Is there a technical reason for low ISO settings to disappear?

Question

Compact superzoom cameras, for obvious optical reasons, have stuck remarkably constantly at the 1/2.3" sensor size. CCD has been superceded by BSI CMOS but light gathering did not change all that much. Pixel counts have gone from 5MP to about 16MP, a factor of about 3 in area. A 2005 vintage DSC-H1 offered an ISO64 setting for best quality. Assuming a similar light yield of the sensor, that would imply that similar quality for a modern camera would require using an ISO25 setting. Yet the lowest that is available on, say, a Panasonic FZ200 (admittedly only 12MP), is ISO100. Reviews point out that you basically have to choose between visible noise, noise reduction artifacts, or both.

Now except for indoor shooting without flash or bad weather conditions, light tends to be reasonably available (and actually, a good external flash can do a lot about near objects in bad weather, and a tripod can do a lot about faraway objects as long as they are basically stationary). But no low ISO settings to use the camera in a manner minimizing noise.

Is there a limit to the charge the detectors can hold before bleeding to adjacent pixels or stopping to absorb photons? Or can the A/D converters not deal with it for some reason?

Or is the removal of low ISO options just a victory of marketing over physics? Because it looks good to offer high ISO settings and bad to offer low ones?

Answer 1

If the sensor is "natively" outputting a signal corresponding to a sensitivity of, say, ISO 200, lowering that sensitivity will be by attenuating that signal, while raising it will be by amplifying it. There seems to be no way with common CCD/CMOS sensors to actually alter sensitivity eg by changing a biasing arrangement - not sure if you can do that with ANY solid state sensor, or even a camera tube, unless it is something photomultiplier like.

Amplification never improves signal quality, with the possible exception of an amplifier very near the source being used to better drive a lossy/mismatched/interference prone cable (already done with modern sensors).

Attenuation also never improves signal quality, unless it is used to avoid overdriving a receiver that would distort or fail if it was to handle a signal that large. If that is not the case, you will essentially multiply noise and low-level nonlinearities of your receiver by your attenuation factor.

From a marketing perspective, offering attenuation-based low iso options might risk people drawing false conclusions about the quality of the higher options ("if iso 25 is already that noisy, iso 100 must be horrible..."). Also, it might invite carelessness - people might get ideas about taking long exposure shots of extremely bright scenes, causing worse performance or even sensor damage because of thermal consequences of doing so.

A limit to "photon capacity" actually does exist in sensors, usually referred to by the term "full well capacity". There has to be - digital image sensors are expected to yield a result based on cumulative exposure, so there always has to be a capacitive element. With a limited operating voltage, a capacitor eventually gets full.

Also, "bleeding over" indeed happens, some CCD sensors are notorious for it.

Answer 2

Yes, there is the trend and, yes, it's because of the increasing light sensitivity.

(and actually, it would be even more apparent if not for the decreasing pixel sizes. Smaller pixels partially offset the effect and save the lower ISO!).

Why?

In the current technology, the sensor works with only one single light sensitivity (the efficiency of converting the light into electrons). ISO is applied in the next stage - while measuring the electrons and producing digital data (amplifier/ADC stage).

In the ideal world the signal could be freely amplified or attenuated producing any desired ISO but in reality there are limitations: the pixel can only hold a given number of electrons before saturating, everything above the number is lost. Lowering the ISO too much clips the highlights in an unacceptable way.

High end cameras are less affected because of their higher pixel well capacity (e.g. 100000, while the cheap point-and-shoot might only have 10000), but even then the lowest ISO setting is usually declared as "extended" and its dynamic range is lower than normal (e.g. ISO 100 performing worse than ISO 200).

A good compromise is when both parties are equally dissatisfied ;-)

So now we have a choice: either make the sensor natively low sensitivity and have nice low ISO or optimize for higher sensitivity and low light. The decision is obvious - everyone wants to improve low light and that's what manufacturers do (and it's not that bad choice considering you can always add ND filter but there are no filters for increasing light in low sensitivity cameras...).

Have we lost the low ISO forever?

Not necessarily, because the technology might change. For example the organic sensors are said to have adjustable native sensitivity (light vs electrons efficiency). This should enable low ISO modes without sacrificing the high sensitivity.

Answer 3

They’ve not disappeared.

Professional, high-end cameras like the canon 5Ds do have low ISO settings (5Ds has an ISO50 setting). A quick google suggest they’re much more common as you move to medium format cameras.

Cheaper cameras aren’t marketed at people who care about or would notice the difference. Generalising, they use them with slow kit lenses which need more light anyway, and display the photos as a 6x4 or on an HD screen, far below the optical quality at which low ISO would make a difference.

However, they’re rarely needed Most professional cameras these days have adequate quality and low noise at ISO100 for the sensor resolution (or certainly for the size images are normally printed even in the professional world). With a 5Ds’s 50MP, at 400ISO or below I can crop into an image significantly, and still give a client a wedding photo with significantly less noise than they’d care about. For commercial use where you really need larger images, we don’t crop.

The purpose of low ISO is more around not needing a ND filter to shoot in high light. An ND filter (or lower ISO) allows longer exposures at the same aperture (aperture controls DoF, so you shouldn’t stop down to get less light). This is useful in three main cases: 1) if you want a slower exposure to allow motion blur (e.g. of a waterfall or lake) 2) if you’re shooting in bright light and you’re up against your highest exposure speed (try shooting at f1.4 in bright sunlight - even at 100ISO your exposure time may be below your camera’s minimum) 3) if you’re shooting video, to observe the 180degree rule of exposure times.