Why is the shutter speed sequence inaccurate by design?

[Larry Huppert]

I was spinning through the numerical sequence of shutter speeds on my meter, and found the actual sequence a bit odd when you get beyond 8 seconds.

In half steps it goes:

1 1.5 2 3 4 6 8 10 15 20 30 45 60 90 120...

I'd guess the accurate sequence would really be:

1 1.5 2 3 4 6 8 12 16 24 32 48 64 96 128...

Some of the differences are insignificant, but some amount to 2/10 of a stop ina ccuracy. I know the accepted sequence rolls of the tongue easier, but given tha t we time these long exposures with kitchen timers or similar devices, why not u se the more accurate numbers. It just seems weird that meters which measure lig ht to within 1/10 of a stop would deceive you by 2/10 of a stop by design.

[David Lewis]

Harry: Great question! I believe the sequence in half-steps should go by the square root of 2 (1.41), i.e. 1, 1.41, 2, 2.83, 4, 5.66, 8, 11.31, 16, 22.63, 32, 45.25, 64, etc. But this still doesn't match your meter. The answer is sure to reveal some interesting history. I'm staying tuned for the answer. Dave

[David Lewis]

Oops, sorry Larry, I can't see straight enough to spell your name. Dave

[Glenn Kroeger]

David: F-Stops go by square root of two since aperture area goes by square of radius, but time is linear, so shutter speeds should go by 1.5 for half stops, and 2 for full stops. On most cameras and shuttered lenses, the full stop sequence gets off-track between 1/8 and 1/15 and then again between 1/60 and 1/125, instead of 60 and 120? Few shutters have half-stops. Larry's second sequence is logical, but then again, that should be the sequence for film speeds, but they diverge after 64 as well... thank goodness the eye is only sensitive to about 1/4 stop.

[Pete Andrews]

No mechanical shutter is going to be accurate to 0.2 of a stop throughout it's speed range. If you're lucky, it'll be consistent to about that sort of accuracy.Electronic shutters as found in modern 35mm cameras actually lie about their speed sequence. 1/60th is actually closer to 1/64th; 1/30th is 1/32nd, etc. This is because it's easier to design a circuit to divide a fixed frequency in steps of 2, and so derive the shutter timing. If you test one of the new shutters, you'll find that's the case.

No-one doing normal everyday photography needs accuracy to 0.2 stops, and accuracy of 0.1 stop measurement of exposure is pure fantasy.Outdoors, the lighting could easily change by that much in the time it takes to transfer the meter reading to the camera shutter.

[Pete Andrews]

BTW. The root 2 sequence is the correct one. Exposure uses a geometrical progression, whether you change the light, or the time.

[Larry Huppert]

I'm concerned with timing in the range of full seconds, not fractional seconds, so the in the sequence I'm talking about the accuracy of the shutter is irrelevant. Out beyond several seconds, I can use "B" or "T" to time an exposure much more accurately than 2/10 of a stop. With regard to metered exposure, the point about exposure changing outside is well taken, however on a sunny-16 type day, the readings are very stable. I'll also do fairly long exposures of interiors where all relevant light is controlled.

[Pete Andrews]

In that case the series ought to go:1,3,6,10,18,35,60....or such like, in order to compensate for reciprocity law failure.;^)

[Glenn Kroeger]

Pete is obviously right, it has to go by 1.4 since its geometric. That will teach me to try to think late at night!

[Glenn Kroeger]

But the real, historic,, question is not what the shutter speeds really are (since most have error) but why did some manufacturer decide they needed to be labeled as 1/8 1/15 instead of 1/8 1/16 and 1/60 1/125 instead of 1/60 1/120. With electronically timed shutters, powers of 2 are easy, but were these "shifts" based on mechanical shutters that switched gear trains at these points?