With the digital age stacking techniques make it possible to shoot a star trail in the middle of a city. But when I got my hands on a great medium format camera, I also ran into a problem: the star trail had strange patterns in the stacked image. Fortunately I found out why this is, and how to avoid it.
I have been shooting star trails for many years. My first star trail was made in the eighties of the previous millennium, using a 400ASA slide film and a remote to lock the shutter in bulb for about five minutes. At that time it was quite challenging to make a star trail as long as possible without over exposing the film. It needed a lot of experience which – by the way – I never had. Film was expensive and I couldn’t afford to use a complete film role for experimenting.
Digital photography made a lot of things much easier, including star trail photography. Exposing for five, ten, fifteen, or thirty minutes is possible, and you can check the exposure afterwards and do another try if the first time was faulty. You just have to be patient. How long you can expose depends on the ISO level, aperture, and the amount of ambient light or light pollution. I managed to expose for half an hour in a dark part of the Netherlands, where light pollution level is among the worst in the world. I had to rely on the dynamic range of the camera to turn down the exposure enough to see the stars. Just like the photo from the eighties, I used ISO400 for this 1800 second shot. The result is shown below.
But a long exposure has a major downside, especially with higher ISO levels. After fifteen or twenty minutes the amount of noise becomes very annoying, with the risk of rendering the image unusable. What the maximum usable exposure time will be is camera dependent of course. Although it might be interesting to found out, it is probably better to use a series of shorter exposures that can be stacked into one virtual long exposure. I think most of the people that do star trails use this method.
Using the stacking method makes it easy to do star trails in pretty much any location, even in the middle of the city by choosing the right combination between shutter times, aperture and ISO values. I used this technique for many years, with different kind of cameras. When I got the change of reviewing the Fujifilm GFX-50s I also made some star trails. To my surprise I stumbled upon a strange effect which I never seen before. There was some kind of moire effect visible in the picture, as shown below.
It took quite a while before I found out what caused it. It turned out to be the lens distortion correction that Fujifilm build into the raw files of the GFX-50s. And the funny thing was; Lightroom lacked the ability to turn this lens distortion correction off. After a lot of experimenting I found out I had to reduce the resolution by 50% to get rid of this effect.
Finally, after searching the internet for a long time, I stumbled upon a raw editor that was able to turn the GFX-50s build in lens distortion correction off. This was a piece of free software called RawTherapee. Even now, I still don’t understand why Lightroom lacks the ability to turn off the lens distortion correction. It means you still don't have full control over the raw files
Sometimes I get a question from someone, that is following my star trail tutorial, about these strange effects in the stacked image. And in all cases they have turned on the lens distortion correction in the raw editor they are using. It deforms every image just a little bit, and it varies with every image, even if the same lens and aperture settings are used. When stacking many images, the small deformations will form the patterns as I witnessed with the GFX-50s star trails.
So next time you are going to shoot star trails, make sure you turn off any lens distortion corrections prior to the stacking. Unless your camera has the lens distortion correction fixed in the raw file, you will be okay.
Have you ever experienced these effects in your star trails? Please let me know in the comments and specify what kind of camera you are using. If you found another solution for this problem, please let me know.