How to let users submit custom fields value?

I have a photo that I want registered users to be able to enter data about. For example I want them to be able to enter the subject or location of the photo.

For example they will go to the single post page and under the photo will be a form with the following text fields and a submit button:

Who
What

When they enter text into the “Who” field and click submit that text needs to be associated with the post and the “Who” field so I can query it for display later. Kind of like a special type of comment entry form. There may be multiple entries under the “Who” field by the same or different users.

It seemed liked custom fields would be the best way to handle this but I am open to suggestions of course.

So the issue becomes how do I code it so a user can submit a value that will become the value of a custom field? Is this even possible?

Any help would be greatly appreciated.