One of the better aspects of this type of lens, is usually the number of blades in the iris.

I have a 150 sitting on my desk as I write this; it has 13 blades and gives a wonderful out of focus effect which I mainly attribute to the amount of blades, almost like a perfect circle.

I also have a 55 Micro Nikkor for my Nikon cameras, it does wonderfully with flat copy, but is a pearl of a lens for general photography as well.

Both of these lenses are excellent over the whole range of subjects that you would shoot, but their ability at infinity isn't as good as a standard 55/50 in 135 or 150 in LF. At least my two aren't.

Basically I see that the resolution, or whatever you would like to call it at infinity, is not as good as a normal lens, I cannot describe it any other way.

Mick.