Re: Shutter speed versus focal length
I agree with Don's answer. I recently saw a posting on another forum in which this question was being passionately debated without a resolution. To me it's easiest to think of it as follows.
Let's assume we have two cameras (a 1Ds and a 20D) each shooting the same scene with a 100mm lens. The 1Ds has a 24x36 mm sensor, and the 20D has a 15x22.5 mm sensor. Let's further assume (using round numbers) that the shutter speed in both cases is 1/100. Finally, assume that while the shutter is open, the photographer's hands twitch to the extent that the camera moves horizontally by an amount we'll call x.
It's easy to see that this movement x has a larger relative effect on the smaller sensor than the bigger one, because x/22.5 is bigger than x/36. In other words, while the shutter is open the field of view of the smaller sensor changes more (in percentage terms) than it does for the bigger sensor. This, of course, means you get a more pronounced blur and loss of sharpness.
What if we set the shutter for the 20D at 1/160? Assuming that the "twitch" moves the camera at the same speed (x per 1/100 second) as in the first example, it will now move a shorter distance due to the faster shutter speed - instead of moving x, it will move x/1.6. Thus, its relative movement now becomes x/(1.6)(22.5), which is the same as x/36 as with the 1Ds. This is why it is appropriate to use the "effective" focal length in the shutter speed rule of thumb.
Of course, this says nothing about whether the heavier 1Ds is more or less prone to movement via "twitching." On the one hand, it takes a more violent twitch to move a heavier camera a given distance than a light camera. On the other hand, the increased fatigue from using a heavier camera all day may produce more violent twitches! I think I'll let somebody else do a detailed experiment on this effect... [img]/ubbthreads/images/graemlins/smirk.gif[/img]