More specificly : why does the camera shoot at ISO200 (instead of ISO100) if the goal is to SAVE the highlights ?I know a lot of things in photography, but on this one I confess, I don't understand the process.Could it be that the camera displays ISO200 as a minimum, yet shoots at ISO100, getting an underexposed image (with lots of retained highlight detail) then pushes the shadows and pretends it's an ISO200 picture ?

RalphDurstwrote:Could it be that the camera displays ISO200 as a minimum, yet shoots at ISO100, getting an underexposed image (with lots of retained highlight detail) then pushes the shadows and pretends it's an ISO200 picture ?That is exactly what is being done. It has been proven that an ISO 2oo HTP RAW file is identical to an underrexposed ISO 100 non-HTP RAW file. The only difference is that HTP will include a tag that alerts the RAW converter to use a different tone curve.

RalphDurstwrote:More specificly : why does the camera shoot at ISO200 (instead of ISO100) if the goal is to SAVE the highlights ? I know a lot of things in photography, but on this one I confess, I don't understand the process.Could it be that the camera displays ISO200 as a minimum, yet shoots at ISO100, getting an underexposed image (with lots of retained highlight detail) then pushes the shadows and pretends it's an ISO200 picture ?ISO 200 with HTP is simply ISO 100 RAW, under-exposed by a stop (exposed for ISO 200), and then when the JPEG is made in the camera or the RAW is rendered in DPP, it is pushed back up by a stop, and extra highlights are squeezed into a compression so that they don't clip. There is no ISO 100withHTP because there is no room in the sensor to do it. ISO 100 uses almost the full DR of the sensor, so there is no lower ISO to get more headroom from. 50 is there, but it offers nothing extra, because it already has a stop less headroom than 100.BTW, it's all pretend. ISO 100 could really be ISO 400 with two more stops of headroom (5.5 vs 3.5), or ISO 10 with no headroom above middle gray . There is no natural sensitivity, per se. It's simply a trade-off of headroom vs footroom, in determining the "ISO" of any analog gain and digitization of the sensor.At this point in marketing and technology, headroom is conservative so that cameras can sport low base ISOs, and because cameras produce significant amounts of read noise, so low exposures spoil the shadows to some degree. The D3x and A900 are steps ahead in this regard, but hopefully future cameras will have so little read noise at base ISO that exposing base ISO for ISO 800 or even more is practical, to keep specular highlights. Of course, other technological improvements could extend highlights without reducing exposure in a single shot.

Karma Travelerwrote:That is exactly what is being done. It has been proven that an ISO 2oo HTP RAW file is identical to an underrexposed ISO 100 non-HTP RAW file. The only difference is that HTP will include a tag that alerts the RAW converter to use a different tone curve.It is also interesting to note that ISO 50 is just the opposite; it has the same RAW data as ISO 100, and is just 100 exposed for 50.