f-number equivalent between m43 and FF

Started 6 months ago | Discussions thread
dethis2
Forum MemberPosts: 83
Like?
Re: That "explanation" is so wrong...
In reply to xpatUSA, 6 months ago

xpatUSA wrote:

xpatUSA wrote:

OK, I used your method same as before but there was a problem or two. A normal scene shows a change of metering when zooming. So I shot a letter-size piece of white paper instead. Then, because it's a Foveon, each pixel is 3 photo-sites so I exported only the green channel from RawDigger to ImageJ.

Image size: 1134x756px (SD9 LO res RAW)

Histogram area selected: 670x552px

35mm zoom: mean 67.185, s.dev 8.551, min 32, max 104

70mm zoom: mean 77.853, s.dev 16.98, min 32, max 140

Numbers above are 16-bit grayscale levels.

Curves were classically bell-shaped.

Oops, the focal lengths were typed back-asswards, should be:

70mm zoom: mean 67.185, s.dev 8.551, min 32, max 104

35mm zoom: mean 77.853, s.dev 16.98, min 32, max 140

. . . making the wider-angle shot the noisier.

Confirmed by a test I just did at HI res (avoids binning obfuscation):

70mm: mean 49.7, s.dev 11.9

36mm: mean 48.4, s.dev 24.4

These last figures straight from RawDigger, whole image, no exporting involved. Numbers are un-scaled raw data values. Much mo' simple!

-- hide signature --

Cheers,
Ted

I think you need to downsample using a good algorithm (say Lanczos) and use for this linear space instead of gamma 2.2. Gamma 2.2 is responsible for the lower mean value at the "70mm" sample.

http://www.4p8.com/eric.brasseur/gamma.html

http://www.imagemagick.org/Usage/resize/#resize_colorspace

Now that I am thinking of this .. this gamma error is in part responsible for the much less noise we percieve after downsampling as (at the dark parts like in your sample) it dampens the pixel differences much more than a correct workflow should ..

-- hide signature --

Ilias

Reply   Reply with quote   Complain
Post (hide subjects)Posted by
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark post MMy threads
Color scheme? Blue / Yellow