Depending on the nature of the letter you would address the recipient as Mr/Ms/Miss/Mrs or by their first name only.
When you are writing a letter to someone whose title is director of photography, it's a little formal to put that in a letter, HOWEVER, if they are in the ASC or another honorary organization, it is with respect, that you would include their membership letters after their name.
When deciding about including a title, think about what you would wear to an interview. Most crew members who interview in a business suit, "freak out" people because camera assistants don't wear business suits. So if you're a camera assistant writing to a camera operator, you probably wouldn't use his/her title, but you would put SOC after his/her name if it applies.
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