The Canon Gelatin Filter Holder III is for 3x3" (75x75mm) gelatin filters.
To use this holder you must purchase an adapter that matches your lenses filter size. Available adapters are: 52mm (C546201), 58mm (C546211), 67mm (C546291), 72mm (C546221) and 77mm (C546231). A specific adapter is available for attachment to Canon's 50mm f1.4EF lens (C546301).
An optional hood is also available (C448211). Multiple hoods can be "stacked" to increase the length for telephoto lenses.