Width Of The Lens

once the iris is opened all the way, the physical limitation is the width of the lens itself.

Can anyone explain me what is meant by width of the lens in the above line ??

It means that if the Iris is not obscuring any part of the lens, then the amount of light the lens lets in is limited only by how big the lens is.

Consider lens 'width' and lens 'diameter' the same in that sentence.

Sometimes I use little ditties to help me remember facts, like this one sung to the song from 'Rocky':

It's the size of the iris,
It's the width of the lens,
Stopping up to the challenge of our level.