Another question that gets lively discussion among photographers' community is model bringing on set with her an escort — mom, boyfriend, or friends. Pros and Cons — don't hesitate to give your point of view in comments.
I will say upfront, models, please make sure you are safe (and feel safe) at all times. It's more important than pictures, no matter how cool they are. Many photographers say that stranger on set distract them. I can relate — photography is often a very vulnerable, creative process for both sides, and having spectator on set really throws me off at times. But I also know what it is to be a young non-ugly girl coming into the space of stranger you met on the internet (photo-clients for me). I never get in any real trouble, but I was uncomfortable more than once. And I know people who got in real trouble back in Moscow, even working through third parties who called themselves agencies. So, photographers, if a girl wants to bring a friend with her, it is her right, and it's not to "steal your secrets" or "turn the shoot into a circus."
At the same time, models, when you bring someone on set, make sure he/she won't sabotage your performance by giggling, staring, talking to you about something else while you pose. I had this client once, a young girl. She came to do her first tests with her mom. While mom was in another room, the girl was posing and connecting fine, but as soon as mom came on set, the girl automatically switched back to her mom' s-daughter self and was awkward and stiff. I tried to go around the corner with her, to offer mom some tea, nothing really helped. One other time an experienced model brought her man to the set and all the shoot I couldn't help but feel as though I am simply an accessory to their flirting game.
So my advice — bring a good friend. Someone who you feel safe with, but who will be able to step aside and not interfere. If nothing else, bring an aspiring photographer you trust, but not his camera — it's not cool to shoot on other photographer's set unless it's a masterclass ;)
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