Can RAW images be already somewhat cooked?
Tuesday, March 10, 2009 News| Fixed pattern noise correction | Noise reduction | Conclusion |
Shooting pictures in RAW gives DSLR users the opportunity to fine-tune their settings, by potentially controlling every step of the RAW conversion. Users often assume that RAW images are directly output from the sensor without any additional processing, but is this really the case? What kind of processing can be applied and what kind of influence does it have on the final image?
Fixed pattern noise correction
Most noise occurs at the pixel level and can be considered as statistically white (meaning that each individual pixel’s noise is independent from other pixels’ noise) and identically distributed. However, there is another kind of noise, usually less common, that violates this “statistically white” rule. It is noise due to irregularities on the sensor, and thus has a fixed position.
Because different sizes of transistors are shared by all the pixels in a single row or a single column, all the pixels of a row (or column) might be slightly too bright or too dark. This is called row or column noise. It is often possible to correct it by leaving a few pixels unexposed to light on the affected side of the sensor. The level of noise is measured for these pixels and the correction is applied on the whole row or column.
Some pixels can also behave almost independently of the incident light: they may stay dark (dead pixels) or overreact (hot pixels). Most DSLRs have very few dead or hot pixels. Since they are isolated, however, when they occur they are easily detected and replaced by a more adequate value. This can be done directly on the sensor.
Further readings for the Can RAW images be already somewhat cooked?
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.
|
Launched in September 2009, the Leica M9 is the first full-frame (24x36mm) rangefinder camera provided by the famous German brand. For its top of the line camera, Leica provides a new 18 Mpix CCD sensor. It is important to note that Leica is the only brand that still makes cameras with CCD sensors; all other main full-frame cameras have CMOS sensors. |
|
The DxOMark Sensor scale is based on RAW data measurements and provides comparable metrics only if no RAW-specific processing has been applied. |
|
The dxomark sensor scale is based on RAW data measurements and provides comparable metrics only if no RAW-specific processing has been applied. |
|
The Sony Alpha 700 with its new version 4.0 firmware scored a 66.3 on the DxOMark Sensor scale, putting it in 17th place on the dxomark.com overall ranking, and in 5th place for APS-C cameras. Given its launch date (Sept. 2007), this is a pretty good result. |