Sharpness is expressed in PMpix and is typically between 50% and 100% of the sensor pixel count, Differences below 1 P-MPix are usually not noticeable.
Best resolutions are usually attained for fixed focal lenses and moderate apertures (depending on the lens, between f/2.8 and f/8).
To compute the distortion metric, we average the absolute value of the maximal distortion over the whole range of focal lengths (distortion being independent from aperture). The result is the DxOMark distortion score that we report.
Zooms usually have negative distortion (barrel) for short focal lengths, and positive (pincushion) distortion for longer focal lengths. The metric penalizes both types of distortion. Distortion is expressed as a percentage: the value 0 is the perfect case; 1% is high, but there is no upper limit. A value of 0.2% corresponds to a noticeable distortion. Wide-angle lenses have more distortion.
For each focal length, we only consider the widest possible aperture. We weigh the vignetting value in the field, and tolerate a bit of vignetting in the corner. This yields a single value for each focal length. We average values over the whole range of focal length, and the result is the DxOMark vignetting score that we report.
Vignetting is expressed in Exposure Value (EV) and is a negative number, as it describes a loss of exposure. No vignetting at all (0 EV) is perfect. Very wide aperture lenses are obviously more likely to show more vignetting (more than 2EV). Variations below 1/3 EV are barely noticeable.
For each focal length, we measure the T-stop at the largest possible aperture. We then obtain the transmission metric score by averaging over the whole range of focal length.
T-stop has the same meaning as the lens f-stop and utilizes similar values. Smaller numbers mean more light. Best transmissions are attained for fixed focal lenses. Zooms cannot usually have very large apertures for long focal lengths. T-stop has a indirect impact on the image, since it will usually change autoexposure behavior. A lower transmission can result in longer exposure times (and motion blur) or higher ISO sensitivity (and more noise). Variation below 10% will not be noticed.
For each focal length and aperture, we first normalize (scale on a 24x36mm sensor) and weigh lateral chromatic aberration in the field, so as to tolerate a bit of aberration in the image corners. For each focal length, we select the largest value over the range of possible apertures, and then average this value over the range of focal length. The result is the chromatic aberration metric score that we report.
Chromatic aberrations are expressed in micrometers (µm). The perfect value is 0; a value of 30 is very high, although there is no upper limit. A value of 5µm is noticeable and represents about 1 pixel for most cameras.