How long does a 30 second exposure take?

This seems like a trick question akin to “How long did the Hundred Years War last?” (116 years, it turns out) or “Who is buried in Grant’s Tomb?”. Ulysses Grant is not buried, but entombed (above ground) and he shares that space with his wife, Julia. So while the question seemed to provide the answer – like these two seemingly obvious cases, the 30 second exposure, it turns out does not take 30 seconds.

I have 3 DSLR cameras: a Canon 40D, a 50D and a 5D Mark II.  The 40D can shoot 6.5 frames per SECOND – that’s faster than anyone can say “click” six times.  The 50D tops out a 6.3 fps (frames per second), and the 5DII is a still-speedy 3.9 fps.

But I take long exposures in night environments. A 30, 60, or even 480 second exposure is not unusual in my nocturnal world. Surprisingly my cameras that can take 3.9 to 6.5 frames per second under good light, actually take an average of 32.6 seconds for each 30 second exposure and it seems to not matter at all whether the exposures are RAW or JPG, whether the card is a speedy one – a 30Mb/second or a paltry 8 Mb/second.

How is it that a 30 second exposure takes 32.6 seconds? I asked Canon about that and didn’t get a very good answer. Something about “longer exposures take longer to process.” Indeed, some sleuthing and testing reveals that on average as exposures go longer than 1 second, they incur a penalty between shots of about 10%.  Two 20 second exposures take about 21.8 seconds. Two 30 second exposures take 65.8 seconds (not 60). Fortunately after 30 seconds, the penalty remains at about 2.8 seconds and does not increase farther.

Why does that matter you might ask?

It may matter a lot. A meteor streaks by every minute or so under a great meteor shower like the Perseids which peaks on August 12/13. A typical Perseid shower tosses out a meteor once a minute on average. Taking continuous 30 second exposures means about 10% of the possible streaks will be missed.  We *could*  take longer exposures, but then two things happen that are not so much fun.

1. The stars begin to noticeably streak.
2. Sky glow and light pollution begin to decrease the contrast between the stars and meteors and the dark sky.

What’s more, the delay between shots also means that a plane flying across the sky will have gaps in successive images. When joining the images together (stacking) the star trails formed by their motion may have gaps as well even though those gaps may not be readily obvious. In the image below, the gaps are not very noticeable until you see the larger sizes.

Granite Park Star Trail - 59 minutes with "gaps"

As a result of the “processing delay” it is necessary to change the way images are captured.

• A delay of 3 seconds may need to be be included when attempting to take successive exposures using a timer on a Canon body. UPDATE: The Canon 5D Mk II apparently is able to buffer the writing to the card so that a second shot can be started with a delay of only one second – however if you use the continuous drive mode a 2.8 second delay is introduced.
• We will want to shoot a long as we reasonably can so there will be fewer gaps.

Considerations like these are just one of the subjects we endeavor to teach participants in our Star Circle Academy Workshops.  I hope you’ll join us on one soon!