There’s a slot on the rear of my EF 1124mm f/4l USM lens, is it for a filter?
Aiden Schwartz, Brussels
Brian Says… Since the front of the lens is so large, there are limited solutions for using a filter on the front. Lee Filters has an adapter that allows its range of 150mm square filters to fit on the lens. However, due to the wide angle of view that is a characteristic of the lens, you may still find vignetting with the lens set wider than 13.5mm when using a filter holder with two slots. To allow simple plain filters to be used with the lens, Canon includes a gel filter slot in the rear of the mount, and this is what you can see. Historically the rear gel filter slot was more useful for film photography, to convert daylight to tungsten film or vice versa. Also for some monochrome landscapes, a yellow filter may be used. With digital the need is less for these, but ND gels for long exposures are still commonly used.
The rear filter takes cut gel filters, but you will need to cut your own strips of 31mm wide filter gels. While the rear filter holder is a good option for single-colour filters, there are no polarizing filter gel sheets or graduated neutral density sheets available; it would be almost impossible to accurately align a polarizer or graduated ND filter in the rear of the lens. If you decide to use the rear filter slot, take care handling the filters as finger marks and distortions in the filter will degrade the images you take.
Over the years the gel filter holder has been incorporated into several wide-angle lenses in Canon’s lineup. These include your EF 11-24mm f/4l USM, but also the EF 8-15mm f/4l Fisheye Zoom USM, EF 14mm f/2.8l USM, EF 14mm f/2.8l II USM, EF 15mm f/2.8 Fisheye, EF 17-35mm f/2.8l USM and EF 17-40mm f/4l USM.