Raw is not raw, and ISO is not ISO Locked

Started Mar 30, 2015 | Discussions thread
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Jack Hogan Veteran Member • Posts: 7,768
Off vs On-chip ADCs and noise

rwbaron wrote:

Referring to "sensor" is misleading as the problem that Canon cameras suffer from is their off-chip ADC, not a deficiency in their sensors.

Canon is currently the only company that uses an off-chip ADC (analog to digital converter) and it's implementation is responsible for high amounts of read noise at low ISO. As the ISO climbs...

... gain is increased so that the initially slightly lower noise out of the sensor is amplified to the point that it overshadows the initially higher unchanging noise it finds at the ADC - from then on what you see is effectively the amplified sensor noise only (the sensor becomes 'ISOless'). For Canon DSLRs this point has been around ISO3200 in the past.

On the other hand with on-board ADCs you have slightly higher initial sensor noise but much lower unchanging ADC noise. As the ISO climbs sensor noise very quickly catches up and surpasses static ADC noise, so you typically get faster to the point where all you see is effectively the amplified (slightly higher) sensor noise. For Sony Exmor sensors in DSLRs until last year this has been around ISO 1000. *

Canon's "sensors" are actually quite good and that's why they are very competitive in both noise and dynamic range at the higher ISO's. If Canon had a contemporary on-chip ADC implementation (as all other sensor manufacturers currently do) Canon likely would have the best DXOMark scores and offer the best DR at the lowest ISO's.

Right. Except that there is a bit of a noise compromise to be had when bringing ADCs on board the sensor. We can't have our cake and eat it too

It's a mystery why Canon has not implemented on-chip ADC as they have done the R&D and have had patents for years. Some speculate that the expense for Canon to revamp their sensor production lines for finer geometry to accommodate on-chip ADC at high pixel density is prohibitive for the relatively small number of sensors they produce as they're used only in Canon's own cameras. In the grand scheme Canon is a relatively small player in the sensor industry.

I've heard similar.

Jack

*Simplified for clarity. Current sensors (A7s, D810 for instance) are more complicated than this.

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