Of course, my first question is, why would you want to?
In the film world, there's 2 main reasons to go slower film.
- Longer exposures. You can get longer exposures with slower film, without using ND filters or stopping down too far (if you can). Good for blurring waves into mush and star trails.
- Higher quality images, smaller grain size means less grainy, better "resolution" (and I'm not so sure about Dynamic range and tonal-response). Either way, the lower ISO/ASA you go, the better your prints.
Now, in the digital world, you have the same arguements, but it's a bit different:
- Longer exposures, yes. But then noise comes into play, unless you get some fancy fan-cooled Medium format back, the longer you're reading the sensor, the hotter it gets, and the noisier it gets (I'm not sure, but it could lead to more or the same noise as a shorter/higher-iso exposure. Depends how long you're talking). There's a school of thought concerning star-trails that it's better to take lots of 30s exposures and merge them all digitally (whether it's better or not, people do it). Of course, with digital you have to worry (more) about diffraction, so you can't stop down too far. But there's always ND filters anyway.
- Higher quality images? Depends what camera you're using. Take a look at this graph
. Assuming the link works, it's a graph of the ISO response of the 5D2. Note how ISO50 is practically the same as ISO100?
Another graph here
, is the Dynamic Range of the 7D. It doesn't get any better going from ISO200 to ISO100, going to 50 or 25 isn't going to do anything to make your images better.
OK, so I chose those 2 examples on purpose, not all digital cameras do that. But it's kind of indicative, you don't gain much in Image Quality by going to a slower ISO, not like you do/did with film.
Even the Phase One IQ 180
(the best sensor at DxOMark thus far) doesn't do any better at lower-ISO-response, but its Dynamic Range
does get better all the way down to ISO25. But then, the price tag is the same as a very nice sports-car ...