Very low ISO's on DSLR's; why not?

[DPD]

I am wondering why camera manufactureres couldn't include ISO settings down to ridiculously low values even if it simply meant a modified sampling of the data being generated by the sensor that has anominal low ISO value of 100 or even 50. Having the ability to set ISO to 10 or even 5 or 1 would be like having a built-in ND filter...would be very handy for getting long exposures without needing to add glass...and for those that don't want any more than one filter attached tot he lens it would give the option of using another filter instead of being locked in to the ND filter. And, with multiple settings, it would be like having a who;e range of ND filters without needing another piece of glass!

Comments?

[Roberto Chaves]

I agree and have had the same thoughts for a couple of years.
It would be great to avoid having ND filters and should be quite easy for implement in the cameras. Maybe even with a firmware update..

[Graeme Nattress]

You can keep pulling the post sensor gain down and down, but at some point, a full pixel will render as grey, rather than white and any lower gain is of no benefit. Most sensors act like this at around the 100 ISO setting I think.

An ND will block light entering the sensor, and hence retain full dynamic range.

Graeme