I was just browsing Canon 's The EF Lens World Catalogue
, single focal length lenses, and came across a reference to a drop-in filter.
As far as I can tell, it seems to be a filter for those lenses whose front element is too wide for a conventional filter. For example, the Canon EF 400mm f/2.8L IS USM has a front element of about 160mm in diameter.
But what exactly is a drop in filter? Could it be that it's placed on the rear element, much as a tele-converter would?
My second question is about DO elements (Diffractive Optics). I understand these are used to cut down the len's weight, but seem to make it slower. But the catalogue states "unrivaled image performance".
For example, the Canon EF 400mm f/2.8L IS USM has "conventional" UD glass (Ultra low Dispersion) + fluorite, and weights 5.37Kg
while offering fast optical speed.
On the other hand, the Canon EF 400mm f/4 DO IS USM has DO glass + fluorite and weights less than half: 1.94Kg. But it can "only" offer f/4.
How do these different technologies compare, in therms of image quality? Which one do professionals prefer?
Er... actually, could you please also explain how diffractive optics work and how does ultra low dispersion glass work?