Limits to ISO

I came across an interesting post on the limits to ISO sensitivity in digital sensors (linked from the online photographer). The assumptions are a sensor with 100% quantum efficiency that is purely limited by photon shot noise (not a bad assumption) and a full-frame sensor with 12 MP resolution. Basically to get similar noise characteristics as a D3s at ISO 6400, it would have ISO 96,000. That's an astonishing conclusion and would be a really fun camera to play with. Read the whole post for the fairly straightforward calculations that went into it. Current sensors are nowhere near 100% quantum efficiency however, in large part due to the Bayer array layout which throws away a lot of light as well as the high reflectivity of the sensor surface. Both can in principle be solved and there are already concepts out there that do so.

14-bit capture; does it do anything?

There is lots of confusion on the web about the benefits of 14-bit RAW capture. on the one hand people test it to hardly make any difference shooting test charts, on the other hand it is claimed that far more detail can be extracted from a 14-bit capture. One of the issues is that most of these tests use test charts or highly contrived circumstances, never a real situation. Therefore I decided to give you a real image. This is our garage in a very sunny day (it was Flag day when I shot this as you can see). I used default conversion in Lightroom of this 14-bit shot. In this sort of lighting, in order to not blow out the flag, you end up underexposing the garage. If you're smart in such a situation you might use strobes to light the inside of the garage. Another person would increase the "shadow fill" in Lightroom. Without those tricks, you cannot see the plethora of bikes and the fact that I really need to clean it out, which is perhaps a good thing.



The often repeated "truth" is that supposedly, with 14-bit capture, you get better detail in the shadows as you have the extra bits to play with. To test this, here are two details from the above guide image pushed 4 stops in Lightroom at 100%. I had to set the blacks to zero, otherwise the area stayed simply black. Guess which one is the 14 bits one!




Well? I'll tell you. The first one was shot at 14-bit. Surpsingly for an image pushed a full 4 stops, apart from being slightly brighter (even though the shutter and aperture are the same 1/500 at f5.6 and ISO 100) they are identical. You might even say that the 14-bit one has slightly less detail, but that is likely due to the slightly brighter appearance which makes the contrast and therefore apparent sharpness lower. To check whether this was due to maybe Lightroom not using the full 14-bits, I tried the same thing in Capture NX with the same result. Clearly, in real situations, using a Nikon D300, it is very doubtful that you will see any benefit from shooting at 14 bits. Not having tested other cameras with 14 bit capture such as the Canon 40D, I cannot tell you with certainty that this translates, but I would very highly doubt that you could see a significant benefit there. The reason of course is that the noise in the shadows is already large enough to be in the bottom bits in a 12-bit capture, so the only thing that happens is that you're imaging the noise with higher precision - not very useful and probably a waste of card space.

Actual resolution of Bayer sensors - You get only half of what they tell you

Almost all current DSLR-type cameras are based on a Bayer-filter array sensor. This means that each pixel on a CCD or CMOS chip has their own color filter. The array is usually arranged in a a square fashion where each square of four pixels has 1 pixel with a red filter, one pixel with a blue filter and 2 with green filters (in one of the two diagonals for obvious reasons. There are two green ones, because the human eye is most sensitive to green light and therefore to noise in the green channel. So a 10MP camera has 2.5 Million green pixels, 2.5 million blue ones and 5 million green ones. The job of the RAW processor in the camera or in the computer software if you shoot RAW is to interpolate between the single colors to generate the missing color values at all pixel locations. Most RAW processors do a pretty good job at this, but there is a physical limitation imposed by this. The actual resolution of these sensors will never be as large as what is claimed. If you would remove all the filters, you would get the claimed resolution in black and white. Another alternative is the foveon sensor such as the ones used by sigma, which has three color pixels all at the same site arranged in layers. This leads to the exact same resolution as the number of photosites. Unfortunately, Sigma chooses to simply count the number of photodetectors, which artificially inflates the resolution number by a factor of 3. For example, the SD14 is marketed as a 14.1 MPixel camera. This is extremely misleading as the actual resolution is only 4.6 Megapixels. The camera generates jpegs at 14.1 megapixels, but the pixels in between the actual photosites are simply interpolated and do not add any extra information. Silly marketing! Anyway, cameras with Bayer-array sensors suffer a similar problem as I explained above. The excellent site dpreview.com actually tests the resolution of all the cameras they can get their hands on. They use high resolution primes to make the comparison fair. Of course, comparing between cameras this way is the hallmark of a measurebator as ken rockwell likes to say, so I am not going to compare cameras, just see if we can draw some conclusions about the Bayer technology. Dpreview gives some numbers for the actual resolutions of the cameras they test in lines per picture height (LPH). For example, for the Nikon D200 the horizontal LPH is 2100 and the vertical 1700, since this is a 6x4 sensor, the actual resolution of the sensor is 6/4*2100*1700=5.36 Mpixels. About half the number of photosites on the sensor. Dpreview also gives an extinction resolution where all detail disappears, but moiré artefacts are visible. For the D200, this occurs at 7.4 Mpixels. To see if we can learn some more about these Bayer array sensors, I've taken the MTF data from dpreview.com for a large array of Nikon and Canon DSLR cameras and plotted the actual measured resolution vs the number of photosites on the sensor (on the right), together with the "extinction resolution". The green line would be if the resolution was the same as the camera megapixels. It is clear that a trend is visible in both the real resolution and the extinction resolution. For all array densities, the actual resolution obeys:

actual resolution = 0.58*MP

and the extinction resolution:

ext. res. = 0.8*MP.

So the actual resolution of your camera, if you have a Bayer sensor, is only about half of what is claimed! Extraordinarly good RAW processors might be able to get slightly more out of it, but never more than 0.8*MP. WIth Foveon sensors it is even worse, you only get 1/3 of the resolution of the number the camera manufacturer claims as they quote you the number of photodetectors, while the only thing that matters is the number of pixel locations, which is only 1/3 of the number of photodetectors.