Originally posted by sinus007 
The realization wasn't about that the lens is too slow: I never dealt with a lens that long.
These facts are related, though - the long lens means you need faster shutter speeds, and a faster lens is one way to get that.
Not sure if you're aware of the old rule of thumb from the film days that recommends shooting no slower than 1 / focal_length in order to have a good chance at a sharp picture. So for a 300mm lens, that means you would want 1/300". Due to the "crop factor" that changes the field of view from film to digital, and since the rule of thumb is *based* on the relationship between focal length and field of view, this means that for our DSLR's, we really want 1/450" to have a decent shot at a sharp picture when shooting at 300mm. And since this rule of thumb was based on typical small print sizes, but we typically "pixel peep" our digital images much more so than most people ever did with film, ideally, you'd probably want an even faster a shutter speed most of the time.
Of course, SR helps, but it doesn't work work miracles, and SR based on moving the sensor (as is the case with Pentax) can be less effective at longer focal lengths than shorter ones.
Since this is just a "rule of thumb" and not any sort of ironclad guarantee, and because there are so many variables involved (including how steady *your* hands are versus the "average"), it's hard to say what shutter speed you really would need. But the point is, there is a time-honored way of guessing, so it shouldn't come as a surprise that 1/100" isn't really fast enough. You can get *some* sharp pictures that way, but your odds increase with faster shutter speeds.
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