Introduction | ISO sensitivity | Noise | Color sensitivity | Sharpness | Distortion | Light transmission | Vignetting | LCA |
Noise in an image is described as random granulation that is particularly visible in uniform areas.
Noise in an image can stem from several different factors:
Noise itself is characterized by several values, among which are Standard Deviation and Signal-to-Noise Ratio (SNR). Other measures are derived from these, such as Dynamic Range and Tonal Range. (Other aspects of interest include color sensitivity, and noise granulation and coloration, which will be discussed separately on this site.)
,
where is the mean gray level measured in the patch.
SNR is independent of any gain applied to the signal since signal and noise are equally amplified.
Dynamic range is defined as the ratio between the highest and lowest gray luminance a sensor can capture. However, the lowest gray luminance makes sense only if it is not drowned by noise, thus this lower boundary is defined as the gray luminance for which the SNR is larger than 1. The dynamic range is a ratio of gray luminance; it has no defined unit per se, but it can be expressed in Ev, or f-stops.
Tonal range is the effective number of gray levels the system can produce. This measure has to take noise into account (indeed, a very thin gray-level quantization is irrelevant if the quantization step is much smaller than noise). The standard deviation of noise can be viewed as the smallest difference between two distinguishable gray levels. The expression of the tonal range is
.
Since tonal range is a number with no unit, one can consider instead , which represents the number of bits necessary to encode all distinguishable gray levels.
To provide photographers with a broader perspective about mobiles, lenses and cameras, here are links to articles, reviews, and analyses of photographic equipment produced by DxOMark, renown websites, magazines or blogs.
On DxOMark, we evaluate and rank many types of digital cameras with image sensors that vary widely in pixel count, pixel size, and digital signal processing. To ensure that sensor performance comparisons between cameras are fair, it is very important both to test under identical shooting conditions and to take viewing conditions into account. |
The Nikon D5000 and the Canon EOS 500D were two main releases for spring 2009. Launched at the same time, and targeting the same market, these two cameras were the perfect subjects to use for an extensive demonstration of the DxOMark Database. |