Answer.
You know, with a more informative headline on your posts you might get more and better answers.
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But, seriously, with a crop sensor camera you are correct... you have to factor in the "behavior" of the lens.
The 1/focal length rule of thumb is based upon 35mm film/full frame cameras specifically. A crop camera magnifies things... including camera shake. So, yes, with a 1.6X (Canon) APS-C, you should multiply the lens focal length that way to estimate what shutter speed you need. With a 400mm lens, this guideline suggests a 1/640 shutter speed would be needed by most people, to have a reasonably good chance of getting sharp, hand-held shots.
Now, as some others have correctly suggested, the lens focal length actually doesn't change with different sensor (or film) formats. A 50mm lens is still a 50mm lens, regardless whether it's on a so-called "full frame" camera or on an APS-C... or for that matter an even smaller "Four/Thirds", or even larger medium format... or anything else.
What changes is the way that 50mm "behaves".
On a full frame camera, 50mm is a standard lens. On APS-C, it's a short telephoto. On medium format it's a wide angle. Heck, on some of the point-n-shoot digitals with tiny little sensors 50mm is a very powerful telephoto. Or, on a large format film camera, it would be an extremely wide lens. What changes are "angle of view" (aka field of view) and subject magnification.
Although it's still 50mm in all those cases, it behaves differently and this includes susceptibility to camera shake blur.
People's ability to hold things steady vary, both naturally and with practice and good techniques they've learned to use. So, this is always just a rule of thumb, what shutter speed you can or should use. With a steady hand and 30+ years experience, I know I can get a pretty high percentage of sharp, steady shots using shutter speeds a little slower than the rule suggest. You might be different... able to use even slower or needing to use faster. You'll just have to try it yourself.
Another consideration is image stabilization. Most modern cameras or their lenses have this feature (called different things, depending upon the manufacturer: IS, VR, OS, OSS, VC, etc.). With stabilization, most people see two or three or even four stops worth of assistance. Two stops of assistance with a 400mm lens would mean being able to relatively reliably hand-hold it at 1/100 shutter speed. Four stops would be 1/25.
I've been shooting with various Canon IS lenses for fifteen years and love it... especially on telephotos that are harder to hand-hold steady. Canon makes a superb 400mm f5.6L... very sharp and fast focusing... but lacking IS. I instead use a Canon 300/4L with IS... sometimes with a 1.4X teleconverter that makes it an effective 420mm f5.6... still with helpful image stabilization. However, you have to keep in mind that image stabilization, great as it is, can
only help with camera shake. It can't do anything about subject movement blur. You still need to use faster shutter speeds to freeze moving subjects.
And, there are some exceptions. For example, high magnification shots such as are done with a macro lens are considerably more susceptible to camera shake, so a faster shutter speed is likely to be needed.
A flash also can be used to freeze movement... both camera shake and subject movement. Typical portable flash units at full power fire a very short duration of bright light... equivalent to approx. 1/720 shutter speed. (Fill flash, where ambient light is also part of the exposure, is another matter... it generally does not freeze motion of any type, or does so only partially resulting in a "ghosting" effect.)
And, of course, there are tripods and monopods that can be used to help steady a shot at slower shutter speeds. (However, in any SLR at shutter speeds between roughly 1/30 and 2 seconds you also have to think about internal vibrations from "mirror slap", which also can cause shake blur in images... Many cameras have a means of locking the mirror up to prevent this... Or Live View shooting might accomplish the same.)
chapjohn wrote:
...Extention tubes change your focal length.
No, they don't. Extension tubes only change a lens' close focusing ability, thereby making possible higher magnification, but at the cost of no longer being able to focus all the way to infinity. There is no change in it's effective focal length, when you add extension tubes behind a lens.
Teleconverters change a lens' effective focal length. (Yes, Canon calls their teleconverters "Extenders"... which might be a little confusing.) Teleconverters do not change a lens' closest focusing ability. That remains the same. However, a TC does magnify the subject and so can give similar end result as extension tubes.
Using
either extension tubes or teleconverters change magnification, which in turn can require faster shutter speeds to help insure a sharp shot without camera shake blur.