We should have also mentioned the trick of holding down the Alt/Opt key while dragging the input sliders. So, starting with this slightly flat looking landscape, we can see that there are gaps at both end of the histogram.

If we hold down the Alt/Opt key while dragging the white point input slider to the left the screen would initially go full black, and then areas would revealed showing the colour of the channel that was being clipped.  You can see from this screen shot that it would be blues so the brightest part of the sky. Unfortunately, I can’t give you a screen shot because neither Greenshot or the Snipping Tool will work while the Alt key is held down.

We can just barely see a few red and yellow content at the shadow end. If you had the Alt key held down we would see it was the two dark green trees on the bottom left third.

This how it looks with the input sliders pulled in as far as I dared without severely clipping channels.

If you had started in Camera RAW or Lightroom with a RAW image file, and made these adjustments there, you would avoid the gappy histogram.  Do this and open as 16 bit in Photoshop and you would have a more robust file that would withstand more radical editing without clipping channels leading to possible banding.

Dag, ​@D Fosse is far more knowledgeable than I am regards colour, as I really don’t care if colour is not perfect with my holiday snaps and the more illustrative work that I like to do.

If you look at the histogram in the Levels Properties you can see that it has very low contrast and no content anywhere near full black and full white.

If we adjust the input sliders to give the image a full tonal range, it will maximise contrast, but stretch out the original tonal range to fill all 256 steps (assuming 8 bit).

If we now add a second levels adjustment layer, but not to make furth adjustments, but just see the resulting histogram we see it is full of holes.

If this was a landscape with subtle blue to lighter blue sky gradient, then that gappy histogram might mean banding if you stretch the tonal range globally.  There was an interesting video on the recent Photoshop Summit 8 from Blake Rudis which suggested using Blend If to target/preclude specific color ranges.  I’d never thought of this before, and he gave a convincing demonstration.  That video is no longer available, but he has a YouTube channel.  This video might cover it.

Taking a guess without seeing a sample of your western blot, unless you’ve used a selection tool (e.g., Marquee) to select a portion of the image, the change is global, not local.

Adjust color and tone with Levels and Curves eyedroppers