How is ISO determined?

[ChadS]

@BK I defer to Daniel's expertise on Canon's A/D converters (ADCs). He stated that for lower pre-amp gains, Canon's ADCs are terribly noise and non-linear. So by his estimate ISO 100 and 200 are basically the same. In a perfect world we could have ISO 10 for really sunny days at f/1.2. However, there's a minimum amount of amplification necessary to actually get the signal moving - hence no super-low ISO. Some medium format digital sensors do offer sub-100 ISO values. This makes sense as their sensor areas are larger as are their pixels (generally) so there's more photons to work with at the lowest levels of amplification.

So, what do we do to get more signal from fewer photons? That's the $1M question. You've heard of backside illuminated sensors? That's one way. Improving the gain/noise in the pre-amps is another. Reducing the spurious noise sources helps (active cooling above), better energy-level matching for the semiconductor junctions to photon energy... There's a bunch of technologies at improving CCDs, CMOS, etc. Alas, the really advanced stuff is not destined for SLR for many, many years after it's available in scientific-grade sensors for quite a while. True, the market is huge for pro / pro-sumer / enthusiast DSLRs but the margins are thin. The scientific instruments have huge margins but very thin sales. But no self-respecting researcher is going to pay a 10x markup (or more) for a camera that's no better than what he can buy from his local box store. So the sci-grade stuff has to be that much better.

[Raid]

Now I know its been a very long time since I went to school (chalk on slate).

Dynamic Range (DR) is a ratio between the maximum and minimum values.

Everybody is focusing on minimum value around the noise. You can also increase the DR by increasing the maximum, that is being able to store more charge in each pixel.

Sorry if I'm being difficult

Tony

[ChadS]

@Tony that's effectively what you're doing when you change the gain. Let's say the maximum is 1000 units. If I'm receiving 1 unit per 1/1000 and I multiply by 100 or if I'm receiving 10 units per 1/1000 and multiplying by 100 I'm still maxed.

Having no idea what the charge well depth is for any individual sensor we can only make broad generalizations. However, I will say that adding a larger capacitor for each pixel is inviting more dark noise so that's not all positive. Given that, every amplifier is tuned to nearly max the storage ability of whatever the well can hold. So one can't really increase the DR by increasing the max - that's already assumed since amplifying to anything less than the max is sacrificing range. Hence, the interesting bits happen in the noise.

[Raid]

@Chads

http://www.andor.com/learning/digita...ras/?docid=321

This link give Dynamic Range and Full Well Capacity of some commercial CCDs. While the well capacity and noise is provided I'm not sure about the units.

Although I do like all of those dB values, wish Canon would do that, it would make sense to me.

Tony

[Daniel Browning]

Ahhh somewhat clearer, it sounds like bits do not help the absolute range, but would help in the gradations between the absolutes -

Unfortunately, the bits don't help that, either. The gradations are limited by noise (photon shot noise) to less than 8 bits, even in the very lowest-noise circumstance (ISO 100, ample light). The only thing that helps gradations that is increased full well capacity (FWC) when shooting in ample light, and sensitivity when shooting in low light.

the question is what is a praticable limit.

I'm not sure, but the ideal would be 1 bit -- to record the location of every photon strike individually. Then your effective bit depth would vary by output size, from 1 bit up to 16 bits or more.

Back in the day.... the zone system (preferable is formats larger than 35mm) we would have many more zones and EV values within a single negative than what I have been able to produce w/ the electronic sensor - maybe it is just me and I haven't figured it out yet.

It's not just you. Right now the primary limitation is Canon and typical raw converters. With non-Canon cameras and a good raw converter (e.g. RawTherapee), you can get a full 13 stops dynamic range -- 7 above middle gray and 6 below -- just like the best film (and careful chemical development). But most photographers prefer to stay in the 5-7 stop range because it gives the image "punch" and "pop".

Back to ISO capability. The lower amplificaton levels would improve total dynamic range - noise floor stays down? i.e. what happened to the 50 ISO setting?

It sounds like you're asking if dynamic range would improve by removing the amplification altogether. In general, yes. That's what a CCD is -- the FWC there is limited only by the photodiode itself (so the lowest ISO setting is fixed by the the second place where the energy goes, after knocking an electron loose from the depletion region). In a CMOS sensor, on the other hand, the FWC can be limited by the next link in the chain, one that CCD does not have, the source follower transistor. With a low capacitance, CMOS can suffer very poor FWC and maybe low dynamic range (certainly low max SNR). But designers also have the choice of high capacitance, and the FWC is generally as good as CCD.

As for the ISO 50 setting, the fundamental lower limit of the ISO setting is set by the full well capacity and sensitivity. If you increase sensitivity without increasing FWC, your minimum ISO must increase. But you can always put an ND filter on there and get the exact same result as before. But if you increase the effective FWC somehow (such as with an HDR pixel or even just software), then you can get significantly reduced noise or increased dynamic range with lower ISO settings.

The next question is for a given level of amplification and light, what is the signal - charge level - produced i.e. measure of sensitivity and ultimately S/N = max iso?

I'm not sure what you mean here, but one example might be that on some cameras, 10k electrons gives you the level that corresponds to middle gray in the default raw conversion of some converters. To calculate the S/N, you factor in read noise and photon shot noise, and then the max "usable" ISO could be based on whatever minimum SNR you prefer (I'm happy with 0dB, but a lot of photographers prefer 10dB minimum).

Now the question is how to get the sensor to be more sensitive needing less amplification???

There's not really much left to be done. If you don't count color filters, Modern sensors are already capturing 70-80% of the incident light (at the green wavelength at least). Even theoretically, it's only possible to increase to 100% -- since you can't see light that isn't there, of course. The only big jump would be if we found some other way of sensing color (rather than the filters we currently use).

Where there is a *lot* of room for improvement is in the read noise -- the part added by the sensor. If there were no read noise, we could shoot at ISO 1,000,000 and still be able to see detail (it would be very, very noisy, but still plainly visible).

[Daniel Browning]

While the well capacity and noise is provided I'm not sure about the units.

The units are "photo-electrons (electrons knocked loose by an incident photon) or their equivalent (e.g. an electron caused by internal thermal energy instead)".

Although I do like all of those dB values, wish Canon would do that, it would make sense to me.

I do too. It would be nice if they also stopped spouting bald-faced lies (e.g. "our 14-bits give improve gradations!"), but I'm not holding my breath.

[ChadS]

Bleh - we need to get rid of CCD and CMOS and replace all 20 million pixels with photomultiplier tubes! Single photon counting is not a problem!

Ok, seriously though, I was unaware that the quantum efficiency of CCDs was >70%.

There is another way to sense color other than filters and the solar power folks are doing it now. Put your highest bandgap doping at the top of the stack, and reduce the gap for each progressively lower layer. Commercial solar power chips use 3 layers but there's no limit - so long as you're not absorbing the photons in the silicon crystal. This matches the energy of the photon to the absorption band of the semiconductor. For solar cells the result is more power, less heat. For cameras it would be a huge increase in range and also provide a better approximation of color - actually it could provide a low-res spectrum at each pixel.

[Raid]

@Daniel

"Well Capacity e-", this is the first time I have seen such a symbol "e-".

I have been doing a bit of reading through their site and some of the videos are very interesting (OK I'm a nerd I like this stuff). A CCD with self calibrating Photon amplifiers for low light, it even comes with a Canon EF mount. While you need to cool it to -100oC, this is something that Dr C would enjoy doing.

Unfortunately, their biggest sensor is only 1M pixel.

As for dB, how about a bit of excessive government regulation, so that the users have a say in standards not just industry (or am I dreaming).

Tony

[Daniel Browning]

There is another way to sense color other than filters and the solar power folks are doing it now. Put your highest bandgap doping at the top of the stack, and reduce the gap for each progressively lower layer. [...] For cameras it would be a huge increase in range and also provide a better approximation of color - actually it could provide a low-res spectrum at each pixel.

There have been a lot of patents on this type of design, but the only one to make it to market (that I know of) is Foveon, and the chief problem is very poor color accuracy. There is no way to shape the crossover between color channels in the spectral response to more closely match that of the human eye (e.g. red and green wavelengths) -- the same problem that plagues 3-chip video cameras, resulting in significant metameric failure. The other issue is poor SNR in whatever color channel corresponds to the deepest absorption layer (due at least to poor QE there). That said, a lot of people don't mind the grossly inaccurate color of Foveon sensors, so if they could distribute the QE more evenly, it would overcome the terrible low-light reputation Foveon has.

[Busted Knuckles]

That dull bump noise followed by the sound of rushing grey matter was my head exploding.