How to politely decline a photography session
A fast, plain no is kinder than a vague maybe. How to decide in advance which work you take, the three-sentence message that declines without insulting anyone, and what to do when you already said yes.

Decline quickly, decline plainly, and point the person somewhere better if you can. Three sentences, sent within a day, without an invented excuse. A clear no is a kindness; the vague maybe that leaves someone waiting three weeks is the cruelty, and it is the one most photographers choose.
Saying no is a business skill, not a character flaw. Every session you take that does not fit you costs a session that would have. It also produces photographs you do not want in your portfolio, which brings you more of the same work, which is how a photographer ends up with a full calendar and no interest in it.
