Star Trails, Timelapses, Long Exposures: Whatever you're doing, you must know this rule!
The 500 Rule
The 500 rule, also referred to as the 550, 450 or 600 rule, is an essential guideline to employ when doing astrophotography. The 500 rule dictates how long you can leave your shutter open before you get ‘trailing’ from stars. Star trailing is when the stars begin to make a curved line or ‘trail’ on your image when the shutter is open for too long during an exposure.
The formula to work out how long you can leave the shutter open for a FULL FRAME sensor camera is as follows:
500 / Lens Focal Length = Maximum Shutter Time Before Trailing
For example, I have a 14mm Samyang lens on a Canon 6D, so the equation would be as follows:
500 / 14 = 35.7 seconds
This means I can leave my shutter open for 35 seconds before I will start to see star trailing. As you may have realised now, the higher the focal length of the lens, the less time you will be able to open the shutter before you see star trailing. Astrophotographers tend to use wider lenses so that they can fit more stars into the frame, additionally, they usually shoot landscapes and subjects in the foregrounds of their star images which demand wider lenses to achieve the desired compositions; not to say that great images can’t be taken with longer focal length lenses.
There’s another crucial variable for this equation which is the sensor size. Sensors smaller than full frame (FF is often wrongly referred to as 35mm when it’s actually 36mm wide; the confusion comes as the size is roughly derived from 35mm still film stock) are referred to as having a ‘crop factor’. For Canon-type APS-C sensors (70D, 80D, 550D) this is 1.6x, Nikon DX (D7200, D3500) it’s 1.5x and the Micro-Four-Thirds sensors (GH5) it’s x2.0. The formula is as follows:
500 / (Lens Focal Length x Crop Factor) = Maximum Shutter Time Before Trailing
For example, if I was shooting with a 18mm lens on a GH5S, the equation would be:
500 / (18 x 2) = 13.8.. seconds
To understand the practicalities of this rule, let’s imagine you are shooting a star timelapse and each star is a pixel. Throughout the exposure length (the time the shutter is open) these pixels will be receiving more and more ‘signal’ from light hitting the sensor making the pixels become increasingly luminated and brighter in your image. Once the recommended exposure time limit is exceeded - dictated by the 500 rule - that pixel will stop becoming luminated, cease receiving a signal from that star and the pixel directly adjacent will now start to become luminated, consequently a star trail begins to form. The star moves from one pixel to the next because of the earth’s rotation, the same reason that makes it look as though the stars are moving across the sky during a timelapse, when in fact, it is the earth’s rotation - chiefly - that causes their ‘movement’
Star trailing isn’t necessarily a bad thing: it all depends on what purpose you’re using your photographs for. If you’re shooting exclusively to make blended star trail composition (like the ones shown above), then some trailing isn’t really an issue; some timelapsers in fact create this star trailing effect by intrepidly breaking the 500 rule and shooting what essentially is a ‘long exposure’ photograph bypassing the need to blend and process them in post-production.
It's important to understand that this 'rule' is essentially a guideline and despite it's mathematical methodology, it's not a perfect science, in fact, abiding by the 500 rule can still result in trails once the image is closely inspected or 'blown-up'. To achieve truly sharp stars with no trailing, other factors must be considered: pixel pitch & aperture. These are yet more variations that are considered in the more advanced and accurate 'NPF Rule' ('N' for aperture, 'P' for pixel pitch/size, 'F' for focal length)
Other astrophotographical rules include the ‘Looney 11’ rule and the ‘Sunny 16’ rule, but we’ll cover those in subsequent blog posts.
If you’re on Facebook there’s some fantastic groups to join where you can view other people’s work and share your own!
The Art of Night Photography ( www.facebook.com/groups/theArtOfNightPhotographyGroup/)
Stars, Star Trails and Timelapse (https://www.facebook.com/groups/864409090259989)
Timelapse, Hyperlapse and Star Photography (https://www.facebook.com/groups/timelapser)
Astrophotography for Beginners (https://www.facebook.com/groups/775235275932996)