Just want to write a short post to show you an easy way to determine the correct shutter speed for Astrophotography. This is also known as the 500 Rule.
You may be seeing all those amazing photos of the core of our Milky Way Galaxy at night and thought to yourself “I have a camera, I want to do that”. Later I will be doing a more in depth look at all the aspects and settings for doing astrophotography but today I just want to talk about one thing; setting your shutter speed and that is where the 500 rule comes in:
If you have a full frame camera and you want to know what the maximum shutter speed you can use and still have all the stars in the photo be points of light and not blurs then just take the focal length of the lens and divide it into 500 to get the maximum shutter speed in seconds.
A simple example with a 20mm full frame lens. Take 500 and divide it by 20 and you get 25, 25 seconds in the maximum shutter speed you can set and still have sharp pin-point stars.
Now maybe you have a camera with a DX or APS-C sensor not a full frame, what do I do then? Say I want to use this Tokina 14-20mm f/2.0 lens on my D5600 or similar camera with an APS-C sensor. First convert the focal length to full frame which in this case would be 14mm X1.5 to give you an equivalent focal length of 21mm in full frame. Now take the 21mm and divide it into 500 and you get 23.8 seconds. Most cameras cannot set 23 seconds so I recommend rounding down to 20. 20 Seconds is the longest shutter speed you can set and keep the stars from blurring.
The 500 Rule a rule of thumb and in my experience its better to back your shutter speed down by 3-5 seconds. So for the 20mm full frame lens we talked about probably 20 seconds would be better than 25 but the 500 Rule will give you a good starting point.