High DR is just the trick

Started Nov 8, 2010 | Discussions thread
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Oleg_V
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High DR is just the trick
Nov 8, 2010

Sorry for not sharing all that hype about fascinating DR results of the K-5 but its nothing more than simple mathematical trick. It was first published by me at the Russian pentax forum almost a year ago http://www.penta-club.ru/forum/index.php?showtopic=69565 . I'm wandering if sony engineers are reading this forum or not since the trick is now implemented in the nikon d5000, pentax k-5 and probably nikon d7000.

The DR definition.

See http://dxomark.com/index.php/en/Learn-more/Understanding-DxOMark-Database/Measurements/Noise

DXO measures the saturation light level L0 and the light level L1 for which S/N=1. Their ratio L1/L0 corrected for the pixel count is DR rating by dxo version. So, whats wrong with them? Mixing the light levels with plain sensor output is prone to tricks. We just consider the simplest of them.

The trick.

Lets add some constant offset S0 to the output signal S = S + S0. In case S0 > = N the DR measured by dxo procedure just goes to infinity. Varying offset one can obtain just arbitrary results. I guess sony is using some adaptive algorithm to obtain close to linear DR curve. The offset seems to be hard limited since it obviously breaks colors so that it becomes smaller than dispersion and so insignificant at high ISO. As a result the DR curve is changing from classical saturated at low ISO form to almost linear with slight kick up at base ISO.

The proof.

The proof comes from Nikon forum recently http://forums.dpreview.com/forums/readflat.asp?forum=1034&thread=36853749 . The d5000 owner demonstrated carefully shot sequence of heavily underexposed shots that clearly shows adding constant offset to the dark level which exhibit itself by gradually reducing image contrast without dropping dark areas to the real dark (0,0,0). The following picture comparing 13 and 14 EV corrected shots just speak for itself.

Why its bad.

The trick just disturbs colors without adding anything useful to the image. Moreover it gives some marketing advantage to those who sacrifice image quality for stupid tests with inadequate method.

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