What is preventing sensor ISOs from going lower than 100? [duplicate]

Question

Possible Duplicate:
Why can't the ISO level on most digital cameras be set below 80

So, take your typical DSLR sensor. It probably tops out at anywhere between 1600 - 6400 ISO.

The reason for this is that (as far as I understand) it gets expensive to make a more sensitive sensor, because all the components need to be more refinded and just generally of a better quality.

The big thing that I don't get though, is why sensors 'bottom out' at around 100 ISO (or 50 ISO, if you're shooting on something particularly expensive like the 1D). Why should it be that sensors are limited to being this sensitive?

Surely it's easy to just set an electronic component to be less sensitive (say, to get 50, 20, whatever ISO)?

I was looking at this Whirlpool Forums thread, and there doesn't seem to be anything on there lower than 50. Then looking at the beautiful film, this blog mentions that there was Kodak film that went down to a phenomenal 6 ISO!

The main reason I'm asking is because if we could set our cameras to a much lower ISO, we could all do daytime long-exposures without ND Filters. Oh, and I'm sure there's some other purposes for it too. Maybe.

Answer 1

This is down to the way digital sensors work. This tutorial is a good introductory article on the way digital sensors work.

For the purposes of illustration, think of the light cavity as a bucket. Let's assume that a well exposed image at the sensor's base ISO of 100 will fully fill the light cavity of a photosite. Therefore, going from ISO 100 to ISO 200 will halve the exposure time and will only half fill the light cavity. Each stop of increment in the ISO speed will result in the corresponding halving of the amount of light filling the cavity. This is why you have a usable ISO range on digital cameras, e.g. the light cavity is full at base ISO and it is "almost empty" or filled to the minimal amount that is usable at maximum ISO.

With most cameras, you get a usable ISO range of about 6-7 stops which corresponds to an ISO range of ISO 100 - ISO 6400. Camera manufacturers are free to shift the sensitivity window of the sensor to any value they like. The reason most cameras choose to have ISO 100 - ISO 6400 is because it's the most useful range for a wide variety of photography as opposed to a camera that went from ISO 6 - ISO 400. It's also doesn't hurt that having a bigger number always impresses!

While you may be able to push the camera below the base ISO, e.g. setting your camera to ISO 100 and overexposing by +2EV. This is equivalent to going to ISO 25 on a sensor with a base ISO of 100. You'll end up with an image that clips the highlights as the light cavities have overflowed and you won't be able to recover them. This is similar to what you get at the opposite end, where if you push a camera beyond it's maximum rated ISO you will end up with clipped shadows. For more information about these techniques, please look for the term "Push Processing".

Answer 2

In rough terms, each photon that impacts a photosite induces one electron of charge to be stored for that sensel. There's a limit on the number of electrons the site can store, which (in turn) acts as a lower limit on the ISO speed rating. That leaves only a few ways to lower the base ISO.

The first and most obvious would be to reduce the number of electrons that can impact the photosite. The primary way to do that would be to insert filtration in front of the sensor. If you used neutral density filtration, this would only accomplish the same thing you already can by putting an ND filter in front of the lens. The difference would be that you couldn't change or remove the filter -- ever. An ND filter in front of the lens is clearly better in almost all respects: it can be removed when light is low, swapped for a stronger filter when light is particularly high, etc.

Another possibility would be to use stronger color filtration. This would at least potentially provide a small advantage in providing a broader gamut. The problem is that current cameras already support a gamut that's as broad as most people care about. To an extent this seems to already happen anyway. Just for example, Sony sensors appear to use slightly stronger color filters than Canon's sensors. This does (at least according to sites like DxoMark that test such things) give a slightly broader gamut. It also, however, gives noisier pictures at higher ISOs. Based on sales of (for example) the Canon 5DII versus the Sony Alpha 850/900, it would appear that a lot more people find other factors like video and/or lower noise at high ISOs more important than the broader color gamut at low ISOs.

There is yet another possibility (that has also been implemented in real cameras). This is to simply simulate a really low ISO by taking multiple pictures and averaging them together to minimize noise. This has a couple of obvious problems. First of all, it's only good (as low ISOs tend to be in general) for still-life type subjects. Using the ISO 6 you mentioned as an example, consider that the "sunny f/16" rule says the normal daylight exposure should be 1/6^th of a second at f/16, or (for example) 1/250^th at f/2.8. with a "pro" level zoom, you'd still be shooting wide-open to just edge into the lower end of the range where you can hope to freeze much movement.

I think from most people's viewpoint, there's a simple lack of need: below ISO 200 (or so) most current cameras have so little noise that trying to reduce it further is simply pointless, at least for most people. Quite a few routinely shoot at ISO 400 (or higher), even in broad daylight, just because dropping the ISO below that doesn't gain enough to be worth bothering, at least in most cases.

Answer 3

Nothing. It's just a function of the fact that when presenting a list of settings, there has to be a cut-off somewhere. There are hacks for Olympus cameras that will take the ISO lower.

If this is bothering you, buy a neutral density filter.