I understand that I should adjust my histogram after I preview so that the scan comes out with a histogram that covers the whole range. This also prevents "picket fencing" or lose of midtones. Is that the correct way to say that?

My problem is that I do not see a levels tool in preview mode, just curves which I am not good at using. Levels has a set method of adjustment but curves is just a matter of what looks good to your eye (my eye is not very discerning). Or am I way off base.

Could you explain the steps you are taking because I don't understand what you mean by "preview mode".

The curves tool is the same as the levels tool, but with more scope for adjustment. If you stick to adjusting the end points in the curves tool you are making the same changes as the levels tool.

You also might have a look at these two episodes of Meet The Gimp as Rolf uses both levels and curves:
http://meetthegimp.org/episode-052-clear-the-sky/
http://meetthegimp.org/episode-090-selling-the-past-on-ebay/

What I man by preview mode is, after you do a preview scan you can adjust your histogram to effect how the actual scan turns out.

That being said, I will let it go for now. Those videos are so amazingly easy to understand. I am half way through the first one and I have learned sooo much. Thanks.

But here is a related question...
It seems to me that you can not truly modify a particular shade with levels or curves without also modifying all shades.
Example...I cut out a section of a image. I then modified that section with levels to get a better looking image. Then I replaced that section back into the complete image (on a seperate layer). So now I have an image with a square section that is more contrasty than the whole image. Now I try to make them match up. I use levels and curves to try to get the contrast and shading of the large image to match the cutout piece. I darken the shadows to match but then the highlites are to dark. So I lighten the highlites and then the shadows get lighter too. And I go back and forth and never get highlites and shadows both correct.
I hope you understand what I am saying.

i really don't get in general and here even less

"I then modified that section with levels to get a better looking image. Then I replaced that section back into the complete image (on a seperate layer). So now I have an image with a square section that is more contrasty than the whole image"



why a square section ? if you want past back will seems better a more exact free selection

"Then I replaced that section back into the complete image (on a seperate layer)."

Did you replaced OR you paste on a new transparent layer the section ? (the 2 thing are in contradiction or you did the first or the second )

I agree with PC on the Free Selection method, it keeps your levels adjustments neater. However, to avoid making the outer-most areas of the selection darker (a.k.a more "contrasty"), I would use Free select with a small feather (experiment with the settings to suit). That way, even though the selection has more of an enhanced area, it will still blend with the rest of the image.

In regards to trying to make two separate layers "match up", why not implement the above technique by first duplicating the original image twice, on the second layer, use levels on the entire image, then reduce the opacity to suit (or change layer modes/adj. opacity), then on the top layer, select the focus area and apply the technique described above (Free Select)with a low feathered area. Again, reduce the opacity of that layer if the adjustment is too strong. You could also, leave the Free Selection on the top layer, invert it, and delete the outside area, if its creating too much darkness with the lower layers.

All good suggestions. I am currently watching the meetthegimp videos. Holy cow! what a lot to learn. They are really good videos though.

Yeah they are , i would recommend Rolfs podcast to anyone who uses Gimp. He mostly teaches the photography stuff, but occasionally he has a effects tutorial thrown in there. =)