Plenty of digital cameras, like the just-reviewed Nikon D600, feature Expanded ISO sensitivities. Briefly, ISO sensitivities are simulations of film sensitivities for digital sensors. Expanded ISO settings are just more of those sensitivities which are not considered part of the normal range. In the case of the Nikon D600, the normal range is 100 – 6400 and the expanded range is 50 – 25600. So sensitivities below 100 and above 6400 are its Expanded ISO sensitivities.
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 the manufacturer does not want users complaining about its performance. For example, if the quality difference between say ISO 12800 and 6400 is much greater than the one between 3200 and 6400. Noise is obviously the most known aspect of image quality which goes down at higher ISOs but dynamic-range and colors are often 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, the ISO is rarely stored in the EXIF of the image. This usually happens because of a drop in dynamic-range at the expanded setting which causes the grey-point or mid-tone to shift.
Manufacturers deal with this differently. Canon and Pentax lock out expanded sensitivities by default and users must enable them in a configuration menu somewhere. Nikon and Canon does not use numeric ISO sensitivities to describe them but use labels instead, for example Lo 1.0 for ISO 50 and Hi 2.0 for ISO 25600 on the Nikon D600. Some Olympus cameras simply shows a comment on the LCD when an expanded ISO is selected.
Fixed-lens cameras often lower resolution at Expanded ISO sensitivities. For example, the 16 megapixels Fuji Finepix HS30 EXR drops to 8 MP at ISO 6400 and down to 4 MP at ISO 12800. This is a technique used by many but perfected by Fuji called pixel-binning where multiple pixels are used to simulate a larger one. Fuji’s EXR sensors have a special color-filter which lets it bin pixels of the same color together, making interpolation easier.
To the right is an ISO 3200 crop from the 12 MP X-S1 and below it is a 6 MP from its expanded ISO 6400 mode. The image is smaller in terms of resolution, so details look bigger at 100% which magnifies noise. On a fixed print size though, they pretty much even out which is not always the case.
When to use expanded ISO settings? The first part of the answer is to know your camera. Now what each ISO looks like in terms of image noise and retention of details. The second part is to consider the intended use of the image being taken. The smaller the print, the less visible the noise. Modern cameras can produce small 4×6″ prints at stellar ISOs now. The most important consideration is what else can be done to get the shot?
- Lowering shutter-speed is viable for still subjects but you may need to support your camera on something, or simply use a tripod. For subjects in motion, it depends on the motion but by the time light is that low, you are probably at the minimum already.
- Opening the aperture is doable sometimes at the expense of reduced depth-of-field.
- For still subjects again, one can also use multi-exposure techniques and blend several under-exposed but lower-ISO shots by software.
- Finally, you can consider moving or adding lights or moving the subject closer to a light source. This of course depends on the compostion and subject entirely. Still, this is one of the easiest tricks to get better light. Some willing subjects will even do the moving themselves!