What does "expanded ISO" mean?

Question

The ISO specification for the Canon EOS 7D reads as follows:

High ISO For handheld shooting in low light, the EOS 7D offers ISO speeds of up to 6400. Expandable to ISO 12800, for low light scenes where using flash is undesirable.

Why is it phrased this way? Is there something extra that is needed in order to "expand" ISO to 12800?

If not, then presumably the camera is capable of 12800 out of the box — so why not just list that as the max ISO speed?

Similarly, the Nikon D5100 uses Hi1 and Hi2 instead of numeric ISO settings above 6400. If these are "real" ISO settings, why not just call them ISO 6400 and ISO 12800?

What about cameras which have an expanded ISO range on the low side? For example, an expanded ISO setting may allow a choice of 50 rather than 100. Generally,the standard high ISO is very noisy, with the expanded ISO even more so. Are these lower ISOs less noisy than the "base"?

How do these expanded ISOs affect image quality on either side? Is it better to avoid them and do the equivalent processing with RAW files later, or is there an advantage to using these settings in-camera?

Answer 1

There are two reasons why an ISO is not made part of the 'normal' range:

1. It is considered a non-trivial drop in quality and you do not want users complaining about its performance. In other words, if the quality difference between ISO 12800 and 6400 is stronger than the one from 3200 to 6400. Note that there may be more changes than simply more noise, colors can be affected as well.

2. The camera meters and exposes for the said ISO, say 12800, but the results do not strictly comply with the ISO standard. When that happens, you will notice that the ISO is NOT stored in the EXIF of the image. This usually happens because of a drop in dynamic-range at the expanded setting.

Answer 2

Basically all "expanded" means is that this is not part of the standard recommended range. Often the expanded ISOs are implemented in software rather than hardware (which is bad)

With ISO 50 you might be getting an overexposed ISO 80 (the native, unamplified ISO) so could end up with less dynamic range. This is done by metering for ISO 50 but actually shooting at ISO 80 (since you can't make the sensor be less sensitive than it's unamplified state) and then adjusting the figures afterwards. This will lead to clipping highlights sooner. At least this is the case with some Canon DSLRs.

ISO 50 is only really useful if you absolutely need to use a certain shutter speed and don't have an ND filter. I wouldn't use ISO 50 for image quality reasons.

edit: found the source, from Canon rep Chuck Westfall:

There is about a stop less dynamic range in the highlights at ISO 50, which is the reason why this setting is normally locked out.

Answer 3

I think it's partly to cover their ass (if people complain that their ISO 12800 image is really noisy Canon can say "well it that's not a normal ISO...") but also because such ISOs aren't implemented by analogue amplification like the others but are instead done digitally by doubling the numbers. They are sometimes referred to as "fake ISOs" for this reason.

This method is akin to underexposing by one or two stops and then boosting the brightness in post, which is partly why the noise is so bad as you're doubling/quadruopling the read noise!

I personally prefer not to use these modes as you don't gain anything compared to doing it yourself in post (which is not true of lower ISOs) and you run the risk of overexposing and losing data.

The same is true of the expanded low ISOs (50, 100). The camera simply overexposes and you lose highlight headroom.

Answer 4

I believe that the expanded ISO setting is achieved in a different manner to the standard range.

With the the standard ISO range the sensitivity is changed on the hardware level, providing more amplification to the signal from the sensor.

My understanding is that with the expanded ISO it is handled on the software level. In your case when you set ISO to 12800 the signal from the sensor is amplified in the same manner as for ISO 6400 but the resulting image exposure is tweaked to get the results as if you were shooting at 12800. Basically the result is the same as if you took an underexposed photo at ISO 6400 and then increased the exposure in the post processing. The expanded range does it for you so you can use the camera setting as you were shooting at 12800

Answer 5

You do have explicitly state that you want to use ISO expansion through the custom functions on the menus (the option tends to move or be renumbered between models, and I don't have a 7D to hand to double check at this moment.)

I understand this has to be manually enabled as almost an acknowledgement that the results may not be to the same quality (noise/colour accuracy/etc.) as you'd enjoy under the "normal" range.

Answer 6

You use the custom function menu I (Exposure), item 3 to enable ISO expansion. Then you change ISO setting as usual, and the extra setting 12800 is available.

It's common to include very high ISO settings protected in this manner, so that you don't use them by mistake as the result has very much noise. The camera manufacturer want to list both settings, one to show what you can reasonably use in normal situations, and one to brag with...

I remember that my Canon D60 went up to ISO 800, but with the expanded mode allowed ISO 1000...

Answer 7

It's mostly marketing (at least IMO). On one hand, their advertising can brag about supporting a huge range of ISOs -- but at the time time, if you try to put it to use and don't like the results, they can basically blame you for it.

Seriously, there is a bit more to it than that -- they're trying to satisfy a wide range of photographers. Back in the film days, different photographers used different films. People who shot landscapes bought Velvia and/or Provia by the brick. Wedding photographers bought Vericolor the same way. Newspapers bought Tri-X in hundred foot rolls. They might all shoot identical cameras, but got results tailored to their own needs by picking their film.

Digital largely eliminates that tailoring, but expanded ISO ranges try to give it back to at least some extent. While it's a bit hard to define the exact "look" of reduced ISO settings (and varies with the camera) you can play with it and if you like that look, you can use it. At the same time, if you get blown highlights, well, they honestly did warn you...

Answer 8

With film, ISO is controlled by the size of the grain (there are other factors, but this is sufficient for this conversation). In essence, the lower the ISO the smaller the grain. The smaller the grain, the more detail the film can resolve, and the less you will see random variations between the grain. In short, with film you can get more detail shooting ISO 25 film than you can with ISO 100 film--at least within the same brand.

With digital, your resolution is fixed. The only difference in quality is the amount of digital noise. Digital noise is caused by heat, and there are two ways of increasing the amount of digital noise: amplify the sensor (higher ISO) and increasing exposure time. With the normal exposure range of your camera, you shouldn't experience any issues due to accumulated noise. It's the long time-lapse photography shutter speeds you have to worry about.

Today's digital SLRs have improved their digital noise performance so well that it is near impossible to tell the difference between ISO 100 and ISO 400. In fact it is just about at the place where I would include ISO 800 on some models with that statement. In short, there is no reason to choose lower ISOs for image quality reasons. There are for artistic exposure, but not so much for noise performance issues.