Why does a 15-second exposure on my camera actually last 16 seconds?

Question

I was doing a test and wanted the shutter to stay open for 15 seconds -- no problem, I have a 15-second exposure setting in my camera. It seemed to stay open for about an extra second, though, so I started timing it. The 15-second setting takes about 16.1 seconds (+/- about 0.2 seconds or whatever my measurement accuracy is) between the first and second set of sounds the mirror makes. I also timed the 4-second setting, and it takes exactly 4.0 seconds (+/- my measurement accuracy); I measured the 30-second setting at 31.9 seconds.

For the sake of this question, I'm assuming that the 15-second exposure is taking exactly 16 seconds and the 30-second exposure is taking exactly 32 seconds, while the 4-second exposure is taking exactly 4 seconds.

I understand why 16 seconds would be more desirable (and probably easier to implement in software) than 15 (more-precisely one stop longer than 8 seconds), and I also realize that for a 15-second exposure, 1 extra second is a fraction of a third of a stop, and I probably wouldn't be able to see the difference between two otherwise-identical pictures taken at 15 and 16 seconds. But why is the setting called "15" when it's actually 16 (and it's relatively easy to measure that delta)?

Is this common among cameras, or unique to my camera's brand (Canon) or model (30D)?

Is there a spec somewhere that calls this out?

Should I be trying to measure more shutter speeds (in the fractions-of-a-second range) to see if my camera is mis-behaving?

If this is intentional, do the shorter shutter speeds that aren't exact halves/doubles also have different shutter speeds than they claim (e.g. 1/60 - 1/125)?

Asked by drewbenn

Answer

In "classic" cameras systems exposures varied by a factor of two between adjustment steps and by a factor of 2 with standard aperture number changes. The aperture f numbers usually provided vary by a factor of square root of 2 as the aperture is proportional to the square of the diameter and stop numbers relate to the diameter.
ie aperture is an area measure which is proportional to diameter squared. Yes, that hopefully makes sense if you read it slowly a few times :-).

The important thing is that a "classic" range of exposure times would be, starting at 1 second:

• 1 2 4 8 16 32 64 128 ...

The question is more why it would be labelled 15.

They may well be wanting to match the times to fractions of a minute so eg

• 1 2 4 8 15 30 60 120 ...

BUT leaving theactual values in a true power of 2 progression from 1 second.

However, going to faster steps starting at 1 second we also "run into problems".

1 1/2 1/4 1/8 1/16 1/32 1/64 1/128 is OK in fractional form BUT when written as decimal we get

1 0.5 0.25 0.125 0.0625 0.03125 0.015... ... you get the idea. Gets messy.

SO along the way people cheated slightly. The series may end up slightly like

1 1/2 1/4 1/8 1/16 1/30 1/60 1/125 1/250 1/500 1/1000 1/2000 1/4000 1/8000 ...

The decimal values get untidy around 1/16th to 1/125th but get tidy again below 1.250th and people largely don't use these.

The slight errors introduced are inside the margin of error in all but the most tightly controlled lighting situations and well inside what any eye-brain can guarantee they have seen (even if the eye can actually resolve it).

The first few differences are:

1/32 : 1/30 = 6.7% longer 1/64 : 1/60 = +6.6% longer 1/128 : 1/125 = +2.4% 1/256 : 1/250 = +2.4% at all settings above here

Your 15 seconds shown, 16 seconds actual is 6.7% longer than shown.

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BUT many modern cameras blow all this away with shutter speeds in 0.3 EV or other steps that seem good to them. Worse, auto ISO or shutter speed or aperture systems may choose semi random settings so you MAY see eg 1/325th of a second. Very hard on classical system sensibilities ;-).

Answered by Russell McMahon

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