Focusing can be one of the more daunting nightscape photography tasks. Since auto-focus on the modern DSLR and Mirrorless cameras is both unbelievably fast and amazing sharp it's a task we usually leave to the electronics. However when the lights go out the auto-focus stops working.
Switch that camera into manual focus and while you're at it make sure you switch the lens as well. There is nothing worse than getting good focus and leaving auto-focus switched on in one of these two spots thus losing that focus the instant you touch the shutter.
Set your exposure somewhere close to what you think you will need. Your aperture should be wide open, shutter speed high enough but not so high that you get tailing in the stars and ISO as high as you think you'll need. You can tweak these settings later but right now you need to be able to at least see some stars on the back of your camera.
Turn on your LCD screen and aim the camera at the brightest star in the sky. If you can't find a really bright star try to find a distant light. Once you have a point of light located, center it in your LCD and magnify as much as your camera will allow.
Reach up and rotate that focusing ring until that point of light become as small and sharp as you can possibly get it. You may want to repeat this a couple of times to you sure it's tack sharp.
Level your camera and capture an image. Now check that image on the LCD, zoom in and pan around a bit to make sure you have good focus over the entire image. I've run into several lenses that had misaligned elements and were tack sharp in the center but crazy out of focus on one side or the other.
Check Focus Often
You are now ready to start shooting. Remember to check that focus every time you move your camera, change focal length, put your lens cap on and take it off, anything that could shake or jostle the camera. Trust me, there are few things worse than setting focus that first time and "assuming" you are still good while you shoot for a couple of hours and then find out once you offload your images that somewhere along the line something moved. I wish I could say that I've heard of this happening to others photographers but I'm afraid I speak from experience. Don't let it happen to you.
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