Right away I must admit that this is not my idea. I know that is true of many photographic tools but I once saw someone do something just like this and I wanted to try it out myself. The question was how to go about it. I improvised a bit and trusted the camera to do what I wanted which it didn’t always do so I have learned some lessons already.
The idea is to take multiple exposures of aircraft on the approach and then combine them into one image in which the aircraft appear multiple times. I would set myself up in a position and then take a sequence of shots as the aircraft moved through the frame. My first mistake was that, because I was taking the same shot each time, I thought the exposure would not change. Not true! The camera will make some minor tweaks and this will make the whole thing less easy. White balance may also vary but I shoot in raw so syncing that afterwards is no problem.
Since I was not using a tripod, the shots are not all perfectly aligned. I took all the images in Lightroom and used the Open as Layers option to Photoshop. Then I used the auto align layers option to get everything perfectly in place. It is surprising just how much you move doing something like this! I originally thought I could just set every layer above the base one to Difference blending mode and everything would pop right out. However, that didn’t work as I had hoped. The aircraft all had an odd color cast.
Instead, I put a layer mask on each upper layer and then painted in the aircraft one layer at a time. This is more time consuming but it did the trick. Of course, if the exposures are perfectly matched, you don’t have to paint too accurately. if not, the sky color is different so a far more accurate painting on the layer mask is required.
This was a fun thing to experiment with. SFO is a great spot since you can get parallel approaches in one direction and parallel departures on the cross runway. This puts lots of aircraft in a single shot which makes it more interesting. Another time, I will take the lessons from this time and try and get them a bit better.