Beginners guide to Photoshop Layers (Part 3) - Changing Exposure

So in the previous episodes we learnt quite a bit about layers, but now we will use them to improve a photograph. In this exercise we are going to look at improving the shadow detail.

In this example I will use a couple of shots I took of The Wibbly/Wobbly bridge which links St.Pauls to Tate Modern in central London. Standing under the bridge in bright sunshine it was obvious that the exposure range from the dark underneath of the bridge to the bright blue sky would be far too much for the camera, so I auto-bracketed my shots +/-2 stops. An alternative to bracketing would be to produce 2 different exposures of the same shot, through the raw converter or duplicate the shot and adjust the brightness/levels to give the effect of two different exposures.

Bracketing took me from having one picture that doesn't look like the scene I saw, to having 2 or 3 that don't! What I need to do is add the light bits from the overexposed shot to the bits of the underexposed shot that have no shadow detail...this is a job for layers....

Open both images and layer them.

Underexposed & overexposed versions of the same shot
(click to view large)

Now click on the darker one with the move tool and drag and drop it on the lighter one - if you hold the shift key down whilst doing the move the 2 layers will centre on top of each other.

You now have the 2 layers in the same image, look at the layers pallet if you don't believe me, so you can close down the image you just dragged from as we no longer need it.

The two images stacked
(click to view large)

Erase through from top to bottom.

Make sure you have the top layer selected and select the eraser tool. Now you can select the darker areas of your image and erase through the top layer to show the brighter layer underneath.

Starting the erase process
(click to view large)

To do this properly you will need to take your time and vary the brush size (use the [ and ] keys on the keyboard). Also its handy to remember the undo key (ctrl+Z) and step backward (alt + ctl + z)can help you here a lot.

Erasing finished
(click to view large)

As you work, switch the layers off and on and you will see how you are cutting a hole through the top layer to reveal the layer underneath.


In this shot I felt the brightness was too much so I added a small levels tweak to make it look more natural.

The finished product (hey give me a break its only a demo)
(click to view large)

Now the bottom of the bridge is revealed in all its glory yet we still have a blue sky...marvellous.


There has to be a better way

If you have tried this technique out you will soon realise that using the eraser tool is destructive. Its a bit of a pain, as if you make a mistake you might have to use undo half a dozen times to get back to where you were. Even worse if you save the file and realise you have erased a bit that you didn't want to you will have to start all over again.

A much better way is to use masks - which we will cover in the next thrilling installment.