Canon's FileViewerUtility exporting 16-bit TIFF from CRW very dark

"Neal Matthis" wrote in message ...
I have a Canon Digital Rebel and use Canon's FileViewerUtility to export the
Canon RAW (CRW) images to 16-bit/channel TIFF. When I open the image in
Photoshop, it is really dark, much darker than it appears in
FileViewerUtility. When I view the histogram in Photoshop, indeed, it is
shifted all the way the the left of the midpoint. Do I need to enable
16-bit mode in Photoshop or something? If I export the CRW file as an
8-bit/channel image, it looks fine in Photoshop. I have Photoshop 6.

Neal

Sounds to me like you are exporting 16-bit Linear TIFF rather than the
16-bit gamma-corrected TIFF you expect. You can perform gamma correction
(2.2) in photoshop (not elements) or see if the TIFF converter has a gamma
versus linear control you can use.

{Both formats have their uses...}

Ah, that did it. I had linear checked. Hmm, so exactly what does that
mean? I did notice that the histogram of the linear 16-bit image seemed to
have all the pixels in teh first half of the histogram, which certainly
explains why it was so dark. Can you explain the difference between linear
and gamma-corrected?

Neal

"Mitch Alsup" wrote in message
m...
"Neal Matthis" wrote in message
...
I have a Canon Digital Rebel and use Canon's FileViewerUtility to export
the
Canon RAW (CRW) images to 16-bit/channel TIFF. When I open the image in
Photoshop, it is really dark, much darker than it appears in
FileViewerUtility. When I view the histogram in Photoshop, indeed, it is
shifted all the way the the left of the midpoint. Do I need to enable
16-bit mode in Photoshop or something? If I export the CRW file as an
8-bit/channel image, it looks fine in Photoshop. I have Photoshop 6.

Neal

Sounds to me like you are exporting 16-bit Linear TIFF rather than the
16-bit gamma-corrected TIFF you expect. You can perform gamma correction
(2.2) in photoshop (not elements) or see if the TIFF converter has a gamma
versus linear control you can use.

{Both formats have their uses...}

In message ,
"Neal Matthis" wrote:

I have a Canon Digital Rebel and use Canon's FileViewerUtility to export the
Canon RAW (CRW) images to 16-bit/channel TIFF. When I open the image in
Photoshop, it is really dark, much darker than it appears in
FileViewerUtility. When I view the histogram in Photoshop, indeed, it is
shifted all the way the the left of the midpoint. Do I need to enable
16-bit mode in Photoshop or something? If I export the CRW file as an
8-bit/channel image, it looks fine in Photoshop. I have Photoshop 6.

Maybe you have it set to give you a linear TIFF when in 16-bit mode?

Linear TIFFs are just the RAW data with the blackpoint bias subtracted,
and scaled to 16-bit values.
--


John P Sheehy

In message ,
"Neal Matthis" wrote:

Ah, that did it. I had linear checked. Hmm, so exactly what does that
mean? I did notice that the histogram of the linear 16-bit image seemed to
have all the pixels in teh first half of the histogram, which certainly
explains why it was so dark.

Gamma-corrected images have luminance levels that represent the base of
an exponent (usually 2.2). This means that mid-scale (128 out of 255)
is not 1/2 as much light, but (128/255)^2.2 = 0.2195, or 21.95% of the
intensity, which is over 2 stops darker than the max of 255. In a
linear scale, 128 will be about half the intensity of 255, or 50%.

A lot of times you'll have nothing in the right half of a linear tiff
from a RAW file, because the cameras expose for JPEG, which usually
clips away the top 1 stop of highlights. If you don't touch the
"exposure" slider in the RAW converter, you will lose them, also, unless
you are using linear mode. So, you get two effects combined; a linear
image will have a histogram squished to the left even if the clipping
point is the same for both linear and gamma-corrected, *plus* the linear
tiff usually has an extra stop of dynamic range, so the content is
squished even further to the left.

The actual levels of what normally becomes 128 (out of 255) in a default
RAW conversion are around 200 to 400 (depending on the color channel)
out of almost 3900 levels (all 4095 are not used in the Canons). That's
only 5% - 10% of the full linear range.


--


John P Sheehy