Canon EF 100-400mm f/4.5-5.6L IS USM - AKA The Dust Pump

The image quality of this lens is too poor for portrait photograph?

5D3ismydream wrote in post #16667941
The image quality of this lens is too poor for portrait photograph?

Its not too poor, but neither is it ideal. Its not the easiest lens to handhold, at shorter focal lengths it doesnt produce the shallow depth of field you might want for portraits, and at longer focal lengths you have to work some distance from your subject. So most of us opt for other lenses for portraits. Most of the issues are not with image quality, but rather the nature of the lens.

Easily put, it isn't the best tool for the job. It is like trying to pound in a nail with the end of a screwdriver instead of using a hammer. Will it work? Most likely you can make it work, but there are better tools suited for the task at hand.

5D3ismydream wrote in post #16667941
The image quality of this lens is too poor for portrait photograph?

actually the image quality of the 100-400 is good for portrait photography --- but it's a little long for indoors (and not 'fast')

Rebel XSi /450D
Canon 100-400