Adobe Photoshop Workshop: Blending Exposures

One major issue every serious photographer has to consider with nature or landscape photography is varying exposure and the lightest and darkest areas of an image. But luckily with modern methods of HDR (high dynamic range) and the ability to take pictures with more than one exposure using modern digital cameras, it becomes very easy to blend images taken with varying exposures together into one single image with the right contrast. This also allows the image to more accurately portray the wide range of intensity levels found in real scenes, may it be with bright sunlight or faint sunlight. Another way of doing the same would be by applying different settings for processing the RAW file taken using a high end digital camera. Let us look into both these approaches in greater detail.


Different Exposures

To illustrate this method we shall consider a scene where different regions are exposed to varying intensities of sunlight. First we have to obtain different images of the same scene, so that each part of the scene has a good exposure in at least one of the images taken. It is best to take the images after setting the camera on a tripod or in a perfectly stationary position. For this example, we have taken two photos of the scene with a shutter speed of 13 seconds and 25 seconds, respectively. In the 13-second exposure image, the sky and most of the ocean looks good, while rocks are darkened because of the shadows. On the other hand, the image with the 25-second exposure presents a good amount of detail in the dark foreground colours of the rocks that make the sky and water extremely bright and practically invisible.


Getting started in Photoshop

With the two images in hand, let us move on to Photoshop to blend in both the images to get our final image. Open both the images in Photoshop and copy one of the images into the file containing the other image. You can do this by right clicking the image layer and duplicating it into the other file or by dragging and dropping the image into the other file. Make sure both images are in different layers and none of them is a background image. You can convert a background to a layer by double clicking on it from the Layers Panel and clicking OK. We shall start by adding the layer with the darker exposure to the image with the lighter exposure, so that the darker one is on top of the lighter one. Make sure that both the layers are perfectly aligned by moving the layer after clicking ‘V’ to get the move tool and then move the layer holding the [Shift] key to perfectly position the layer in the image window.


Adding a Layer Mask to the top Layer

The next step is the most important step in this process. We shall add a layer mask on the top layer to mask the regions that do not have a good exposure and show only the good parts of the layer. The mask will cover up the bad regions and will let the regions with better exposure in the layer below blend into the final image. Now if there are three or more images with varying exposure instead of just two like we have taken in this illustration, based on the type of the images you are blending and the complexity of the edges in each of them you will be using the same process on each of them adding a mask to each of the layers on top. The mask makes sure that only the good parts from every layer are visible. To add a mask to a layer, first make sure the layer is selected from the Layers Panel. Then click the Add Vector Mask button at the bottom of the Layers Panel or add a Layer Mask from the Layers Menu. A mask which is displayed by a white thumbnail appears next to the layer thumbnail. The white colour of the mask indicates that the mask is opaque. Painting a region of this mask black makes it transparent depending on the amount of black and the layer below becomes visible through the mask. The amount of grey on it indicates how transparent or opaque the mask is. So the mask transparency changes based on the colour range between white and black painted onto the mask


Paint the Layer Mask

Now select the Brush Tool from the tools menu on the left or hit the B key on your keyboard. Set the Foreground Colour to Black and from the Brushes options bar on top set the Opacity of the Brush to 50%. Select a soft edged brush or just change the hardness of a solid round brush to zero. Choose a Brush size large enough to be able to paint over the entire bottom part of the image. In this case, we do this since the view of the rocks where there is good detail is only from the lighter image. Now select the Layer Mask and paint over it using this brush. Keep repeating the brush strokes multiple times to show more colour tones from the layer below. Reduce the brush size if needed and work on other parts of the layer as required. In this case, the entire bottom of the mask is painted black and the other regions where the rocks meet the water should be painted a little more carefully. If you need to reverse any of the changes on the mask, just change the brush colour to white and paint over the region of the mask you want to change or revert back.


Add an Adjustment Layer

In this particular illustration we lowered the opacity of the brush even further to 20% and painted over the region of the hill so that it blends properly with the image below. Now we will try to bring out even greater contrast by adding an adjustment layer on top. [Alt]+Click on the Create New Adjustment Layer Icon, i.e. click on it keeping the Alt key on the keyboard pressed and then choose to add a Curves Adjustment Layer. Make sure you have checked the Create a Clipping Mask checkbox and then click OK. Make sure to adjust the curves as shown in the image. Ticking the Clipping Mask makes sure that the Curves Adjustment Layer affects only the layer directly below it.


Making the Final Changes

Now to verify the exact regions of each of the layer that are visible or the percentage of the image that has been blended in to obtain the final outcome, click on the individual layers Eye icon and note which parts of the layers are made visible due to the layer mask on the top layer. The compound of both the layers after masking and adjusting the curves gives rise to the final image. You can also click the Layer Mask while holding down the [Shift] key to temporarily disable it and view the original layer. [Shift] + click it again to enable it again. Notice since the mask has soft edges; we don’t have to specifically work on lines where the darker regions intersect the lighter regions.


Dual processing of the Raw image

There is another way to do the exact same thing as illustrated above. If you have a high-end digital SLR camera, you can make sure you take only a Raw file of a picture and not process it for a final image. You only need a single exposure and make sure the exposure is from an unprocessed Raw file. You can basically create the same effects as above using this process. This is not possible though, if the still contains moving objects or if the picture was not taken while the camera was perfectly stationary. The primary method here is to bias the histogram to the right as much as possible without clipping the bright spots with respect to the exposure. Ensure that you have best exposure for the darker areas. Now process the first edition of the image using Adobe Camera Raw software for a particular region of the image. Select the blue Options for Work flow button located below the preview button and open this file in Adobe Photoshop as a smart object. In Photoshop, create a new smart object by selecting the New Smart Object option under Layers > Smart Objects. Duplicate the smart object Layer. Double-clicking it will let the duplicate smart object get affected by the Raw dialogue box. Make adjustments as needed. You can use exposure, light fill and brightness by making the appropriate changes in the slider. Use the same process of masking above to blend in the two images.

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