D90 vs. Canon 500D Which one?

Started Mar 27, 2009 | Discussions thread
rhlpetrus Forum Pro • Posts: 25,859
Re: It's not about dynamic range

Dan wrote:

rhlpetrus wrote:

The issue between 12 x 14 bits is related to RAW DR and ADC path.

You're not saying that 14 bit has greater dynamic range are you?

DR is a sensor property at first, related to signal to noise ratio (SNR). You likely know about that, that's why you asked.

12 bits or 14 bits is the choice for digitization at the analog to digital converter (ADC) stage an later path (people think the sensor is a digital instrument for gathering light, no, it's analogic, current proportional to light gathered).

But the choice is related in the sense that SNR is given in DBs, each 6 DBs means twice the luminosity. Most sensors are engneered between 72 and 84 DBs of SNR, which correspond to about 12 to 14 stops of ligyht from noise floor to saturation.

Since first few bits have about no info, 12 bits is mostly enough to spread the signal evenly, but 13 or 14 may bring some extra info. There's still the issue of the post ADC conversion curve, which will say if midrange is more or less spreaded (with less or more tonal contrast).

For example, for the D90, the jpeg output shows about 3-4 stops from midtone to HL clipping and about 5 stops from midtone to almost black (check the tonal curves tested here at DPR).

So, answering your question: if sensor is not able to provide SNR higher than 72DB, it doesn't help to use more than 12 bits, one is just splitting signal into more steps, without any extra tonal info. But if sensor is campable of higher SNR, then it's true that the use of more bits in conversion will help to preserve that property.

I hoope this clarifies my short assertion.

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Renato.
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