On 24/08/07, Toralf Lund <[EMAIL PROTECTED]> wrote: > Actually, Wikipedia says: > > ISO Standard 12232:2006 > > <http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=37777> > ("Photography — Digital still cameras — Determination of exposure > index, ISO speed ratings, standard output sensitivity, and > recommended exposure index") defines ISO speed in terms of the > amount of light needed to achieve a certain "quality" in the sense > of a per-pixel signal-to-noise ratio. > > but also > > However, this standard ISO speed "rating" for a digital camera is > not necessarily very related to the ISO "setting" or "exposure > index" used on the camera. > > As I suggested earlier, I always thought that what was referred to the > native ISO of the camera was the actual rating according to the ISO > spec, and that selecting another ISO value in reality only meant that > the signal from the sensor was amplified by (setting/native ISO), but > perhaps it's not quite as simple as that... > > The actual ISO standard is not available for free, of course.
The problem is that the strict definition of ISO film speed can't be directly applied to direct digital image capture. They record light levels in a completely different fashion, film being non-linear and digital very linear. The "ISO" bestowed on a digital sensor is an ISO that mimics the sensitivity of a film of the same speed for a middle gray based on the absolute white point (just before saturation) a black point (before noise becomes the larger signal) from what I have read. -- Rob Studdert HURSTVILLE AUSTRALIA Tel +61-2-9554-4110 UTC(GMT) +10 Hours [EMAIL PROTECTED] http://picasaweb.google.com/distudio/PESO http://home.swiftdsl.com.au/~distudio//publications/ Pentax user since 1986, PDMLer since 1998 -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
