shutter durations - not what are marked

Started Mar 26, 2009 | Discussions thread
Marianne Oelund Veteran Member • Posts: 7,765
Exposure time - Rationale and Numbers

closeupfanatic wrote:

Trying to get interval timing to work for star trails and such, I
finally found the source of my problems: D700 shutter speeds seem to
actually go by factors of two from 1 second. Specifically, "15s" is
actually 16 and "30s" is actually 32 ! Meanwhile, the interval timer
works on real times, so an interval of 33s is needed for a continuous
series of "30s" exposures.

Has anyone been able to measure the short times to see if they do the
same?

After testing with a D3, I can confirm that they go by factors of two, but with a couple of little twists.
1. Compromise for 1/50 and 1/60

Accuracy for these speeds has high priority, which is problematic for designers as they don't fit the cube-root-of-two ratio for 1/3 stop separation. Nikon's solution for this is to split the error; 1/50 is slightly slow and 1/60 is slightly fast. In order to achieve this, the entire speed schedule is stretched about 3%, so that the 1-sec. exposure is 1.030 sec.
2. 30-sec. special case

If one extrapolates the power-of-two scale starting with the 1.030 sec exposure for nominal 1-sec, the time for 30-sec nominal would be 33 sec. Nikon have actually reduced this one value to 32 sec.

Although the nominal values displayed by the camera (i.e., the ones we know, love and use every day) aren't precise 1/3-stop steps, the actual implemented times follow this step size quite accurately (except as already explained above). The following lists give the nominal shutter time and the measured values for the D3.

Slow shutter sequence (Nominal -> actual):
30s -> 32.0s ..... 25s -> 26.1s ..... 20s -> 20.8s
15s -> 16.5s ..... 13s -> 13.1s ..... 10s -> 10.4s
8s -> 8.24s ..... 6s -> 6.54s ..... 5s -> 5.19s
4s -> 4.12s ..... 3s -> 3.27s ..... 2.5s -> 2.59s
2s -> 2.06s ..... 1.6s -> 1.63s ..... 1.3s -> 1.297s
1s -> 1.030s

Fractional shutter speeds (Nominal -> actual):
1/1s -> 1/0.970s .... 1/1.3s -> 1/1.224s ..... 1/1.6s -> 1/1.542s
1/2s -> 1/1.94s ...... 1/2.5s -> 1/2.45s ..... 1/3s -> 1/3.08s
1/4s -> 1/3.88s ...... 1/5s -> 1/4.89s ....... 1/6s -> 1/6.17s
1/8s -> 1/7.76s ...... 1/10s -> 1/9.78s ..... 1/13s -> 1/12.32s
1/15s -> 1/15.5s ..... 1/20s -> 1/19.6s ..... 1/25s -> 1/24.6s
1/30s -> 1/31.1s ..... 1/40s -> 1/39.1s ..... 1/50s -> 1/49.3s
1/60s -> 1/62.0s ..... 1/80s -> 1/78.0s ..... 1/100s -> 1/98s
1/125s -> 1/124s ..... 1/160s -> 1/156s ..... 1/200s -> 1/196s
1/250s -> 1/247s ..... 1/320s -> 1/310s ..... 1/400s -> 1/392s
1/500s -> 1/490s

If you get out your calculator, you can check these intervals against the exact 1/3-stop ratio (1.25992), and see how closely that has been followed. Measurements are highly repeatable, within a small fraction of 1%.

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