That is only theory and doesn't apply to everyone or every lens. And, doesn't hold true for lenses that are oversize or heavy. But according to the theory, 1/500 second would be the answer.

Yes, it is a matter of angle of view or equivalency to a standard 35mm camera focal length. Thus with Canon's 1.6 crop factor on a number of its cameras, a 100mm lens would require a minimum shutter speed of 1/160th of a second.

While it is a good rule of thumb, there is nothing inherently magic about the number. You will not find that 1/159th is blur-city and 1/160th sharp as a tack. It is a good place to start in finding where your own threshold speed lies. If you are shooting fuzzygraphs at 1/160th, then you need to use a higher speed, no matter the rule.

Thank you guys for your prompt replies.

I had a feeling it was an angle-of-view issue (sigh). It just seemed that shake-blur would be related to a-o-v.

My wife actually has the 20D. I got it for her last Christmas, with a Sigma 28-300mm lens. She birds, and needs a long lens.

This year, I got her a Sigma 170-500mm lens, which is just beautiful in both construction and performance. She got her lens a bit early (she's had it a week, now), because I am terrible about holding back goodies.

I suggested she use 1/800s second if hand-holding, and 1/500s if using her monopod, when out in the field. She has a steady hand and does quite well even hand-holding at 1/500s.

As an aside, both she and I have found that the added mass of the monopod significantly steadies the camera even if the monopod is not touching the ground. Kind of like a poor-man's image stabilization. The drawback, however, is that it is added mass...

For blind work, or open work where the birds are used to people, she is getting a tripod and ball head (they're under the tree at this moment). That will effectively eliminate the shutter problem.

As serious photo-birders will tell you, though, a tripod is just not always practical in the field. Those darn birds just hate to pose! I swear, if a bird spots a camera in your hand, you'll capture nothing but a bird butt!

Thanks again.

Roger

Because I received other advice from an "expert" in another (unnamed) forum, I checked with a professional optical engineer. Without going into technical details (which were extensive and related to the circle of confusion calculations for depth of field), the minimum hand-held shutter speed rule should be stated as:

The minimum shutter speed for a hand-held camera should be the reciprocal of the effective full-frame 35mm focal length.

For a Canon EOS-20D, this would be S = 1/1.6f, where S is the minimum shutter speed and f is the true focal length of the lens.

Thank you all for your assistance.

Roger

Something you might consider, if you're willing to invest some money, is that the Nikkor lenses with vibration reduction (VR) let you throw this rule of thumb out the window. I bought the 18 - 200 (24 - 300 after the conversion) 3.5 - 5.6 lens with my Nikon D80 and the VR works better than I ever imagined. Even at higher ends of the zoom range I can use shutter speeds down to 1/15 (and sometimes lower!) of a second and get sharp results. Obviously this won't work if you're shooting fast moving subjects, but I imagine you're more interested in the birds when they are at rest than in flight. I bought the lens because everything I read is that it was a very highly regarded zoom in terms of sharpness and quality, but after two months of using it, I have to say that the VR is amazing, and has enabled me to get many, many shots I couldn't have gotten without it or without carrying a tripod around in the field. They can be had for about $800, which sounds a bit steep, but having spent more than that on the lens in Europe while traveling, I still feel like I got a heck of a deal.

Just a thought,

Stephen