“With the 1.6 multiplier the effective focal length was 640mm” – that is not true – crop factor has nothing to do with focal length – that is common myth (mistake).

Actually it’s neither a myth or a mistake, at least not any more than how photographers treat focal length being a useful measure of anything. It’s not, at least not until you define a frame size and start looking at angle of view.

Focal length, and effective focal length in this case, are being used, as they generally are, as a proxy for angle of view.

It is still 400mm, but just a cropped field of view relative to a full frame – that is why it looks like 640mm!

I never said it wasn’t still a 400mm lens, hence qualification.

If U have IS (usualy it is 3 stops) in the lens shot with 1/80 of second is really 1/160, 1/320, 1/640 – 1/640 in 3 steps IS so U have more than rule of thumb says U need.

You’re over estimating the capabilities of the IS system in this particular lens. Officially IS system on the EF 100-400 f/4.5-5.6L IS USM is good for 2 stops. My experience is that 2-stops, like most IS ratings, is generous and that it’s closer to 1.5.

Also, to reiterate what I said in the article, the first image was shot with a shutter speed of 1/25th, which is what I called out in the article.