Both JPEG and TIFF files from Sigma Photo Pro contain EXIF headers
thus the only conclusion is that they have been output as TIFF,
They say on their home page that they have saved the images from TIFF
in "100% JPEG".
loaded into an old copy of Photoshop or other Photo package and
output as JPEG.
Obviously not Photoshop, but another Photo package, because there
is still the IJG default 2x2 color subsampling in the JPEGs, seen as larger
16x16 Pixel "Block Grid" in Jpegcrop.
The Photoshop JPEG encoder (which is not IJG based) uses a purer 1x1
color sampling in its higher quality levels, corresponding to a smaller 8x8
Block Grid.
So beware that color is cut in these samples by limited JPEG encoding,
but I hope it is hardly noticeable.
With cjpeg from the IJG tools I would use the option "-sample1x1" for
the Sigma/Foveon images just to be sure to retain the full color.
As said, Photoshop from level 8 or so does this automatically, so Phil
is on the safe side when saving his outputs with PS
.
What you lose with the JPEG 2x2 color subsampling is exactly 3/4s of
the color data (half resolution color horizontal and vertical compared
to luminance), or 1/2 of image data! That's quite a lot, although in a
much more reasonable way (just color downscaling) than with Bayer
'distortion'.
Almost all other digicams with direct JPEG output use an asymmetric
2x1 (horizontal only) color subsampling due to common video standards.
This loses half the color information, or 1/3 of image information.
As noted, this was probably no big deal for traditional digicam images,
but for Sigma/Foveon images I would recommend to be on the safe side
and use the 1x1 color sampling option for best quality representation.
Regards
Guido
