The most desired ISO setting is really contingent on the type of photography you do, and what ISO setting you need to support your photography. A low "base" ISO of 100 is more a matter of conforming to international standards than anything else. The base ISO of a camera is not really the critical factor in the quality of native ISO settings in general, though.
First, to group photographers into some really rough buckets, you generally have the following:
- Landscape and Still Scene photography
- Lower ISOs most preferable...usually ISO 100 in almost all cases, artificially lower ISOs used when long exposures are needed without filtration and DOF needs to be controlled
- Macro photography
- Lower ISOs tend to be preferable...middle ISO settings useful with small apertures
- Moderate action photography
- Middle to higher ISOs tend to be used exclusively (except in exceptional light)
- High action photography
- Higher to Very High ISOs tend to be used almost exclusively (except in exceptional light)
- Low-light photography
- Higher ISOs tend to be used most of the time, although it is often a game of trade-offs...available light vs. grain/noise
- Wedding & Portrait photography
- Low to High ISOs? Really depends on the moment, the lighting, and the acceptable levels of noise
For your type of photography, which really falls into the Moderate action and Low-light buckets, I would say middle to higher ISO settings are ideal. I'd say ISO 800 is probably the sweet spot, but again, it really depends on your style, your goals, and to a degree the capabilities of your gear.
Modern camera equipment, especially the new models from Canon like the 5D III and, if you have the cash, especially the 1D X, can really expand the range of usable ISO settings for your kind of work. With native ISO settings up to 25600 and 51200 respectively, ISO 800-like quality jumps up to the ISO 1600-3200 range at least, and in the case of the 1D X, I've seen shots taken at ISO 16000 (yes, sixteen THOUSAND) that look PHENOMENAL.
So, what ISO is best for low-light street photography has a fuzzy answer at best. Stylistically, if you like grain, you could probably get away with much higher ISO settings, and the noise just becomes an artistic element of your work. If you do not like grain, but have the option of using modern gear, then it is very likely you could get quality photos at one, two, maybe even three stops higher ISO.
The rest of this answer gets into the fundamentals of ISO, sensitivity, and IQ at any ISO setting. If you are interested in knowing how increased IQ at higher ISO settings is possible, keep reading.
What is ISO and sensitivity?
A little bit of low-level 'gritty on ISO and noise characteristics at higher ISO. The quality of an ISO setting is not really dependent upon the "base" ISO. By base ISO, I mean the lowest native setting. These days, pretty much ubiquitously, ISO 100 is the "base" native ISO setting, and conforms to one or more interpretations of the International Standards Organization photographic sensitivity specification for which its namesake is used.
Oddly enough, ISO, which generally refers to the "sensitivity" of a film or sensor, is really nothing of the sort in the case of digital sensors. As ISO is increased, the "sensitivity" of the sensor is also supposedly increased, however what actually happens is the maximum saturation point is simply halved over the previous for each stop of ISO increase above ISO 100. That is kind of a difficult concept to understand without further explanation, so...
Digital Sensors: Sensitivity
A digital sensor is an electronic device that uses certain properties of silicon to detect light. Sensors use a photodiodes to convert light (photons) into charge (free electrons in a circuit). To be more precise, every photon that strikes the photodiode of a sensor pixel will free up an electron in the diode. When an exposure is made and the sensor is read, the freed electrons of each pixel produce a measurable charge, which is converted into a digital number during readout. Each pixels digital number is saved to a RAW image as a pixel value in either red, green, or blue.
Every sensor has certain intrinsic properties for each pixel. These properties determine the pixels rate of conversion of photons into electrons. There are a variety of factors that affect the rate of conversion, but to simplify it all, we just call it quantum efficiency, or Q.E. Most cameras have a Q.E. in the range of 40-60%, which means that for all the photons incident on a pixel, about half of them actually release an electron and increase the pixel's charge. The rest are either reflected, or converted to heat.
Fundamentally, to be accurate, Q.E. is really a measure of a sensor's true sensitivity. At ISO 100, a sensor with 60% Q.E. is more sensitive to light than a sensor with 40% Q.E. The sensor with 60% Q.E. is capable of capturing more light at any given ISO setting than the sensor with 40% Q.E., and as such, requires less signal amplification to produce the same exposure.
Digital Sensors: ISO and Exposure
Another factor in ISO and exposure is full well capacity (FWC), or the amount of charge each pixel in a given sensor can hold as a physical maximum. The amount of charge held, or FWC of a sensor is determined by the area of the pixel. Larger pixels have greater surface area, and are capable of holding a greater charge than smaller pixels. This is the fundamental reason why FF sensors with the same number of pixels tend to have better IQ than APS-C sensors. For any given sensor with the same pixel size, one with a higher Q.E. will produce better results than one with a lower Q.E., despite having the same area per pixel.
So, if Q.E. is really a measure of a sensor's sensitivity, what is ISO? ISO is more of an instruction you give to the camera that tells it how much to amplify the charge accumulated in the sensor when you take an exposure. It changes the maximum saturation, or the charge level that is assumed to be "fully exposed", from the base ISO setting. If we have two sensors with the same pixel size, one with 40% Q.E. (Sensor A) and one with 60% Q.E. (Sensor B). We could assume that, at ISO 100, the former has a FWC of 50,000 electrons, while the latter has a FWC of 75,000 electrons.
If we experiment with two hypothetical cameras with our hypothetical sensors, assuming we expose photos with both cameras at ISO 100, the maximum charge at read time is 50,000 e- for Sensor A, and 75,000e- for Sensor B. Any pixels at read time that have reached FWC are read out with the purest form of that color. By pure, I mean if a green pixel is exposed to FWC, it is 100% green. If a red pixel is exposed to FWC, it is 100% red, etc. Changing ISO simply changes what charge level is converted to the purest form of a pixels color. To simplify, we'll just call that "white", and a pixel that has reached white has reached "maximum saturation".
If we bump up to ISO 200, the maximum saturation (Max Sat) is halved from FWC. For Sensor A, "white" is read when a pixel has a charge of 25,000e- or more, and for Sensor B, when a pixel has a charge of 37,500e- or more. At ISO 400, max sat is halved again, Sensor A to 12,500e-, Sensor B to 18,750e-. And so on, for each successive ISO setting. By ISO 25600, for example, Sensor A's max sat is ~195e-, and Sensor B's max sat is ~293e-.
It is critical to point out that increasing ISO does not actually improve a sensors sensitivity. For any given exposure value (shutter speed and aperture), the same number of photons will reach the sensor regardless. A digital sensor's sensitivity is a fixed attribute set in stone at the time of manufacture. The only way to actually improve sensitivity is to manufacture it with better technology that maximizes Q.E., thus increasing the amount of photons that actually result in an increased signal strength for a given exposure (vs. those that reflect or convert to heat.)
Digital Sensors: Noise and IQ
At lower ISO settings, the differences are not all that relevant, as the full well capacities are huge. However, as ISO is increases, and the charge level for maximum saturation drops, the differences, even small ones, can be very meaningful from the standpoint of noise and IQ. For all the attention the various forms of electronic noise get from photographers, and the amount of effort that has gone into developing debanding algorithms...the vast majority of noise in a photograph, and the primary cause at pretty much all ISO settings for the low midtones and brighter, is photon shot noise.
The random nature of light results in random photon strikes. You cannot really predict where or when any given photon may strike the sensor and increase the charge of a pixel. As such, the fewer photon strikes you have, the more noisy your image will appear. At lower exposures, or higher ISO settings (which, in the absence of read noise, are exactly the same thing), some pixels may receive a lot of photon strikes, while some may receive few or none. This results in an evenly distributed amount of noise in the charges of each pixel in the sensor.
At high charges, the relative differences are small, so the amount of noise is low. At low charges, the relative differences are large, so the amount of noise is high. As ISO is increased beyond a certain limit, and max sat shrinks more and more, smaller and smaller differences in the amount of charge acquired during a given exposure become more and more meaningful. Sensor A has a charge of ~195 at ISO 25600, while Sensor B has a charge of ~293. That is a difference of 50%!!! That difference should mean a similar reduction in the apparent amount of noise in an exposure at ISO 25600. Technically speaking, the absolute difference is 50% at all ISO settings, so all ISO settings should have a similar reduction in noise, however as the relative differences shrink as ISO is reduced, the apparent amount of noise normalizes as you approach ISO 100. You could probably still notice a slight difference, however it would not be as apparent as at higher ISO settings.
Can ISO 800 get better? Can a higher ISO be as good as ISO 800?
So, in response to the following statement:
As an amateur stills photographer, I hate using anything below ISO
400/800, but would rather use even higher if the IQ were better.
YES! You could use a higher ISO setting and get just as good, if not better IQ if you upgrade to a modern camera that supports higher ISO settings. I am not sure what kind of camera you have, but we can use the Canon 60D and 1D X as examples. The 60D has a Q.E. of 40%, while the 1D X has a Q.E. of 47%. The 60D has a FWC of 24322e-, while the 1D X has an FWC of 90367e-. The 1D X has the benefit of both 7% better Q.E., as well as a 61% larger pixel pitch, or 2.6x times the pixel area for each pixel.
The maximum native ISO of the 60D is 6400, and is quite noisy at that setting. The 1D X, thanks to improved real sensitivity (higher Q.E.) and much larger pixel area, has a maximum native ISO of 51200, and at ISO 25600 is about as noisy as the 60D @ 6400. At ISO 6400, the 1D X has 3.3x the maximum saturation as the 60D, and as such should have about half the apparent noise.
If you moved to a 1D X from a 60D, photos at ISO 3200 on the 1D X should look at least as good as photos at ISO 800 on the 60D, and ISO as high as 6400 should be quite usable for street photography. The 5D III, a much more accessible camera than the 1D X, should offer similar results, albeit with one stop less native ISO range. Instead of ISO 3200 as on the 1D X, ISO 1600 on the 5D III should produce similar IQ as ISO 800 on the 60D, and ISO 3200 should be quite usable. If you are upgrading from a camera even older than the 60D, or from a camera that has a much lower Q.E., then the differences could be even more extreme. ISO 3200, 6400, maybe higher on a camera like the 5D III might be just as usable as ISO 800 on your current camera.