Accessing CustomFields progmatically

Hi,

I'm working on a small module which requires per-photo data. Rather than starting modifying the schema extensively, I thought that it might be easier to just use the customfields module to add the fields I need. However, I'm not sure how to retreive an array of the custom fields from within my PHP code, given either the entity itself or it's ID (I know how to get it from Smarty based on the code for the CustomField block)

Thanks