You may have heard it elsewhere as the “600 rule”. I first heard about the rule while visiting the Looney Bean in Bishop, California in 2008. Five photographers sitting in a coffee shop pouring over their laptops reviewing what they recently bagged are bound to start talking. It was my good fortune that one of those present was the very talented Brenda Tharp who first quoted the 600 rule to me.
I, however, have repeated the rule as the “500 Rule” because I think 600 is overly optimistic. What is the rule? The rule states that the maximum length of an exposure with stars that doesn’t result in star streaks is achieved by dividing the effective focal length of the lens into the number 600. A 50mm lens on a 35 mm camera, therefore would allow 600 / 50 = 12 seconds of exposure before streaks are noticeable. That same 50 mm lens on a 1.6 crop factor camera would only allow 7.5 seconds of exposure.
But Wait. The Rule Isn’t All That Great!
The real number is quite subjective. A little math reveals that on the Canon 5D Mark II (a full frame camera), with a 16mm lens a pin point star on the celestial equator moves from one pixel* to the next in 5.3 seconds. But the 600 rule would allow 37 seconds of exposure and the 500 rule 31 seconds. Both rules will produce streaks on the sensor! The visibility of those streaks will depend on the finished print size and viewing distance. Print it large and stand close and the streaks will be obvious.
So what does a 30 second exposure look like at the pixel level:
Clearly those stars are streaking across about 5 pixels* just as the math would bear out.
What is going on here? The Canon 5D Mark II images are 5634 x 3753 pixels* from a sensor that measures 36 x 24 millimeters. Dividing 36 by 5,634 reveals that the distance from the center of one pixel* to the next is a scant 0.00639 millimeters (or 6.4 microns).
The formula for calculating the distance in millimeters (d) that a star travels across a sensor due to the earths rotation looks like this:
d = t * f / 13750
Where t is time in seconds, f the effective focal length and 13750 is, well 13750. Is the math scaring you a bit… don’t worry… we’re almost done. Earlier we calculated the pixel* to pixel distance as 0.00639, what we want to find is how long (t) it takes for a star to move that far on the sensor.
0.00639 = t * f / 13750
Solving for f = 16mm we get a t value of 5.3 seconds as I asserted earlier.
But how does that calculate out on a different sensor, the Canon 50D, for example?
The Canon 50D has 4770 pixels across 25.1 mm or an inter-pixel* distance of 0.0053 millimeters. Substituting into the earlier equation we find that a star marches across a pixel on the 50D with the same 16mm lens in 2.83 seconds. With a 50mm lens on the same camera… the bad news is the star is speeding from one pixel* to the next in less than a second!
What does an image look like with a 30 second exposure at 16mm on a full frame camera? Remember that the streaks will be 40% longer on the cropped Canon 50D.
Scaled down to only 16% of the original image size or seen from a distance no streaking is obvious! We will try not to twitch knowing – because we pixel peeped – that the stars are really dashes not nice round pinpricks of light. And indeed only the eagle eyed are likely to notice the dash-like nature of the stars until the photo is printed large, say at 20 x 30 inches.
What Can We Conclude?
- Streaking starts a LOT sooner than any rule you may have learned.
- The time it takes to streak depends on the inter-pixel* distance (sensor density / mm) and the focal length.
- How much streaking to allow depends on your aesthetic tolerances.
- You can not get more or brighter stars by exposing longer; starlight has already given up on one pixel* and moved on to the next in just a few seconds.
- The longer the focal length, the more impossible it becomes to prevent streaking.
- Gaps in your star trails may be unavoidable if the inter-shot delay (normally 1 second) is long enough to skip pixels*.
I carefully added asterisks* to every location where I wrote the word “pixel” in a way that might imply your camera collects light in pixels. You might be wondering why I did that. The answer is: your sensor is comprised of sensels, not pixels. It takes 4 to 9 sensels to create a single pixel depending on the de-mosaic-ing algorithm your camera uses. Maybe you aren’t that picky, but I didn’t want to hear complaints from the purists.
I particularly relish this epiphany because I reported long ago that “longer exposures do not result in more stars“. I just never got around to doing the extra bit of math – or the experiments – to prove out my assertions.
Real Final Note
A commenter has rightfully taken me to task by pointing out that the perception of a streak is dependent on many things other than just the actual sensor values recorded. In particular, if the image is not enlarged much some streaking will be scarcely or completely unnoticeable because the feature will be too small for the eye to perceive. The problem with this assertion is that it assumes a lot of preconditions: e.g. how large the print is, how far from the print a viewer stands, and the subjective experience of the viewer. My real world experience has led me to conclude that it is a reasonable goal to keep the streaking to 2 to 3 pixels or less because that will provide the greatest possible usable magnification (finished viewing size). There would be no point to collecting a high megapixel image if you can not produce a print proportionately larger or more detailed than a lower megapixel image!
Here is an example that makes my point. I love this image captured on a Canon 5D Mark II. When printed at 20×30″ and viewed at 4 feet there is some streaking. Perhaps only a critical eye would notice, but even an untrained eye will notice when viewed from two feet away.