Here's some specifics of the blend modes.
https://en.wikipedia.org/wiki/Blend_modes
http://www.linuxtopia.org/online_books/graphics_tools/gimp_advanced_guide/gimp_guide_node55_002.html

For your example pictures, screen will over brighten the darker areas of your picture, so dodge produces a better result.  In a different picture, screen could be the better choice.

Brian

On 03/23/2016 12:56 PM, Michael wrote:

please, what is the difference between the two modes?

On Wed, Mar 23, 2016 at 2:35 PM, Brian Cluff <brian@snaptek.com

<mailto:brian@snaptek.com>> wrote:

    Oops, use the Dodge layer mode instead of screen.  You'll get better
    results.

    Brian


    On 03/23/2016 11:22 AM, Brian Cluff wrote:

          > On 03/23/2016 04:49 AM, Michael wrote:
          > Thanks Brian. I am putty in your hands!

        I get the bounces from the PLUG mailing list, so I got your
        images even
        though they were too big for the queue... nobody else did
        though, so my
        answer will be the only one based off your image.

        Anyway, looking at your pictures, it appears that the darkened edges
        aren't due to fading over time rather they are caused by what
        appear to
        be a flash going off that lit the center of the image more than the
        edges. This can also be caused of lower quality lenses giving you
        darkened edges in what is called a vignette.

        If you want to try and reduce/eliminate it, in gimp you would add a
        layer above your picture, but before you do that, if you have the
        option, go to Image -> Precision and set it to 16 ot 32 bit, so
        that you
        have more color data to work with.  Unfortunately you'll only
        have that
        option if you are running the development version of GIMP, but
        it will
        still work OK if you can't do that setting. Next use the
        gradient tool,
        changing the gradient type to radial, and make a gradient that
        goes from
        black in the center fading to white at the corners. (draw the
        gradient
        from the center to one of the corners).

        Then set your new gradient layer's mode to "screen".

        At this point the effect will probably be too strong, so you
        will need
        to play with the opacity till it looks better and you can also
        play with
        the color curves moving the line on the lower left back and
        forth across
        the bottom to adjust the shape of the gradient till it better
        matches
        the vignette in your image.  You can change the gradients layer mode
        back to normal to get an idea of what the color curve adjustment is
        doing to the gradient.

        Brian Cluff

        On 03/22/2016 06:42 PM, Brian Cluff wrote:

            Can you send an example so that I can see what you mean by
            "darkened
            around the edges"?

            I'll see if I can easily correct it and then send
            instructions on what I
            did.

            Brian Cluff

            On 03/22/2016 04:18 PM, Michael wrote:

                how does one use a transparent layer? is it only for
                drawing things on?
                I have some old pictures that have darkened around the
                edges. How I
                think I can fix it is to duplicate the layer then
                lighten the exposure
                value on the lower image, then paint black and uncover
                the lighter image
                over the section that has darkened. How do I do this
                with gradients and
                Darktable?

                --
                :-)~MIKE~(-:


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