One of the issues that all photographers have to confront when photographing other people is getting a model release. If you're thinking of selling, or even just displaying, photos of other people you really should have a valid release. The actual legal need for a model release (especially for purely editorial, noncommercial usage) is frequently debated (and often misunderstood), but from my personal point of view: if you have recognizable people (or property) in your images, it's worth getting a signed release.
The key word here, I think, is recognizable. I'm certainly not an expert on model releases, but it's generally accepted that if a person isn't readily recognizable, then you don't need a release. The subjects in this photo may recognize themselves, of course, because they remember being at the beach or they may recognize their beach pails or their clothes, but without their faces showing, it's unlikely anyone could ever prove it was them in this photo.
That's why, when I'm out shooting in public places (like beaches) and I am too far away (as I was in this case) to get to the subjects in time to have them sign a release, or when I'm working where it's almost impossible to get a release signed (a busy city street in Manhattan), I consciously look for moments when the subjects' faces are turned away from me. Does it rob me of some of the best shots? Yes, I think it does. But does it simplify life and provide good photos in situations where getting a release would be a major hassle or impossible? Yes. I watched this interesting father and daughter gathering (I think) periwinkle shells at the beach for about 20 minutes or so, but I was a few hundred feet up on a rocky overlook and getting down to them with my tripod and cameras would have been pretty dangerous. So as I watched them, I tried to wait until they were just generic humans on the shore. Again, it's a trade-off that probably isn't even legally necessary, especially for editorial usage, but it makes my life easier.
By the way, the other downside of not making contact with your subjects is that you can't offer to send them prints of your photos. I would love to have shared a print with this father/daughter since they seemed to be having such a nice time together, but by the time I had finished shooting they had moved on and I lost track of them down the beach.
There are some good books available on model releases, of which my friend Dan Heller's A Digital Photographer's Guide to Model Releases: Making the Best Business Decisions with Your Photos of People, Places and Things is certainly one of the most exhaustive and broad views of the subject. Dan isn't a lawyer, but you'd be hard pressed to find someone who knows more about the business of photography.
The best advice in my view is to carry releases with you, get them signed when you can (and get a phone number and address for your subjects) and consider consulting a lawyer about this subject if you begin to sell or display your photos.
2 weeks ago