As I often get requests to explain how to manually blend exposures in photoshop, I figured I would finally post this older article of mine straight on my blog (this post appeared on Bret Edge’s blog some time ago) and make sure it is available to my readers. This alternative method can not only replace GNDs in some circumstances, but offers significant advantages over both GNDs and the automated HDR options. This technique involves blending multiple exposures using masks and layers in Photoshop. Now this method is not the end all be all, mother all of all options, it doesn’t work in every circumstance. If you dislike spending time on your computer, this is probably not for you. However, if you believe that you should use every tool you have at your disposal to make the best image you can, then you ought to give this method a try.
Enter Exposure Blending
Let me first start with a disclaimer: as I indicated above, this is not a panacea, or some miracle recipe. Exposure blending is very useful but does have some limitations: it simply does not work with scenes containing moving objects. Also, when dealing with trees or grasses blowing in the wind, it can get tricky and sometimes even unworkable. That said, I find that exposure blending beats HDR programs by a long shot because the results are more realistic looking (personal choice here) and it avoids the muddy toned images that HDR programs often result in.
So, what is exposure blending exactly? As its name suggests, it involves taking several exposures of the same subject and “mixing” them in Photoshop using layers and masks. It is imperative that the framing be identical for each exposure, so it is important you use of a tripod, and ideally a remote release to minimize vibration and motion. The exact bracketing required varies from scene to scene and depends in great part on the dynamic range of your camera. Using my Nikon D700, I simply set it on “matrix metering” and take 3 exposures: 0, -1 and -2 stops. If you are just trying your hand at this, I would definitely recommend trying at least +2 to -2 brackets (5 images total), then adjust accordingly once you get more comfortable. Ultimately, however, only two exposures are usually necessary (0 and -2). I will sometimes blend in parts of a third one if I need more details in the shadows, but that is rarely required.
Now on to the blending.
1) Once the images are uploaded on the computer, process each exposure separately in RAW then open both exposures in Photoshop (here 0 and -2 stops).
2) Go to the darker exposure, select all (command+A), copy (command+C) and paste it on top of the lighter image (command+V).
3) Select the top layer (dark) and while pressing the alt button, click on the “add layer mask” button. This will create a “see-through” mask that will come in handy later to “paint-in” details from the dark exposure onto the lighter one.
4) In the layers panel, select the channels tab. Photoshop allows you to create a selection based on the luminosity values in the image. In essence, at the click of a button you can select all the “light” pixels. You can do so by pressing the command button and simultaneously clicking on the RGB channel icon.
5) You should now see a set of “marching ants”, indicating a selection appear on your image. While this can be used as your mask, I find that going one step further will provide an even better selection. To do so, press shift+option+command and click on the RGB channel icon one more time. This will intersect the previous selection with itself and select a narrower set of light values. You can repeat this as many times as you want, but I find that two selections (step 4+5 combined) are sufficient.
6) Now move back to the “layers” tab and make sure to select the dark mask we created earlier in step 3, by clicking on it.
7) Select the eraser (press E), select a large brush (400 px) with 0% hardness and ensure the opacity and flow are set at 100% (top panel). Make sure that black is your foreground colour (you can toggle between foreground and background colours by pressing X, if black is not already one of your colours, press D to revert to default colours, then press X to select black).
8) Here is where the magic happens: simply start erasing over the overexposed portion of the image (sky and/or reflections for example). You will now see that the darker layer is literally being painted over the lighter one. Because the selection is completely self-feathering, you need not worry too much about being very precise with your painting. I usually find that a few passes with the eraser are necessary to bring all the colours out. Notice that while the sky is being uncovered, the areas in the shadows (e.g. buildings or foreground) are hardly touched.
9) That is it. Once the result satisfies you, you can merge your two layers and continue on with your usual post processing routine.
10) If you feel that you are still lacking a bit of detail in some of the shadow areas, you could use an overexposed frame (+1 or +2) to paint in the necessary detail. To do so, simply copy the overexposed frame on top of your other layers. Then, create a “see-through” mask as done in step 3. Select the Eraser, set the opacity between 25 and 40% and flow at about 50%, make sure your foreground colour is black. Then select the mask and start painting over your image in the areas where you would like to reveal more detail. Since the opacity of your eraser is low, multiple passes may be required to achieve the desired result.
The process may sound a bit daunting at first if you are not familiar with masks and layers but once you give it a go you will see that it is fairly straightforward and the results will surely speak for themselves!