How To Replace a Blown-Out Sky Using Affinity Photo

by Sep 15, 2022Photo Editing Tutorials

Robin Whalley Landscape Photographer

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How To Replace a Blown-Out Sky Using Affinity Photo

We often use filters in landscape photography to prevent the sky from blowing out. When a sky blows-out it overexposes, turning white which is distracting and can make photos look ugly. Whilst using filters like Neutral Density Graduates can help to avoid problems, it can still happen. In this article, I’ll explain how I saved this photo by replacing the sky using Affinity Photo.

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Finished image with the sky replaced in Affinity Photo

Whilst I could have replaced the sky in the photo with any sky from any photo, this can be difficult. Most of the time, when you use a sky from one photo, the lighting doesn’t match the image you add it to. It’s much easier if you can use a sky shot at the same time as the photo that you’re repairing. That’s why you always need to check your histogram.

Always Check Your Histogram

It’s tempting when photographing the landscape using filters to ignore the exposure. Most of the time an ND Grad filter fixes any exposure problems, but it’s a mistake to assume it always will. When I shot the image above, I was using a 3-stop ND Grad filter, but the sky still overexposed. You can see what happened to it in this next image.

Image showing an overexposed and blown out sky that needs to be replaced

The camera couldn’t handle the dynamic range of the scene in a single exposure. If I hadn’t been checking the histogram on the camera, I might have assumed the exposure was fine. As it turns out, I was extremely surprised when I realised how overexposed the sky was despite using a filter.

My solution to the problem was to take a second frame of the same scene with a reduced exposure, ensuring I captured a good sky.

I was careful when doing this not to change any of the camera settings other than the exposure. The aperture and focus point are the same. I left the filter in place on the lens and didn’t change its focal length. My only adjustment was the exposure compensation dial on the camera which increased the shutter speed, creating this darker exposure.

Darker exposure of the same image. We will use this to replace the sky in the overexposed image.

The camera was also on a tripod to shoot both images so that the sky from the second will line up with the foreground of the first when combined.

Replacing the Sky

To replace the sky from the first image with the sky in the second, I opened both images in Affinity Photo.

Combine the Photos in the Same Image

I then copied the underexposed image which had the good sky by pressing Cmd + A. This selects the entire image layer followed by Cmd + C on my Mac keyboard to copy the selection. If you are using a Windows PC the keyboard shortcut keys are Ctrl + A followed by Ctrl + C.

After that, I switched to editing the overexposed image which had a good foreground but needed the sky replacing. I was then added the copied image to it as a new layer by pressing Cmd + V on my keyboard. On a Windows PC that’s Ctrl + V.

Two images as seperate layers in the Affinity Photo Layers Studio Panel

You can see in the screenshot of the Affinity Photo Layers panel that both photos are in the same image, but on separate layers. The bottom layer contains the photo with the overexposed sky and the top layer contains the replacement sky.

Add and Invert a Layer Mask

We now need to add a Layer Mask to the top layer to hide it. We do this so that we can use the Layer Mask to reveal only the new replacement sky from that layer.

To add the new Layer Mask, click the top layer in the Layers Studio Panel. You should see the layer becomes highlighted to indicate it’s selected. Now click the new mask layer icon found at the bottom of the Layers Studio Panel (1).

Adding a new layer mask to the layer containing the sky

When you click the icon, you should see the new Layer Mask (2) appear in the Layers Studio panel slightly inset below the top layer.

To invert the Layer Mask hiding the layer, click on the Layer Mask’s white thumbnail in the Layers Studio Panel. This ensures that only the Layer Mask is selected. I can then press Cmd + I on my Mac keyboard, if you’re using a Windows PC that’s Ctrl + I.

You should now see the Layer Mask thumbnail turn black, hiding the top layer to reveal the overexposed image.

Selecting the Area of Sky to Replace

We now need to select the overexposed area of the sky to be replaced by the sky from the hidden layer. We do this using the Channels Studio Panel.

The Studio Panel in Affinity Photo

In the Channels panel you will see three “composite” channels being Red, Green, and Blue. When you click one of these channels you will see a black and white image representing its values. To reset the view back to the colour image, click the curved arrow icon to the top right of the panel.

Here are the three channels in this image.

Image colour channels as seen in the Affinity Photo Channels panel

The colour channels you can see are:

  1. Red
  2. Green
  3. Blue

They appear similar but look carefully and you will see some differences, mainly in the distant hills and fields. We want to find the channel with the best separation between the hills and sky. In this example it’s the blue channel but that isn’t always the case so check each channel carefully.

Next, select the Composite Blue channel in the Channels window by clicking it with your mouse. We then need to load the channel as a selection so we can use it.

To load a channel, right click on it with your mouse. This displays a small popup menu where you can select the option “Load to Pixel Selection”. Once the channel loads, you will see animated marching ants appear on the image indicating the selection.

You can now return to viewing the colour image rather than the channel, by clicking the curved arrow icon to the top right of the Channels panel.

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Paint Through the Selection

We will now use the Affinity Photo Paintbrush tool to paint through the selection and onto the Layer Mask we added earlier. This will reveal the replacement sky from the top layer in the areas where the sky from the bottom layer is overexposed.

Before starting to paint, first hide the marching ants to make it easier to see what’s happening. You can do this by clicking the View menu at the top of the Affinity Photo interface. Then in the View menu click the “Show Pixel Selection” option which will toggle the marching ants off.

Select the Paintbrush tool from the tools palette on the left of the screen.

Configuring the paintbrush tool

You can then set the paint colour to white in the Colour Studio Panel. After this, configure the brush to have a medium Opacity of around 50% and a soft edge by setting the Hardness to 0%. You are now ready to paint onto the Layer Mask to reveal the replacement sky.

In the Affinity Photo Layers Studio Panel, click the Layer Mask icon which we previously turned black. This selects the Layer Mask to ensure we only paint on that.

Now paint over the sky in the image where you want to replace it. As you paint you will see the top part of the Layer Mask thumbnail begin to turn white. At the same time, you will see the new replacement sky appear in the image. Usually, a few brush strokes are sufficient to replace the sky.

Refine the Image Exposure

Whilst the image with it’s new replacement sky should look well exposed you can further improve its appearance. By using a Levels adjustment attached to each layer, it’s possible to refine the exposure of the sky and ground separately to help them blend.

To see the entire process of replacing the sky as explained above, together with the levels adjustment mentioned, watch this short video.

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Summary

In this article we’ve now looked at how you can replace a blown-out sky using the tools in Affinity Photo. To make this technique work well, remember to shoot two versions of an image where you see exposure problems. This technique of extending the dynamic range of an image can also be used to reveal detail that’s lost in shadows.

This tutorial has replaced an overexposed sky by creating a luminance selection from a colour channel. To take the luminance selection technique to the next level in Affinity Photo see my Luminosity Masking tutorial. It can give you total control over your selection.

More Affinity Photo Tutorials

You’ll find more high quality, free tutorials on my Affinity Photo Tutorials page.

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