# Why not 16 bit files?



## K-amps (Feb 15, 2015)

I do not claim to be as technically savvy , but the question hounds me, why does Canon not start using 16 bit files rather than 14 bit, why this 14 bit brickwall? Color depth, DR can certainly benefit from doing this. I accept that there will be an increase in file size, but many would happily take that if it gave them more latitude or IQ.

Can someone tell me what I am missing?

(Apologies, I came out of the 80's CD redbook thinking, where 16 bits were not enough, then came 18 bit converters, then 20... and finally we were happy at 24bit before DSD ruined the party...... There seems to be a reluctance in upping the Ante in Digital photography).

I believe Canon's banding has to do with the algorithms they use, its not a hardware issue. Granted there is some noise at the hardware level, but the banding is a software issue or so it seems.... an optimized algorithm would produce more uniform/ random grain that is not as ugly as vertical/ horizontally focused bands...


----------



## Marsu42 (Feb 15, 2015)

K-amps said:


> Can someone tell me what I am missing?



Obviously the max. resolution they can get out of their current sensor design is 14bit, so why blow it up to 16 (essentially adding random noise or clipping) and put more burden on the in-camera raw post-processing chain? What you want is 16bit *data*, not just 16bit *files*, right?



K-amps said:


> I believe Canon's banding has to do with the algorithms they use, its not a hardware issue.



Definitely not, it's about they read the analog sensor data - faster and/or dual readout is worse (see 6d vs 5d3 or 7d vs 60d).

If would be just a software issue that can be done in camera, even Canon would have solved it long time ago considering how important the banding issue is for their already limited dynamic range. There have been some ideas for banding removal in post by Magic Lantern, but they don't have the resources to develop the algorithm.


----------



## nvsravank (Feb 15, 2015)

You need more than 14 bits if the data cannot be represented in 14 bits. As far as I understand what is possible from the canon sensors can be represented in 14 bits. Some say even if 14 bits is too much as approximately 2 bits are really noise. 

I think it is related to the dynamic range captured and the gradations that can be captured from the sensor.


----------



## jrista (Feb 15, 2015)

Canon's banding is absolutely due to their hardware. This is a master bias frame from my Canon 5D III:







Note the vertical banding? Here is a better view of it:






This is a superbias frame, where the original master has been run through a special noise reduction algorithm to leave behind just the base bias signal itself. Note...bias SIGNAL. This is IN the sensor, due do the voltage applied to each column of pixels.

Every sensor has this. There are ways of reducing this banding, such as per-column ADC with adaptive adjustment to balance the offsets, producing a flat bias. 

The bias signal itself is easy enough to eliminate...you simply offset. The problem with Canon's bias is that it varies a lot...so you cannot offset the entire thing. Each column can often deviate significantly from others, and when they do deviate significantly, that leads to the fixed banding that you can see in photographs. 

This is just one source of banding, but it is the source of fixed banding. It's not the most pronounced banding, a lot of the horizontal and vertical banding that shows up in Canon cameras is largely random, and has a significantly larger standard deviation. These other sources of banding come from the row and column drivers, from readout electronics, and from signal interference. 

Canon cameras can also still have problems with amplifier glow. For example, a master dark frame from my 5D III:






The amp glow is obvious. The horizontal banding is more obvious in this image as well, as I have subtracted the superbias frame from it, leaving behind only thermal signals and read noise.

As for why not use 16-bit. Canon cameras, due to their noise levels, can't even make full use of 12 bits of data, let alone 14 or 16 (or more). Again, if we use my 5D III as an example, the full well capacity at ISO 100 is (according to sensorgen) 68151e-. The read noise at ISO 100 is 33.6e-. The number of discretely discernible tonal levels in my 5D III is 68151/33.6, which comes out to 2,028.30, or 2028 discrete tonal levels. Noise affects the entire signal...not just the shadows. A 12-bit number can represent everything from 0 through 2^12-1, or 4095. Since 2028 < 4095, Canon is technically wasting bits by using a 14-bit ADC. At best, they are representing the deviations caused by noise more accurately, but they haven't actually increased tonal resolution to any meaningful degree. They are just wasting two bits. If they went to 16-bit ADC, they would be wasting four bits.

The above should also be fairly obvious due to the limited dynamic range of Canon cameras. Canon DSLRs thus far have around 11 stops and change of dynamic range. Dynamic range is limited by the bit depth of the ADC, so if Canon had significantly more than 12 stops (i.e. enough to actually make use of a 16-bit ADC), every single Canon camera would have 11.99 stops of DR. Since most Canon cameras have 10.9-11.4 stops of DR, it is safe to conclude that they are unable to effectively use the full 14 bits of data their RAW format supports. When Canon cameras are all delivering 13.9 stops of DR at ISO 100, then we can start to wonder why they aren't using a 16-bit ADC.


----------



## Mt Spokane Photography (Feb 16, 2015)

Its like putting a 0-200mph speedometer on a car that can only go 90 mph. Its deceptive. Once we get images that actually need a 16 bit output, it will be there.


----------



## Marsu42 (Feb 16, 2015)

jrista said:


> At best, they are representing the deviations caused by noise more accurately, but they haven't actually increased tonal resolution to any meaningful degree. They are just wasting two bits. If they went to 16-bit ADC, they would be wasting four bits.



Interesting - and depressing :-\



jrista said:


> Canon cameras can also still have problems with amplifier glow.



Never heard of that one - and certainly never seen that. It only seems to matter on astro or lens cap shots? Looks a bit like this, probably again for astro shots :->


----------



## K-amps (Feb 16, 2015)

Jon, I was hoping that you would take a stab at this... I am a believer now.

Marsu, as always helpful, thanks everyone.


----------

