How To Use In-Camera Blend Modes For Multiple Exposure Photography (Pt I: Average Mode)

Interested in making multiple exposure photographs? One of the most confusing aspects of this photographic genre is understanding how the various in-camera blend modes work. For this reason, I thought I would do a series of posts on what each of the available blend modes do and how to use them effectively.

To start with, what are these blend modes? Mirrorless and dSLR cameras offer four in-camera blend modes at this point in time: Average, Additive, Bright (sometimes called Light), and Dark. However, there are two caveats. The first is that not all brands/models of cameras offer all four of these blend modes. The second is that if you are using a smartphone for your multiple exposure photography, you might have more than these four blend modes, depending on the phone and/or the multiple exposure app you are using. I have seen apps that offer up to 17 of the 27 blend modes that are available in Photoshop. However, for this series, I am going to focus on the four that are currently available in ‘regular’ cameras. If you are using a smartphone, these four modes work the same way in phones as they do in cameras.

Enough intro…..let’s get down to business. For this post, I am going to focus on the Average blend mode. This mode, along with the Additive mode, are the ones that produce the kind of results that most people think about when they imagine a multiple exposure photograph. More specifically, they yield results that are similar to what you would get from making a multiple exposure in the film camera days (assuming you are old enough to have used film cameras :).

So what happens when using the Average mode? When making multiple exposure images using this mode, the camera takes the exposures you make and simply blends them together. But because your camera is also a computer, it does something very helpful. It calculates exactly how much ‘weight’ to give each exposure so that they contribute equally to the final image and so that the combination of images yields a properly exposed final photo (taking into account what your exposure compensation setting is). This is important because each exposure allows more light into the final result of the multiple exposure. If the camera didn’t make this calculation for you, the final result would be overexposed.

Why would the final result be overexposed? With your exposure compensation set to zero, the first exposure would be normally exposed, just as if you were taking a ‘normal’ photo (exposure compensation set to 0 means that the average of all tones in the photo would be middle gray, because that’s how metering works). However, each subsequent exposure would add more light to the image, meaning that the tones in the final multiple exposure would no longer average out to middle gray. Add enough images and ultimately the entire photo will be a ‘blown out,’ overexposed, white mess. Thankfully, in Average mode, the camera internally calculates how much to underexpose each of the component images so that the final combined image comes out correctly exposed.

What is this calculation, and how can it be emulated in Photoshop? You will want to know this if your camera doesn’t have the ability to do multiple exposures or if you want to combine single images in order to turn them into multiple exposures. The answer lies in the Photoshop opacity slider. To emulate the result of the Average blend mode (approximately, since different camera manufacturers tend to mix some secret sauce into their calculations), the opacity of any given layer should be set to 1/the layer number expressed as a percent. The Photoshop blend mode for each layer should be set to Normal. At first blush, that sounds complicated, but it really isn’t, so hear me out.

Let's say, for example, that you want to combine four images into a single ‘pseudo-multiple exposure’ image in Photoshop, emulating the Average blend mode. The first image is the first layer. So the opacity should be set to 1/layer number or 1/1, which would mean the opacity, set as a percentage, should be 100%. The next image you bring in would be on the second layer, so the opacity should be set to 1/layer number or 1/2, which would mean the opacity, set as a percentage, should be 50%. Easy, right? The third image is brought in on layer 3, so we get 1/3 or 33%. Do the same for that fourth image, and you get 1/4 or 25%. Add as many image layers as you want, but adjust the layer opacity appropriately and you have a very close approximation of the Average blend mode (approximate because, again, camera manufacturers tend to add a bit of their own flavoring).

Let’s look at a real-life example using three photos I took of black and white foam core.

© Howard Grill

© Howard Grill

© Howard Grill

These three images are the ones that make up the three-image in-camera multiple exposure using the Average blend mode that is seen directly below.

© Howard Grill

If I take the three component images and load them into Photoshop as three separate layers, with the first photo being on the first layer set to 100% opacity, the second on the second layer set to 50% opacity, and the third on the third layer set to 33% opacity, I get the result shown below. It is very close to what came out of the camera in Average blend mode, right?

© Howard Grill

The result is actually just a tad darker than what came out of the camera (there’s that secret sauce I mentioned), but with the addition of a simple curve in Photoshop the results become indistinguishable, as seen below.

© Howard Grill

I will end this little foray into the Average blend mode by reiterating that using this blend mode instructs the camera to lend essentially equal weight to each of the images that the multiple exposure image is composed of. That may or may not be what you want for your final result. Maybe you would prefer that one, or more, of the images that make up the multiple exposure become more dominant. What then? Then you switch to the Additive blend mode. And that will be the topic of my next post in this series. So stay tuned!

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