500 Rule, Left Brain Thinking!

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We have a problem. The earth is rotating! It’s making the stars in our astro images blurry!!! Don’t worry, we have a solution. There is a simple rule to follow to avoid these streaky stars. Don’t get me wrong, star trails can make some really cool images but what if you want to get that tack sharp shot of the Milky Way? FYI- Right now is prime time for MW images in the northern hemisphere.

Here is where the rule comes in to play. It’s pretty easy to remember….all you need is the focal length of your lens and the number 500. Now simply take 500 divided by the focal length of your lens. Lets use an example where you have a 24mm lens. 500/24 is 20.83. This number is the slowest shutter speed you can use without getting movement in the stars. The key here is to always round down. In this case the number would be 20 seconds. Make sense?

Perfect, now let me throw in the kicker. This rule is based upon using a camera with a full frame sensor. If you are using a camera with a crop frame sensor you have to take that into consideration. The magic numbers for crop frame sensor cameras are as follows:

– Nikon = 1.5

– Canon = 1.6

– Micro 4/3 = 2

Now let’s use another example. Say you are using that same 24mm lens on a Canon 80D. You have one more step. First take the focal length and multiply it by the magic number. 24 x 1.6 is 38.4. Now use the 500 rule. 500/38.4 is 13.02. So now you have to round down to 10 seconds with the Canon crop sensor. That’s it! No more blurry stars.

Here is a fun Milky Way image from our recent Field Trip to Joshua Tree National Park where this rule came in handy. I used a Canon 6D (full frame) with a 16-35mm lens at 16mm. What was my max exposure time? Tell us the answer in the comments.


P.S. We have a cheat sheet for all of this over on our Patreon page.

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