The general rule is 1/(focal length)(crop factor). For example, say you're shooting at 200mm with a 40D (crop factor 1.6). You should be shooting at 1/(200*1.6) = 1/320s.
And we need to remember that this rule of thumbs heritage comes from the days of 35mm film.
General rules are changing....with VR/IS and...
I'll get to that in a minute
I don't beleive the part about the crop factor. The geometry is determined solely by the focal length.
True, but the rule of thumb was based on the focal length geometry of 35mm systems.
As such, whenever the effective field of view is "X", you need to convert the actual focal length to the 35mm equivalent to apply the 35mm-based rule of thumb.
But if you don't want to do this conversion to 35mm(equivalent), then the new rules of thumb are:
For a Nikon 1.5x crop body dSLR:
{0.667/(focal length)} sec
For a Canon 1.6x crop body dSLR:
{0.625/(focal length)} sec
YMMV on if this makes for easier math.
How slow you can go is very much a factor of how steady you are -- all of the above is a decent rule of thumb if you are using film and need to wait a few days to develop the photo and see if it turned out.
Agreed, and it was the cost and slow turn-around of film that had an influence on what was an acceptable risk, based on the expected probability of success, and its associated costs: you didn't want to waste money on shots that would fail, nor did you want to get your film back a week later and discover that you had failed, so the level of acceptable risk was lower relative to what we're willing to risk today (see further below for more).
Do note however, that the question of "how steady you are" is a very important one. There can be a lot of skill transfer applicability between being steady for photography and being steady in weapons marksmanship. This includes body positions that provide better support (and self-support) for a steadier platform, and to similarly brace yourself against objects. There's also "trigger squeezing" techniques (to not jerk) and even breathing (breath hold) techniques.
Part of military research on aiming accuracy has to do with a a known phenomina of a human tremor / oscillation ... IIRC, its at ~1.6 Hz...and a lot of their training techniques for steadier aim has to do with minimizing these movements. For example, holding a camera with both hands against your forhead (think 'Optical Viewfinder') forms two triangles with relatively short 'moment arms', and the camera will move around a lot less than when even a smaller camera is held out at arm's length (think P&S with LCD display). Bottom line here is that if you get firmly braced and "sandbagged in", your technique can be nearly as good as a full blown tripod.
For example,
this night photo was taken at 1/6sec @ 38mm(e), which means that versus the rule of thumb which would have called for a 1/45sec to 1/30 sec ... 1/15... 1/8 --> it is around 2-2.5 stops under the rule of thumb. How/why? Because the "hand held" technique was able to compensate: the support position was firmly braced against a streetpost.
And getting back to
H2Ockey, there's been some changes in technology that have allowed the rule to become less important. I think that the major ones are:
(1) Optical path correction/stabilization ...ie, VR/IS stuff.
Some systems claim to be as good as 4 stops. This improves your number of keeper shots.
(2) Instant feedback from digital & the LCD display.
You can try a shot, give it a quick review to see if seemed okay or was obiously trash, then decide if you want to try again. This lets you improve your technique rapidly, as well as improve your number of keepers.
(3) 'Bottomless' film, from digital.
Didn't like the above blurry shot? Delete & try, try, try, try again...unlike film, there's virtually no 'per shot' cost, other than your time and battery charge status. This similarly lets you improve your technique rapidly, and improve your number of keepers.
-hh