What does "expanded ISO" mean?
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
Hi2instead 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?
@DragonLord - It is highly doubtful that you find a reference that tells you what the expanded ISO is called on every camera, maybe another camera reviewer but it would not be any more authoritative. Since I try several dozen cameras per year, I have have access to all DSLRs and pretty good coverage of non-DSLRs (except for Ricoh and Samsung which I have no relationship with).
There are two reasons why an ISO is not made part of the 'normal' range:
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.
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.
**@Itai** I wasn't aware the standards said anything about dynamic range. The latest version of he standard lets camera manufacturers supply their own definition of what is a well exposed image, which is why ISO sensitivities not only differ between camera manufacturers, but also between models from the same manufacturer! It seems a bit pointless having a standard at all...
@Matt - Yes, the clause specifying what is a proper exposure is exactly for this, allowing a metered 18% brightness value to no longer be reproduced at 18% brightness which indicates a change in dynamic-range, although the standard does not explicitly say so.
I think the reason the ISO is not stored in the EXIF is because the "expanded" ISO 12800 is not implemented as an analogue amplification but is really ISO6400 underexposed by a stop with the raw data "pushed" to maintain the correct exposure.
@Matt got a reference for that? I agree that image quality does significantly suffer. I think these arguments very often undervalue the loss of color detail, which can be quite large in these cases.
@Paul I can't find the original source, but basically if you look at the raw values at the highest ISO, they are all even numbers, a sure sign that the digital figures have simply been doubled. There's a discussion on this topic here: http://photography-on-the.net/forum/showthread.php?t=282393