Advanced techniques for replacing a dull sky

What you learned: Use Blend If sliders to replace the sky in a photo

Set up the new sky and a reflection of the sky

1. With the Rectangular Marquee tool, select the sky. Then click the Add Layer Mask button to create a mask that hides everything but the selected sky.
   
2. Select the Move tool, and drag the sky layer up until it is about where the sky in the original image is.
3. Duplicate the sky layer by pressing Control+J (Windows) or Command+J (MacOS).
4. Flip the duplicate sky layer upside down by pressing Control+T (Windows) or Command+T (MacOS), right-clicking the layer, and choosing Flip Vertical.
5. Drag the inverted sky layer down until it lines up with the bottom of the original sky layer.

Use Blend If to reveal pixels from the underlying layers

1. You’ll use the Blend If feature to replace the sky. First, identify the best channel to use with Blend If by going to the Channels panel and clicking through the Red, Green, and Blue channels. Look for the channel with the most contrast between the light sky and the darker foreground. In a daytime photo with lots of sky, this is likely to be the Blue channel.
2. Take note of the channel with the most contrast. Then click the RGB channel and return to the Layers panel.
3. In the Layers panel, select the layer that contains the original sky you want to replace, and drag that layer above the two sky layers.
4. Click the FX icon at the bottom of the Layers panel, and choose Blending Options to open the Layer Style dialog box.
5. In the Layer Style dialog box, click the Blend If dropdown menu and choose Blue in order to reference brightness levels in the Blue channel.
6. Still in the Layer Style dialog box, go to This Layer and drag the white slider to the left to reveal the new sky. This hides the original sky on the selected layer, revealing the more dramatic sky images on the layers below. The original sky is hidden because its brightness values in the Blue channel are higher than the value the white slider is now pointing to. 
   
   Tip: Alt-click (Windows) or Option-click (MacOS) as you drag the white slider to split the slider in half. Then spread the half-sliders to create a smoother transition between hidden and visible areas in the image.
   
   
7. Click OK to close the Layer Style dialog box.

Bring back details you want to keep

Using Blend If to hide the sky might also hide other details in the original image that you want to keep.

To bring those details back into view:

1. Duplicate the original image layer by selecting it in the Layers panel and pressing Control+J (Windows) or Command+J (MacOS).
2. Place the duplicate layer above all other layers.
3. Remove the layer style from the duplicate layer by right-clicking the double-square icon on the duplicate layer and choosing Clear Layer Style.
4. Using the Quick Selection tool, drag across the details that you want to keep. Then click the Add Layer Mask button to create a mask that hides everything on the duplicate layer except the selected areas. This can bring back into view some details that were hidden by Blend If on the original image layer.

Match luminosity

1. Click the Create New Adjustment Layer button in the Layers panel, and select Levels.
   
2. Drag the Levels Adjustment layer above the two sky layers.
3. In the Properties panel, drag the black Output Levels slider to the right to brighten the new sky until it matches the brightness of the rest of your composite image.
4. Finish up by dragging the gray Input Levels slider to adjust the contrast of the new sky.