captivecookie wrote:
I've heard one way to be sure is to always shoot double the lens. For instance if you are shooting with a50 mm lens, you would try to shoot at least 1/100 speed. If course this is a very general rule and as such there are many ways it can lead you astray.
That's way more than is usually necessary, but certainly would be safe.
The old rule of thumb with 35mm film (i.e., "full frame") was 1/focal length. So for a 50mm lens it would be 1/50. For a 100mm lens it would be 1/100.
With a crop camera, you have to take that into account too... with a Nikon, Sony, Fuji, etc. it's a factor of 1.5X... so a 50mm lens would be 1/75 and a 100mm lens would be 1/150. Canon crop factor is 1.6X, so a 50mm lens would need 1/80 and a 100mm lens 1/160. Micro 4/3 cameras (Olympus, Panasonic) have a 2X lens factor, so the rule says a 50mm lens would need 1/100, while a 100mm lens would require 1/200.
However, many cameras or lenses or both today have image stabilization which can help. It varies, but is usually good for at least 3 stops of assistance, which means with a full frame 100mm lens you should be able to get a reasonable number of sharp shots as slow as 1/12 (calculated by 100 ÷ 2 ÷ 2 ÷ 2 = 12.5).
It also varies a lot from person to person and even from camera to camera. For example, a DSLR has "mirror slap" that causes internal vibrations at slower speed (such as 1/12). A mirrorless camera doesn't have to worry about that, but still may have some "shutter shake" if using a mechanical shutter.
When it comes to people, our skill and technique matter. I often "get away" with using 1/2 focal length even on a crop camera. Good technique means keeping our elbows tucked in, cradling the camera and lens properly, pressing the shutter release rather than jabbing at it, standing with our feet about shoulder width apart, and even using something we find locally to help steady our shot. For example, the first shot below was done with a 50mm lens on a Canon APS-C camera where the rule of thumb would be to use a minimum of 1/80.... I shot at 1/30 by resting my elbows on a table (sort of a human tripod). The second shot was done with an 85mm lens, also on a 1.6X crop camera where I should have needed 1/136, but was able to lean against a door frame and instead use 1/80.
Another "trick" is to set your camera to continuous shooting mode and fire a burst of several shots. At least one of them is likely to be sharp.
Some good practice is doing panning shots of some sort of action. For this autocross shot I was hand-holding a zoom set to 90mm focal length on an APS-C camera (so the rule says I need 1/144 shutter speed). I shot at 1/100 in order to deliberately blur down the background, the faster rotation of the wheels and other stationary objects in the image. This is called "dragging the shutter". Usually I get fewer "keepers" doing this, there are some shots that just don't work out... but it can make for interesting images that capture the sense of speed. Subjects moving at different speeds can require some experimentation to find a shutter speed that renders just the right level of background blur effects
Finally, "dragging the shutter" can be combined with fill flash, the short duration of which acts like a fairly fast shutter. To do this with a moving subject the flash needs to be set to "2nd curtain sync" (with normal sync the motion blurs are recorded after the flash, so they make it appear the subject is going backward)... This was a zoom lens at 52mm focal length, handheld at 1/30 shutter speed. I don't know for certain, but estimate the fill flash duration was equivalent to 1/720 or faster shutter.