| |
In message ,
"Zed C. Pobre" wrote:
>JPS@no.komm wrote:
>>
>> There is no ISO 3200 mode, per se, on the 20D. It is using ISO 1600,
>> but exposing for 3200, and doubling the digitized RAW numbers ... they
>> are all even except for half of the bad pixels, which are interpolated
>> to odd numbers. They are aboout 99.9% even.
>
> This is really, really interesting. I'll have to play with this a
>little. ISO3200 is visibly noisier than ISO3200 level-adjusted from
>RAW, though, and I don't know why that would be if it's basically the
>same mode. If anything, the interpolation should smooth things, no?
I'm not sure what you're talking about here, or if you understood what I
wrote. Let me restate it ...
When the 20D is set to "ISO 3200" the metering system works as expected;
it meters for ISO 3200. When the data is read off of the sensor,
however, the same gain is applied as is applied for ISO 1600, and then
the numbers are doubled after digitization. 0 stays 0, 1 becomes 2, 2
becomes 4, 3 becomes 6, ... 2047 becomes 4094, and anything higher than
2048 becomes 4095. Also, any mapped out pixels, interpolated from
nearby pixels, have a 50% chance of being odd, as they are interpolated
after the doubling.
>> ISO 3200 on the 20D is really a JPEG mode, as shooting RAW at 1600 and
>> -1 EC captures everything RAW 3200 does, plus it gives you one more stop
>> of highlights.
>You mean a +1 EC?
I mean -1 EC, and ISO 1600, in the camera. The midtones and shadows
need +1 EC in the RAW converter, or post-processing The highlights can
be clipped or squished at will.
>Or you mean playing with the EC in
>post-production by one stop? If you mean going for +1 EC on the
>camera, the extra light has to come from somewhere, either from
>aperture or shutter speed, and you wouldn't be playing with even
>ISO1600 unless those options were already blocked off.
No, that wasn't what I was talking about. Setting the 20D to ISO 1600
and +1 EC is basically "shooting at" ISO 800, with the top of the
highlights clipped one stop lower than if the camera were set to ISO 800
and ) EC, but with one bit more depth in the range captured; IOW, it's
like having ISO 800, but instead of 0 to 4095, having 0, 0.5, 1, 1.5,
... 2047, 2047.5.
>In that case,
>your option is between shooting ISO1600 one or more shots underexposed
>(thus wasting the extra RAW data space in the highest stop), or
>forcing interpolation in the camera to move data out of the lower
>stops.
???
>> odd numbers, but the bottom line is that any position in the RAW bitmap
>> can only be one of 2048 odd or even numbers.
> This is true, but when half of those numbers are in the top stop
>alone, you want to force your data over there if possible.
That's called shooting at ISO 1600 and 0 EC. You shoot at ISO 3200, or
1600 with -1 EC, because of lower light (or the need to stop action, or
increase DOF).
--
<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
|
|