The lens is the most important factor in the image quality of a landscape astrophoto. We outline all the most important traits of a great astrophotography lens in this complete guide.
There are a number of lens traits that will determine the quality and usability of a camera lens for astrophotography. Let me explain what sort of thinking should go into choosing and using a lens for making astrophotography and Milky Way nightscapes.
There are two basic traits of a lens that will affect how to take your landscape astrophotos: focal length and clear aperture size.
What Focal Length?
For simple non-tracked landscape astrophotography and nightscape images, you will generally want a wide angle lens. I usually suggest something 24mm or shorter on an APS-C camera or 35mm or shorter on a Full Frame Camera. Finally, about 16mm and shorter on a 4/3 camera will do best. These wide angle lenses offer some advantages when shooting images of the Milky Way:
- Wide angle lenses have a larger field of view (FOV) and allow you to frame more of the Milky Way. This trait lets you collect light from a larger area of the sky and offers a balancing compromise to a typically small clear aperture for light gathering capability. More on clear aperture later.
- Short focal length, wide angle lenses produce a smaller image size at the sensor allowing you to use longer shutter speeds without creating star trails from the Earth rotation.
The shorter the focal length, the wider angle the lens. Most APS-C sensor digital SLRs like the Nikon D3100 or Canon EOS T5i come in a kit with an 18-55mm focal length lens. It can zoom from a relatively wide angle 18mm to a medium telephoto 55mm.
Star trails (as shown in the images below) are caused by the rotation of the Earth. For any given angle of view, or any given lens, there is a certain amount of exposure time before the Earth will have rotated enough to start to “smear” or “trail” the stars across your image frame. 18mm on an APS-C sensor is considered a relatively wide angle lens but even so, the angle of view is narrow enough that you will start to see star trails on exposures longer than about 20 seconds.
It tends to be more difficult to take landscape astrophotos with longer lenses like a 50mm or 85mm because the narrower field of view makes movement of the stars due to the Earth’s rotation more apparent. This can be solved by tracking the stars but in turn adds complexity and extra expense for the equipment required to track the stars while make your photos.
Tracking is possible with the use of a manual barn door tracker or motorized equatorial mount , sometimes controlled by an autoguider that provides feedback for the motor mount movement. Star tracking is an essential technique for imaging of deep space objects with lenses and telescopes that have comparatively long focal lengths. For nightscapes, however, where we are usually capturing the landscape as well, tracking the stars will in turn start to streak the landscape in the foreground.
When getting used to taking untracked astrophotos, I highly recommend that you check whether the stars are trailing by reviewing the image and zooming all the way into the detail.
Rules to Prevent Star Trailing
I’ve heard of several rules that different astrophotographers use to determine how long your shutter speed should be to prevent star trailing. For full frame cameras, the chart below roughly uses the so called “500 Rule” which means that you take the number 500 and divide it by your focal length to determine the maximum number of seconds of your exposure before star trails are apparent. For example: If we have a 24mm lens on a full-frame camera, we can take 500 and divide it by 24 to get 500/24=20.8 or about 20 seconds.
Note that differences in sensor resolution, pixel size and even the direction you point your camera in the night sky will change how the rule works. APS-C cameras and cameras with higher resolutions sensors need shorter focal lengths to achieve similar shutter speeds without star trailing and the rule becomes something closer to a “300 Rule” for APS-C sensors the guide below. Basically, it differs by camera.
Also, pointing your camera toward the celestial equator line will cause more star trailing than near the poles due to the larger arc length swept by the stars in that portion of the sky. The important thing for you to do is to generally determine what maximum shutter speed will work best for your particular camera and lens combination. Start with the recommendations here for your lens and then adjust accordingly.
Once you have determined the maximum shutter duration with no star trailing for your lens or focal length of choice, remember it. That shutter duration will always tend to work for that particular lens on that particular camera. For instance, at 18mm on my APS-C cameras, I have found that 20 seconds works for most photos of the milky way.