On Fri, Mar 27, 2009 at 05:58:13PM -0500, Nick Wright wrote:
> Would someone care to explain a little further about the 800 iso thing?
If you do some research on the dpreview pentax slr forum, Marc
Sabatelle pointed out that someone had written a raw converter (I
foreget his name, it's in my archives someplace) and he had discovered
that above ISO 800 the circuitry doesn't change, the raw values are
just multipled by 2 or 4.
ISO real binary hex dec raw file values
200 000000010110 0x016 22 000000010110 0x016 22
400 000000101100 0x02C 44 000000101100 0x02C 44
800 000001011001 0x059 89 000001011001 0x059 89
1600 000010110011 0x0B3 179 000010110010 0x0B2 178
3200 000101100111 0x167 359 000101100100 0x164 354
You'll see that at ISO 1600 and 3200 that data in the last bit, or two
bits is just lost. Mind you, there's a lot of noise in the analog
signal anyways, so the actual information you're losing at the bottom
end isn't that much.
The problem is if you have a pixel that is close to full scale at 800:
800 100000000000 0x800
at 1600 and 3200 it just goes to
111111111111 and clips
So when you increase the ISO above 800, not only do you not get any
more information from the lowest bits on the darkest pixels, but you
clip the information on the brightest pixels.
But, don't take my word as gospel, try shooting in some very low light
situations, with the camera in manual exposure mode. Assume that at
3200, the correct exposure is 1/10 second f/4.0. Shoot at f/4.0 at
ISO 800 1/2.5 1/5 1/10
ISO 1600 1/5 1/10
ISO 3200 1/10
And compare the quality of the shots. I've found that I get as good
of a shot at 1/10 f/4 in ISO 800 as I do ISO 1600 and ISO 3200.
Given the choice, it's better to expose properly, but if you're
shooting dancers or musicians, people who are moving, you may get
better results trading noise for shutter speed.
--
The fastest way to get your question answered on the net is to post
the wrong answer.
Larry Colen [email protected] http://www.red4est.com/lrc
--
PDML Pentax-Discuss Mail List
[email protected]
http://pdml.net/mailman/listinfo/pdml_pdml.net
to UNSUBSCRIBE from the PDML, please visit the link directly above and follow
the directions.
