Focus on Stars Focusing Tool
THE PROBLEM
One of the greatest frustrations for many night photographers is focusing accurately at infinity for astro-landscape images. Infinity markings on lenses are often inaccurate, autofocus doesn’t work under starlight, and live view can be noisy at high ISOs in low light, making it difficult to confidently focus on stars.
THE SOLUTION
Focus on Stars is a next-generation precision focusing tool. It uses a Bahtinov filter that has been designed by a photographer specifically to be used with the wide-angle lenses that we typically employ for astro-landscape photography.
Focus on Stars is not the first photography-oriented Bahtinov filter, but I believe it is the best one.
The Focus on Stars filter provides a bright and clear diffraction pattern that makes it easy to confirm accurate infinity focus––even with 14mm or 15mm lenses. Simply place the filter in a standard filter holder, choose the brightest star in your scene and magnify the live view image to 100 percent magnification. You’ll see three pairs of bright spots around your target star, and as the image comes into focus, the pair of bright spots in the center will become equidistant between the pairs on either side. If this idea seems a little “fuzzy,” see the image below, as well as this video demonstrating the concept.
This image shows what a sharp star looks like through a Focus on Stars filter.
The Breakdown
WHEN THIS IS NOT THE RIGHT TOOL
As with all Bahtinov-type focusing aids, the Focus on Stars tool works best with normal to moderately wide lenses, because the diffraction pattern appears progressively smaller with ever-wider lenses. It is still useful with lenses as wide as 14mm, but it’s slightly more difficult to see the effect. If you don’t have much difficulty focusing on stars or planets, or you primarily use hyperfocal focusing, then you probably don’t need this device.
WHEN THIS IS THE RIGHT TOOL
If you struggle to get accurate focus at infinity, or don’t have confidence that your images are sharp until it’s too late to do anything about it, you should give Focus on Stars a try. It adds a step to your workflow, but adds a good measure of reassurance that your images will be sharp. It’s easier to spend a few extra minutes while you are out there on location than to go back and hope for the same conditions on another night.
