A Stop is not a Stop

[Jon Ruyle]

So 1.4 - 2.2 is a 'stop' and 16-22 is a 'stop' but each stop does not representthe same


amount of light being blocked.

That's right. A stop does not refer to a quantity of light being blocked but a fraction of light being blocked. Exposure values are an exponential scale (in which one notch on the scale represents multiplication by a constant factor), not an arithmetic one (in which one notch on the scale represents an addition or subtraction of the same amount).

Changing and aperature by one stop has no direct translation to changing


speed by one stop.

I think you're confused. The idea is that closing the lens one stop (cutting area in half) has the same effect on exposure as doubling the shutter speed. A stop, in this sense, actually is a stop.

IS improves hand held by 3 stops (stops of speed not aperature)

Speed *and* aperture must change to get the same exposure. Most people in most situations fixate on one or the other, but of course if you change one without changing the other, you'll get a different exposure.

do we cosider this true over the entire stop range ie 1 sec exposure is improved by 3 stops


as well as a 1/60 exposure improved by 3 stops. NO

IS improvement is an estimate. The amount of improvement depends on the way you shake. If your hand shakes with a super high frequency but low amplitude, IS would provide no benefit. But this isn't how people really shake.


There may be some shake patterns for which IS can correct at higher shutter speeds better than at lower shutter speeds (and some people have said IS does not give the full benefit at low shutter speeds, ie, short focal lengths), but my experience is that a 3 stop IS does give 3 stops of improvement. With my 24-105, I can shoot about 1/4 second exposures at 24mm, or about 1/16 second at 105mm. But this is just an empirical thing, YMMV. (Eg, may be that the 24-105 does not have enough travel in the IS system to give the full 3-stop benefit at the wide end for some people's way of shaking).