When we shoot the stars, Milky Way, northern lights, or subject in very dark environment, it is challenging to adjust the focus accurately. This note suggest several tips to obtain sharp focus in practical manner. Followings are the steps to adjust the focus to infinity (such as the stars, the moon etc.).
Step #1: Use Manual Focus mode and adjust the focus to the farthest object such as mountain in far distance while still enough sun (ambient) light remains. Then use a tape to prevent from movement. If you use a zoom lens, use the same focal length as your intended shooting. Use gaffers tape so than there is no sticky residues after removing.
Step #2: If Step #1 is not possible or you loose the Step #1 adjustment position, or change the focal length, you will need to do the followings.
Step #2a: Set the camera exposure as your intended shooting, and align the infinity symbol to the center of reference line. Then take one test shot. You may use 1-2 stops bright exposure setting for this adjustment.
Step #2b: Review the image with the LCD monitor in the back of camera by using 100% or larger view of your intended object such as stars. Observe if it is taken with accurate focus. If not, remember how it looks in the LCD with large size view.
Step #2c: Adjust the focus ring to right slightly so that left side of infinity symbol is aligned to the reference line. Then take a test shot and observe. Is the image better focus than Step #2b result? Then ideal focus point is in the left side of the infinity symbol. Remember how it looks in the LCD with large size view (same magnification to be used).
Step #2d: Adjust the focus ring to left slightly so that right side of infinity symbol is aligned to the reference line. Then take a test shot and observe. Is the image better focus than Step #2b result? Then ideal focus point is in the right side of the infinity symbol. Remember how it looks in the LCD with large size view (same magnification to be used).
Step #2e: Depending how the image shows in the LCD display, you will be able to mallow down to the ideal focus point by repeating above steps. Once you find the ideal focus spot, put a tape to prevent from movement and remember where the infinity symbol is aligned to the reference line. If you lost the adjustment for some reasons, you can adjust the infinity symbol to the ideal focus point (from your memory or memo). You may need to repeat these processes if you change the zoom (focal length).
Step #3: Enjoy shooting.
Step #4: Enjoy the results.
Below images were taken with fine focus adjustment using above steps.