[wlesieutre]

Worth noting that on the 600mm with melted iris, the user did use a solar filter.

Unfortunately for the lens, the 600mm lens's drop-in filter slot is behind the iris. So the camera was fine, the lens not so much.

http://shuttermuse.com/canon-drop-in-filter-guide/

[ars]

Why do they even make a drop in filter rated for the sun, if it's going to damage the lens?

What use could you make of this filter?

[nullc]

There is a drop in filter holder that takes 52mm filters designed to screw on to the front of lenses.

[throwanem]

(For those unfamiliar - it works this way because the objective element of this lens is far larger than anyone makes filters for. 52mm is a very common diameter, so placing a drop-in mount further along in the focal path means you can use a wide range of filters with minimal cost and inconvenience, and to identical effect - except, of course, in the rare case where you have enough energy coming down the barrel to damage the iris.)