Open Access is a global movement to make knowledge freely accessible. This includes research, in its variable forms (journal articles, datasets, code), as well as visual knowledge such as photographs, diagrams and images. 

As per SPARC, a non-profit advocacy organization, open access is "the free, immediate, online availability of research articles combined with the rights to use these articles fully in the digital environment. Open Access is the needed modern update for the communication of research that fully utilizes the Internet for what it was originally built to do—accelerate research. - Open Access - SPARC

Creative Commons is a nonprofit organization that works "to increase the amount of creativity (cultural, educational, and scientific content) in “the commons” — the body of work that is available to the public for free and legal sharing, use, repurposing, and remixing." (http://creativecommons.org/about/what-is-cc).

Creative Commons licenses provide a way for people to share their work and make it available for others to build on and reuse.

Check out the Creative Commons website for more information and details about CC licenses.

Open access photographs can be used more freely as opposed to commercially licensed photos. Photographs, visuals, diagrams, tables etc. all contribute to scientific knowledge. If this knowledge is restricted, it impedes creativity and innovation. Photos in the public domain, i.e. those that are the most openly accessible, aid thousands of people around the world.

You may have used open access photos for your own research poster or presentation. You may have been cautioned by your professors to only use images from Wikimedia Commons or Unsplash in your presentations. Have you ever wondered why? It is because they wanted to make sure you were using photographs that you had permission to use.

Open access images allow users to download, share and re-use images a lot more freely than images that have stricter licenses. By making photographs more widely and freely available, open access photography contributes to the wider goal of making knowledge open. This is why many museums and archives — Qatar Digital Library, Smithsonian Institution, The Metropolitan Museum of Art, Harvard Art Museums —make their images openly accessible.

	This is an example of an open access photo. It uses a Creative Commons license that has no use restrictions. This image came from the Smithsonian Museum's Open Access Collection.
	This image of Her Highness, is also an open access photo. It uses a Creative Commons license that has certain use restrictions: the photo can be used if the creator (the UNDP) is attributed. Bilateral Meeting with Qatar | Flickr
	This image is an example of a photograph that has a very restrictive license. It has to be purchased from Getty Images in order to be used.