Gamma - Scanner & Image

[davidgordon]

I am using SF 5.5 with a Polaroid 4000 on Mac OS 9.2, Photoshop 5.5. SF is using the IT8, my colour matching is fine.

I notice that I can always make my image better after opening in Photoshop. I find my scans a little lacking in contrast. Not by a lot. Sometimes a quick "auto contrast" from PS will make the image better. I find that I have to set the gamma control on SF high to get good contrast - by which I mean you can see all the tones and is visually pleasing. If I prescan and click "auto exposure" SF provides me with a _very_ flat image. I never use "auto" now. I set the black and white points using the levels and then gamma/contrast with curves.

The resulting image always needs a little work in Photoshop though. I would like to have a perfect scan from SF so I don't have to make any further adjustments with PS.

What might I be doing wrong? Recallibrating with my IT8 doesn't seem to help.

Further, I recently read (but cannot find again) an article about scanner gamma which suggested scanning an IT8 at various gammas in order to find the setting which produced a certain RGB value for the mid-grey. I wonder if this is done by the IT8 callibration? Or what gamma setting should I be using in SilverFast? Did anyone else read or see this - can you tell me the URL?

[ilyons]

You don't change the gamma after calibration otherwise the calibration is meaningless.

Mid grey on your IT8 is Grey-scale patch 9 and it should ALWAYS have a value of Lab - 49 or VERY close.

LaserSoft gave you tools to adjust contrast - gamma gradation was NOT one of them.

[davidgordon]

You don't change the gamma after calibration otherwise the calibration is meaningless.

Yes, of course. I understand that any changes I make will require me to re-calibrate!

LaserSoft gave you tools to adjust contrast - gamma gradation was NOT one of them.

Contrast? Gamma? Pretty much the same thing? That's the trouble with SilverFast, not everything is what it seems...

So what is the "Gamma-Gradation" setting in Options -> General? As described in your tutorial at http://www.computer-darkroom.com/sf6-it8/sf6_1.htm

I think what I am asking is does this setting have an impact on the overall contrast of the scanned image. I'm getting a slightly lower contrast result from the calibrated prescan than I would expect and I have to increase the gamma (midtone separation) and contrast to obtain a nice scan. What impact does the Gamma-Gradation setting have. And why do you recommend a setting of 2.0?

On the other hand maybe my "Auto Threshold Highlight" and "Shadow" are wrong. My impression from the manual is that they come into play when using "Auto Exposure", perhaps this setting works at other times? Remember I find I need to adjust the levels in Photoshop to get a perfect scan.

My colour is spot on by the way!

[ilyons]

David,

I think what I am asking is does this setting have an impact on the overall contrast of the scanned image. I'm getting a slightly lower contrast result from the calibrated prescan than I would expect and I have to increase the gamma (midtone separation) and contrast to obtain a nice scan. What impact does the Gamma-Gradation setting have. And why do you recommend a setting of 2.0?

I don't recall recommending gamma 2 specifically but a range between 1.8 and 2.2. The screenshot shows 2 but that is the value I use - it might work for others and it might not. So far as the gamma-gradation value is concerned it is used as apreset and since you are using IT8 it will have no effect on image contrast or brightness. Once IT8 calibration comes into play you only ever adjust contrast/brightness on an image by image basis using the supplied tools.

I have a set of notes that I use for scanner lectures and demos. I've tidied it up a bit and posted it to my web site. I'm not sure if it's what you're looking for but have a look and let me know.

http://www.computer-darkroom.com/gamma/gamma_1.htm

Ian

[davidgordon]

I have a set of notes that I use for scanner lectures and demos. I've tidied it up a bit and posted it to my web site. I'm not sure if it's what you're looking for but have a look and let me know.

Sorry for the delay in saying "Thanks"!

Thanks, this has answered both my questions. I'm now going to have to look again at why I need so much pre-scan curves correction. I don't have the same problem with negative film. Perhaps its just the way I expose my tranies! Mostly dark and pushed...