Here is another reason to use a filter. Quote from Canon site on EF50mm f/1.2L USM:
"By utilizing dust- and moisture-resistant construction (Fig. 3) in the lens mount, switch panel and focusing ring to prevent dust and water from reaching the interior, the lens can be used in harsh environments.
*This applies when the lens is used with a dust- and moisture-resistant EOS body and a filter is attached to the front of the lens."
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