When you book a photo session, you will be asked to sign a contract. The contract defines the timeline (when you are being photographed, when your gallery should be delivered, by when you will have to select your favorites, etc). It also defines the ownership model for the photographs. As a rule, the photographer retains the copyright (rights to use the photographs whichever way they fancy), which ranges from low-volume promotional materials to high-volume wholesale.
An example of low-volume promotional materials is using a photograph in the portfolio. Portfolios are viewed by a smaller audience, have very few photographs (usually, less than 30) in them, and photographers constantly update their portfolios. Result is such that only a few people will see it, hence "low-volume". An example of high-volume use is selling a picture on Adobe Stock Photos. Potentially, completely unrelated 3rd party, say a postcard manufacturer, could buy the right and put the photograph in every supermarket of every town in your country. Some people like it, others are less comfortable with such scenario.
As with any other commercial activity, always seek to obtain a photo session contract, and read it somewhat carefully. Make note of every question you have.
Remember that family photographers genuinely love people---they will work with you to make you be comfortable. Even if you did find something in the contract that gives you a pause: reach out and ask the photographer about their stance on the issue, explain your uneasiness. Your photographer can always modify the contract to address your particular concern.
Professional photo sessions create memories you will enjoy for years to come---they will help you decorate your workplace and living room. They are fun and designed to enrich your life, not make it more complicated. So, glance over the contract, discuss if you have questions, and:
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