Why is low base ISO so desirable in stills cameras, when the opposite seems to be true for digital motion picture cameras, for instance Arri Alexa, Sony F65 and F55, Canon C300, C500?
As an amateur stills photographer, I hate using anything below ISO 400/800, but would rather use even higher if the IQ were better. For low-light or quick response street shooting what would you suggest?
Desirable Depends
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:
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.
TL;DR;
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.
The idea that a lower base ISO is is automatically desirable stems from a mistaken understanding of the relationship between ISO and noise, and harks back to the days of film. Amongst modern emulsions those with lower ISO rating (i.e. they react more slowly to light) had a smaller grain size this in turn yielded less grainy images, making low ISO film very desirable for some purposes.
However with digital sensors it turns out noise is not directly related to ISO. It is in fact possible to increase ISO and decrease noise!
ISO is simply a measure of sensitivity. It tells you how quickly the sensor saturates (gets to the point where it can't create any more charge) i.e. how quickly the photo will become overexposed. That's all. It doesn't necessarily determine the amount of noise in an image. Now let's imagine two cameras, A has a base ISO of 800 and B has a base ISO of 200. According the formula for exposure this means if you shoot camera A at f/8 for 1 second to get a well exposed image, then you must shoot camera B at f/8 for 4 seconds to get the same exposure. Which would be better for shooting a static subject using a tripod (or when there is plenty of light)? Most people would say B, because ISO 800 has to be noisier than ISO 200 right?
Wrong.
Now I will reveal that camera B has exactly the same sensor as A but with really poor microlenses which waste 75% of the incoming light. Because of this you need four times the shutter speed to account for this loss and reach the same exposure. This means that according to the definition of ISO that the base ISO drops to 200, as it takes longer to saturate to due light loss. It also means that in a 4 second exposure the photosites in camera B receive exactly the same amount of light and thus the images produced will contain exactly the same level of noise.
This is why I say there is no hard relationship between ISO and noise. The amount of noise depends mainly on the strength of the signal, i.e. the amount of incoming light.
Now there are ways to reduce base ISO and give less noisy images, that is by raising the saturation point (known as well depth), i.e. allowing the sensor to collect more light before saturating. Slower saturation = lower ISO, but more light = more signal = better signal to noise ratio = less noise. Likwewise there are ways to increase base ISO whilst having no effect on noise, i.e. better microlenses, or more efficient pixels. These will saturate faster, so ISO is higher, but still collect the same amount of light at saturation, so same noise.
Note that any steps you take to increase the efficiency of the sensor, will increase the base ISO as the more efficient sensor will saturate faster! Thus the digital motion picture cameras you mention simply have sensors which saturate faster that the corresponding film cameras, they are not intrinsically nosier (though they might be as there are other factors involved).
First I would not compare digital motion picture cameras to film photography, apples to oranges.
Second it all depends on how you want your picture to look like; dark, a lot of grain. Wether you are using color or black/white film. Also your lighting conditions, do you have a tripod, what lens you will be using.
Generally I use 100 ISO black/white film, it's how I like my pictures. For indoor I use 400. Rarely do I use 800 iso, but there are times that I push 800 to 1600 to get insteresting pictures.
I suggest reading up on ISO, aperture, focal length, pretty much the basics. As it stands your question is too vague to give a specific answer.
Ideally, you want the ISO that is 'native' to the sensor. Anything 'non-native' whether higher or lower will introduce noise.
For Nikon and Canon cameras, the native ISO is typically ISO 100 or ISO 200. You will find many sources of comparison images showing situations where ISO 50, or ISO 125 on these cameras typically produces more noisy shots than those done at ISO 100. This is somewhat analogous to film cameras, where you usually set the camera on the ISO indicated for the film in use. This ensures the best quality and lowest noise. But you can push the film, by setting the camera on a higher ISO than the film is rated, but as you may know, this introduces much noise.
Therefore, to have the lowest possible noise, its best to start at the native ISO, and then adjust shutter speed and aperture to acheive the desired effect. If you are constrained, then move the ISO up (usually by whole stops) until you are no longer constrained.
Many of the newer cameras feature sensors that are feature very low noise, such that these issues are getting less impactful.
Lets talk digital cameras first:
This is quite complex question, but if you plan to use the sensor's full resolution, the lower the better, as then each individual photo sensor will have a change of being hit by enough photons to cancel out random effects (aka noise). Bigger photo sensors are also, generally speaking, better than small in this respect, so a 12MP phone camera sensor (4x5mm) isn't as good, noise-wise, as a 12MP FF camera.
Same is true for film cameras, of course, a Minox spy camera will never take as good shots as Hasselblad, or a Rollie, no matter how advanced lenses you use.
At the sensors base ISO they have the best dynamic range, so if you want to take good shots in low light lower the ISO, and use support - true for digital, true for film (then you have to change film quality, and/or use a different developer, but generally, higher ISO gives lower dynamic range.
If you down-sample you shots, like a pro video camera can do, a lot of the noise is canceled out. Also when a lot of pictures are displayed very quickly, as you do when watching a movie, the individual shots noise is much less visible, as they pass before your eyes too quickly! So, again, noise is not as problematic as when pixel-peeping a still digital shot. Most movie cameras do not have very fast lenses, but again, using a higher ISO is not quite the same problem as when using a camera to take photos.
Not that long ago ISO 200 was the upper limit for useable ISO, before noise became too much of a problem. Today most cameras can handle ISO 3200 very well.
If the subject you're photographing has low contrast, by all means bump up the ISO (high ISO means the camera's sensor will have less dynamic range). At the shipyard I was working in my younger days the photographer only took to the air when it was a cloudy, heavily overcast day, as the contrast was then the least, thus ideal for aerial photography of the big dark hulks being built.
My D3200 with its magnificent 24MP sensor isn't very good as it is (the photo sensors are pretty small in that camera), but using it in 12MP mode will give you outstanding shots! In video mode it just uses slightly over 2MP, thus does an amazing amount of down-sizing to record video.
My D600, with its twice as big 24Mp full-format sensor, outshines it every day, due to its physically bigger photo sensors. A Hasselblad, with an even bigger sensor goes one step further, again!
The sensitivity of a sensor is fixed, just as the sensitivity of each type of film was fixed when film ruled.
ISO settings do not change the sensitivity of the sensor.
I will say that again, because the myth is that fiddling with the ISO somehow makes the sensor more or less light-sensitive. It does not.
ISO settings do not change the sensitivity of the sensor.
Setting a higher ISO merely allows a shorter exposure time (or smaller aperture) hence less light, and hence a decrease in the S/N ratio. On readout the levels that are read from the sensors are amplified. This boost amplifies both noise and image data resulting in more visible noise at higher ISO settings.
It is somewhat analogous to underexposing film, then increasing the development time, or using a different developer.
Of course if there is very little noise to start with then there is not much after amplifying it, and all kinds of cleverness are deployed in hardware and software to identify noise and mimimise it.
