Although it is very important to
get exposure right at the time of image capture, there are times
when even the best of us accidentally get the exposure wrong.
Sod's law says that this will happen on your best shot.
If you shoot in RAW there is a much bigger exposure latitude
than if you shoot jpeg. This offers a better chance of rescue,
but what we are talking about here is really trying to make a
silk purse out of a pigs ear.
Under exposed image
There is often a lot of hidden detail hidden in the shadows of
an under-exposed image, but in opening up the shadows (e.g. with
curves or levels) a lot of ugly digital noise will be revealed.
In the first example below, I was trying to photograph a hawkowl
attacking a vole in very poor light - it was beginning to snow.
I knew that if I did not achieve a shutter speed of around 1/3200th
of a second the picture would contain motion blur. I was shooting
at high ISO with an old 1DmkII camera and even with my f4 lens
wide open I could not achieve this shutterspeed without under-exposing
the image. So what can be done in post-process to correct it ?
I tried using the highlight slider in Levels but this badly exposed
noise and burnt out all detail in the snow.So I used the following
In Photoshop, open the layers palette (Window>Layers)
Next duplicate the background layer by dragging it over the Create
New Layer icon (second from the right on the bottom of the layers
Next set the Blending Mode to Screen in the drop down menu. (See
image below). You will instantly see the image brighten. Repeat
by dragging the background copy over the New Layer icon until
the image is just a bit brighter than you would like it to be.
Finally reduce the opacity by clicking on the arrow to the right
of the Opacity box and adjust the amount to taste. When you are
satisfied, click the small down arrow (above the Opacity box)
and select Flatten Image from the drop down menu.
I put a few finishing touches to the image by adding some shadow
detail to the snow with the Shadow/Highlight tool (which is covered
in another tutorial), tweaked the midtone slider in Levels, sharpened
it and saved as usual. This was the finished result below:
To deal with an overexposed image an identical method is used
to that in the underexposed example above, but this time the blending
mode to use is Multiply.
This image is of a wildebeest crossing in Kenya and is overexposed.
Once again I duplicated the background layer but this time I chose
Multiply from the drop down menu. I lowered the Opacity to 53%
which looked good to me and flattened the image. After tweaking
in Shadows/highlights and Levels I sharpened the image and saved
it. Some of the brightest highlights are still a little burnt
out but I think you will agree that this is a massive improvement
to the original image.