Well this may be my first and last blog post – we’ll see!
This post outlines how I would go about improving exposure on a troublesome image. The idea to write something down came from someone posting up an image on a forum.
Health warnings – I’m very amateur both as a photographer and Photoshop (7.0) user – this may be rubbish and there may be easier ways. I will try to make this reasonably self-contained but you may also need to refer to Photoshop help to understand how to do some things. Finally, these areas are covered elsewhere on the ‘Net (which is where I learned), but I’m covering again here in the hope that a different perspective will help some people.
This is the starting image.
The main problem here is that the ducks heads are relatively dark/under-exposed and the water is arguably over-exposed.
I believe that the picture was spot-metered from the feathers (a reasonable technique), but because the ducks are in shadow and the water is reflecting light, it makes for a really tricky exposure. This is not an exposure tutorial. See here for explanation of metering types and here for a good article on reading histograms and managing exposure.
So assuming we captured an image with some exposure issues and we don’t just want (or can’t) have another go at capturing the scene what can we do with this? Here’s the histogram for the original image as shown in the Levels dialogue box.
The annotations I’ve added show roughly which parts of the image correspond to which parts of the histogram. The horizontal axis represents dark to light and vertical shows how much of the image is at that brightness.
For less troublesome images, we might normally just use this levels dialogue to tweak things. The usual process process would be to drag the right-hand (highlight) slider in until we are the end of the histogram. In this case we might decide to sacrifice a little of the lightest parts of the feathers by dragging the slider in to the end of the mountain which represents the reflections on the water.
TIP: You can see which parts of the image are affected by this by holding down the Alt key (Mac Option) whilst dragging the slider. The image will go black and the bits of the image which are being “chopped off” (turned to white) will appear as white areas as you drag the slider in.
We would then drag the left-hand (shadow) slider to the start of the histogram on the left. The Alt key trick can be used here too. Finally we might drag the middle slider right to darken the image or as in this case left a little to lighten things up (it actually affects the mid-tone areas of the image). Here’s the histogram with the new slider positions.
And here’s what the image would look like.
This has improved contrast and lightens things up a bit and in many/most cases this would be adequate. We might be happy with this in most cases. For our purposes here, I’m going to try to demonstrate a different technique.
We’re now going to look at three concepts:-
- Adjustment layers
- Curves
- Layer masks
1. Adjustment layers
Adjustment layers are special layers in Photoshop which contain only an image adjustment of some kind. They act (in Adobe’s words) as a veil through which the rest of the image (i.e. the layers below) is viewed. Their power is that unlike just doing an Image>Adjustments>whatever, they can be altered afterwards. So you can go back and tweak the levels, curves or whatever later on. This is also useful because sometimes a later adjustment will mean you wished you’d done an earlier one differently (for example adjusting tone can often affect colour saturation).
2. Curves
Curves are a way to make tonal changes to an image. They allow a greater degree of control than Levels. They can also be used to adjust colour by working on individual colour channels, but we won’t cover that here.
3. Layer masks
Layer masks can be used on all types of layers and they can be used for instance to allow parts of layers underneath the layer to be seen (i.e. they can cut holes in a layer). Their relevance to us here however is that when combined with adjustment layers they allow us to adjust parts of the image below whilst leaving other parts untouched. This will allow us to adjust the ducks and the water separately. Sounds complicated but is worth a little effort to understand as they open up huge possibilities.
To summarise, we are going to create two adjustment layers, one for the ducks and one for the water. Both will be curves adjustment layers and we’ll use a layer mask on each to control which parts of the image are adjusted.
Curves Adjustment Layer for the Ducks
Starting with the original image, we’ll create a Curves adjustment layer for the ducks.
Either go Layer> New Adjustment Layer>Curves… or click on the half-black, half-white circle at the bottom of the Layers palette and then select Curves.
This creates the layer and brings up the Curves dialogue.
As on the levels histogram, the horizontal axis represents dark to light. In this case, this is the original image (or Input). The output (vertical axis) is what the curve changes the inputs to. To start with the output values are all the same as inputs. If you’re not used to graphs, using just your eyes, pick a point on the horizontal axis, go vertically up a line until you meet the curve (which is actually a straight line at the moment) , then turn left and proceed horizontally until you hit the vertical axis. You’ll find that your position up the vertical axis is the same as how far along the horizontal axis you started.
Now, make sure the Preview box is checked and click and drag a point on the curve upwards and to the left (North-West). A point near the middle of the curve is good to try. You’ll find that this has the effect of brightening most of the image without affecting the deep shadows and highlights too much. This is worth knowing by itself. You’ll probably also have noticed just how big an effect a small movement of the curve has. It’s powerful stuff and this is worth bearing in mind when you start working with your own images.
TIP: If you hold down Alt (Mac Option), the Cancel button chances to Reset. Click this to reset the curve without having to leave the dialogue box.
In the ducks image, here is the curve I used.
The first point right in the bottom left corner is the standard point which can’t be removed. The next one, just a little further up has the effect of lightening some of the deep shadow areas so that the heads of the ducks are a bit more visible. The other two points are there to make sure that this lightening effect doesn’t make the rest of the ducks too light. You can see how Curves gives quite fine-grained control.
TIP: to remove a point, just click and drag it off the graph.
So now we have adjusted our ducks, but we now have really bright water.
TIP: Another great thing about adjustment layers is that unlike just doing an Image>Adjust>Curves or whatever you can go back and tweak your curve as much as you like. Just double click on the black and white circle in the Layers palette for the relevant layer and the Curves dialogue will open.
To address the bright water we’re going to use a layer mask on the curves adjustment layer we just created. If you look in your Layers palette you’ll see that the adjustment layer is represented by a black and white circle, then a link symbol and then a white rectangle. The white rectangle represents the layer mask.
With it all white as now it means that the adjustment layer affects the whole image. If we paint on the layer mask, we “mask” off bits of the adjustment layer so that these parts of the image underneath are not affected by the adjustment layer (it’s easier to see and try than it is to think about!).
We will want to paint out the water so that this is not affected by the ducks curve. There are lots of ways to approach the painting and it will depend on your subject. You might create a selection on the background image, then click on your adjustment layer and fill it with black for example, or you might just paint directly.
Either way, the trick is to paint solid black up to close but not right up to the edge of your selection. Then if you’re painting, reduce the brush flow right down using the brush options (try values between 15 and 30%) and paint over the edge with a largish soft brush. You can also reduce opacity if needed and also try progressively reducing flow and opacity to soften your edge. The great thing about the adjustment layer is that if you make a mistake, just paint white back and try again. You can of course also fill the whole area black and then just paint white on the parts of the image you want affected. That’s what I did in this case.
TIP: “X” swaps between background and foreground colours for painting so if you have black and white set up as your foreground and background paints, you can easily swap between the two as you work on the layer mask. Also worth knowing – “[” makes the brush head smaller and “]” makes it bigger. CTRL & “+” to zoom in CTRL & “-” to zoom out.
If you Alt+click (Mac: Option+click) on the layer mask in the layers palette you can see the mask itself. Do the same to return to the normal view. I also find CTRL+click on the layer mask handy as this creates a selection based on your layer mask painting. It’s handy for showing the mask another way whilst you’re working on it (Ctrl+D to cancel the selection).
Here’s the layer mask I ended up with for the ducks. With this in place things look much better as we have bright ducks and the original water. Using layer masks sounds tricky but if you use the reduced flow and opacity approach you can achieve a surprisingly good result even if your painting is not that accurate which is reassuring for people like me with shaky hands and no graphics tablet.
We can go a bit further easily if needed. We can create an adjustment layer for the water and darken it a little . What’s more, we don’t need to paint the layer mask again as we can re-use the mask from the ducks curves layer. To do this:-
Make sure you have black as foreground colour and white as background colour (click the little black and white squares symbol if needed to set this).
Now CTRL+click on the ducks adjustment layer we just created. Then invert this selection (Select>Inverse or Shift+Ctrl+I). Then create your new curves layer for the water as before (Layer>New Adjustment Layer>Curves… or click on the black and white circle on the Layers palette and select Curves…).
Adjust your curves to achieve the effect you want. You’ll notice that the ducks are not affected and you can also see in the Layers palette that your new adjustment layer has a layer mask attached which is the inverse of the ducks one. Here’s a detail from the the final image compared to the original.
If you’ve not worked with these things before, it can be a bit hard to get your head around and a bit of practice is needed to learn how you like best to create the layer mask. It’s worth persevering though as this is a really powerful technique which you can use in lots of situations.
For example, you can achieve spot colour this way too by creating a Channel Mixer layer mask, or you can selectively adjust saturation.
Let me know if you have any comments (particularly better ways which might help me and others). Please also let me know if anything is unclear or incorrect.
Occasional musings 