Change Form Input Text Using Dom?

how to add text to a hidden field in a HTML form. I want to add text from an array, the array holds the information for the caption placeholder for the images.

Basically i have an image viewer which lets you scroll through images and they have a text caption below them telling what the images are. When the user clicks an image this brings up the form and hides the imageViewer. I want the caption of the image clicked to automatically be entered into the hidden field of the form when a user decides to choose a particular photo, so that it sends this to the server script for processing.

At present i have the function
function insertText()
{
//getElementById('camera').value=getElementById("captionPlaceHolder");
document.getElementById('camera').value = document.getElementById('captionElement').value;
}
i have commented some out to try various things.

My form input box has an id="camera" and i was told to do something like this. Give the hidden element an id="someName"...then in the JS you can change the value of it by the usual way... getElementById('someName').value=??? where ??? is the name of the camera the user has