I believe that those who photograph at night provide us “day dwellers” with a totally different perspective on the world. Star trail photography is a technique that many people may never attempt but is not as difficult as it may seem. This topic will be presented in two parts. In this article, I will discuss the steps necessary to take star trail photographs in the field. In the part two, I will discuss the steps to merge a series of images in Photoshop CS5 to create the final image.
Most photographers have seen an image in which the stars make beautiful arcs and patterns through the sky.
Capturing these images on film was a simple process with great results, one simply needed to point their camera to the sky, lock open their shutter (using a cable release) and wait for a few hours (depending on how much movement you wanted to show).
This same process can also be used with today’s digital cameras, however the results will not be nearly as clean and sharp as those on film. The biggest problem with using a digital camera to shoot star trails is that they are subject to producing an undesirable characteristic in the image known as “noise.”
Digital cameras depend upon an imaging sensor to capture and “see” light coming in from the lens. To capture this light, the camera sends power to the sensor which allows it to see the light. This power and energy in and on the sensor will cause it to heat up. When your sensor heats up, some pixels on the sensor will change color or not appear similar to the rest of the photograph. This effect is referred to as noise.
Two things that produce more heat buildup on the sensor and as a result more noise are long exposures and high ISO’s. During a long exposure, the power needed to capture light is needed throughout the exposure, this means that the longer your exposure is, the more heat is produced across the sensor and the more noise. A high ISO will also bring about more noise in your photograph. To put it simply, by raising your ISO, all you are doing is pumping more power into your sensor to make it more sensitive to light. Because of noise, the process of creating satisfactory star trails with a digital camera becomes much harder.
It is here that we need to turn to more alternative methods of creating these images with our digital cameras. This process that I explain below can be done in any newer versions of Adobe Photoshop. However, I will be giving the steps in Photoshop CS5.
The way we get around the problem of noise in the digital age requires us to combat the two things that cause noise. We do so by shooting at a low ISO and by avoiding long shutter speeds. Do you see a problem here? How are we going to see the movement of the stars through the sky if we can’t keep the shutter open for longer than 30 seconds? Simply put, we need to take multiple images and merge them into one.
I like to set my camera to ISO 100 or 200 for shooting star trails; this will minimize any noise that would be created using a high ISO. A good rule of thumb for shutter speed is to not go much longer than 30 seconds. I also set my camera to shoot compressed JPEG files so I can get more images on the card and it will take less time to process later on.
To capture a timed series of images, you will need a cable release to lock open the shutter and force your camera to keep photographing (even after you go to sleep). Once you have your camera set up on a tripod and your image has been composed, make sure that all your settings are in order (f-stop, shutter speed and ISO). Also make sure that your camera is set to “continuous” drive mode which will ensure that after each exposure is completed, the next will automatically begin. If you forget to do this, you will have large gaps between images as the stars move.
Once you return to your computer, download the images to a folder on your desktop. This makes it much easier to find them when you want to merge them together. The process for merging these images together is quite simple despite the number of JPEG files you end up with. To process or make global adjustments to this files you will use the “actions” function in Photoshop, a little set of instructions that tells the computer to batch process all 500 of your photos automatically and consistently. All we need to do now is load the appropriate action onto our computer and into Photoshop so that we can use it to create our final star trail composite image.
How to complete this process will be the topic of my next article (arriving in two weeks).
Tags | Digital Noise, Forest Chaput de Saintonge, Night Photography, Photoshop CS5, star trails