Since embarking on my journey of creating multiple exposure images in Photoshop I have been both confused and intrigued when setting opacity over multiple layers. This has largely been witnessed when processing a large number of images (>20) using techniques developed by Pep Ventosa, but this would apply to varying degrees depending on the number of layers involved.

One of the anomalies was witnessed when applying a similar opacity to each layer. Even though the opacity setting was consistent throughout it became clear that the resulting impact diminished as I moved down the layers. That is to say that when enabling/disabling the top layer a change was clearly evident, but the further I moved down the stack of layers any change became less and less evident. In some cases, this came to a point where lower layers in the stack (including the base layer) showed no discernible impact at all when turned on/off.

Secondly, depending on the opacity set to layers above, the impact of the base layer could vary widely, from having zero impact (as above) to having an impact that was far too great.

I have seen a lot of confusion over such settings and have been unable to find anything that might offer an explanation. Following my own experiments, I have come up with the following theory. Disclaimer - I do not claim to be knowledgeable on this subject, all information comes from my limited understanding of the process and opinion based on my own experiments. However, with that being said I am very confident that even if this theory is flawed it will help readers better understand the relationship of opacity when applied over multiple layers.

Example scenario

Note that I make no claim that the math is accurate (math is not my strong point), but results do seem to correlate with what I witness in Photoshop.

Let’s say we have 10 layers in Photoshop and apply the same opacity of 10% to all but the base layer (NOTE- it is assumed the bottom/base layer remains at 100% opacity)

impact of layer 10 opacity = 10%

cumulative impact of remaining 9 layers = 90%

effective impact of layer 9 opacity = 10% of 90(%) = 9%

cumulative impact of remaining 8 layers = 81%

effective impact of layer 8 opacity = 10% of 81(%) = 8.1%

and so on for remaining layers.

It can be seen that there is a diminishing return for each layer working from the top. But what does this mean in real life? The most obvious impact may be that results can change wildly as layers are ordered differently (ie those put to the top would have a more significant impact), but as long as this phenomenon is understood then opacities can be modified to suit (ie lower opacity at the top, increasing throughout layers).

To demonstrate this graphically I have plugged this data into a spreadsheet (for 20 layers) and produced the following charts. Note the relationship between opacity setting and the impact of the base layer (last in chart), and how this effect changes dramatically as opacity increases.

Video option

Still feeling like you are in the dark, or confused by this wordy explanation? The following video may help your understanding and visual an end result. Although I can’t say I’m great in front of a camera I think the medium may make the key points easier to understand.

YouTube video

Conclusion

I believe that the relationship between opacity setting, layer ordering and the resulting impact of the base layer should be understood. Doing so will allow more educated decisions to be made during the process of creating multiple exposure/multi-layer composites. While I make no claim on the accuracy of the math (if indeed flawed I would appreciate feedback), I do feel that work may fail to meet expectations due to a lack of this understanding.

If anyone has discovered another source for such information or has questions, please contact me via the site contact form.

Opacity set to 5% across all layers (base = 100%)