Vuescan : what gamma value in 24 bits RAW ? How to change brightness ?

[Jean Delmas]

Followin an recent excellent advise given by Nikita, I try to evaluate
optimum gamma for calibrate my Nikon 4000ED with lcms.

So, I have to scan my Wolf Faust IT8 Ekta target in a 24 bits RGB RAW
file for several increasing gamma values between 2.2 and 3.0.

Because, default Vuescan gamma correction for saving 24 bits RAW is
2.2, I scan my target with differents Color|Brightness from 1 (for
gamma=2.2) to 1.3636 (for gamma=3.0).
When I look at scanned images in Vuescan, brigtness of image is well
increasing when Brightness option is increasing. But, when I look
scans in Photoshop, brightness of images is about constant !
I probably make a mistake. Probably I dont understand what does option
Brightness means, or how Vuescan save RAW 24 bits files...

I could follow an alternative advise by Nikita : apply gamma
correction curve in Photoshop, but it is more complicated workflow...
And I have to know anyway what gamma value Vuescan apply when it saves
24 bits RAW files...


Jean

 

[SJS]

Because, default Vuescan gamma correction for saving 24 bits RAW is
2.2, I scan my target with differents Color|Brightness from 1 (for
gamma=2.2) to 1.3636 (for gamma=3.0).
When I look at scanned images in Vuescan, brigtness of image is well
increasing when Brightness option is increasing. But, when I look
scans in Photoshop, brightness of images is about constant !
I probably make a mistake. Probably I dont understand what does option
Brightness means, or how Vuescan save RAW 24 bits files...

I don't think the Color|Brightness settings affect the raw file because
the raw file is output before any of the modifications are done.

I could follow an alternative advise by Nikita : apply gamma
correction curve in Photoshop, but it is more complicated workflow...
And I have to know anyway what gamma value Vuescan apply when it saves
24 bits RAW files...

I believe the 24-bit raw files are always gamma 2.2.

-- Steven

 

[Jean Delmas]

Steven wrote

I don't think the Color|Brightness settings affect the raw file because
the raw file is output before any of the modifications are done.


I believe the 24-bit raw files are always gamma 2.2.

You are probably right. So, Vuescan makes a gamma (= brightness)
correction on RAW files before output.

So, I dont see why it does'nt accept a different gamma correction thru
Brightness option, which is a simple coefficient to multiply gamma...

Jean

 

[nikita]

Jean,

the 24 bit RAW is not effected by the Brightness slider. Only the
preview and if you do output a "cropped" tiff set to non and locked in
all ways possible. The RAW 24 bit is a 2.2 like SJS already mentioned.
The RAW 48 bit is linear 1.0 and in need of the curves.

I would go for a smart use of Photshop Action buttons loading the
gamma curves into the Curves dialog and hit the image with that. Then
– later – let the same action include ASSIGN and CONVERT to Workspace.
It will go within a blink of your eye regarding speed. If you need a
24 bit flow let those action automatically end with a downsample to
that from the highbits....

nikita

 

[Dan Honemann]

I believe the 24-bit raw files are always gamma 2.2.

I recall reading that the gamma depends on the output space; if Adobe
RGB or sRGB, then 2.2; if AppleRGB or ProPhoto RGB, then 1.8.

Dan

 

[SJS]

I recall reading that the gamma depends on the output space; if Adobe
RGB or sRGB, then 2.2; if AppleRGB or ProPhoto RGB, then 1.8.

Hi Dan,

I have read that somewhere but testing didn't confirm it. I asked Ed
recently and he said 24-bit raw file gamma is always 2.2.

-- Steven

 

[Ed Hamrick]

I have read that somewhere but testing didn't confirm it. I asked Ed
recently and he said 24-bit raw file gamma is always 2.2.

That's correct - raw 24-bit files from VueScan are always gamma 2.2.

Regards,
Ed Hamrick