The EPSON scan software's "Histogram Adjustment" allows you to change the Gamma as well as the endpoints.
Would 1.0 be a normal Gamma ? What setting will result in a "linear" interpretation of the negative ?
How does this relate to the option of using either 1.8 or 2.2 as the Gamma, in the general settings ?
Hard to say for sure about the Epson setting as we don't know for sure what encoding is set behind the Epson curtain. My guess is that "1" in the box above would be no gamma adjustment. A standard setting is 1.8 to 2.2. But again we don't know for sure if there is some magic being done that Epson doesn't tell us about. Gamma is used to adjust the luminance values of the machine image to human vision.
In settings such as above, obviously the film densities are going to have a large influence on what is captured, so I don't think there is a set answer that fits for all negatives, rather an individual answer for each. Just like adjusting the negative to fit your darkroom paper, its a good idea to adjust the negative to your scanner through a personal testing process. On the other hand once you get all the information into the computer you do have a good bit of lee-way in distributing the values.
I personally think that you're almost always better off getting a full luminance range on input. In the above example that would look like a full histogram window with values from left to right edges and not a clump in the middle as shown. Then gamma is applied to adjust the values of the image to vision. And your intention plays a big part in all this. You may have a completely different way of seeing the picture in your mind from what someone else might choose -- your correction might be very different from mine.
To your specific questions:
"1" should be no gamma and linear, 18 or 2.2 would be "normal".
General settings of what? If in Epson scan, play with the settings and look at the changes in the "Tone Curve Viewer" in the screen you supply above. That shows any changes in the adjustment being made.
The Gamma in this case is a relative number (1.0) used to express the midpoint between the black point and the white point (no relationship to gamma discussion of 1.8 or 2.2).
When using the default black and white points (0, 255), a gamma of 1.0 would translate to a midpoint value of 128. But as you change the black and white points, the actual value would also change. The 1.0 is simply a relative way to express the midpoint value, without having to see it change every time you move a black or white point - 1.0 will always represent the midpoint. You will also notice that it is logarithmic, with a range of .1 to 9.99 where 1.0 is the midpoint.
Perhaps the best approach is to scan a step-wedge and see what happens as different adjustments are made.
Have people done this already ?
What is it that you are trying to accomplish? Your step wedge results will be different every time you change the black point, white point or gamma.
The gamma simply represents the midpoint between the black point and white point. It is a relative number, not an absolute number. The black point and white points are absolute numbers. Changing the gamma simply makes the midpoint brighter or darker (it has no effect on the black point or white point), but it is directly related to the black point value and white point value.
Thanks for your help.
I'm trying to scan a negative in the most linear fashion, so I can compare the results of film speed and development.
I'm trying to view the negative without the scanner imposing its own curve on the result.
Leave the gamma setting alone. This goes double when you are scanning printed paper targets for calibration of digital negatives. See Mark Nelson's manual about scan settings since you are a PDN user.Originally Posted by Ken Lee
Don
1.0 is unity—no change. The 1.8 / 2.2 setting relates to how images are displayed on your monitor, not how they're scanned and saved to disk.
On vuescan you have the option to lock settings, so scan both negatives with the same settings and compare the files in photoshop.
David Cary
www.milfordguide.nz
When comparing negatives, pull the end points out and have the middle slider set at the midpoint 128. In other words, ignore the contents of the histogram and let the negative drop in where it falls. The scanner should hold this each subsequent scan if you save it as a setting.
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