Possible Clues:

• Photograph ruiner, perhaps
• Cause of some missed shots
• You can't leave it on when you shoot
• It's a measure of the print that's endless in exam for agency - the view would be better without it!
• Camera lid
• Item a photographer might forget to remove
• Say Hutton's better being cover
• French city has better device for photographer
• Cover for part of a camera
• Accidental 'subject' of certain snapshots