How do I disable document caching in ram for certain collections?

I have a collection which stores documents worth 1 or 5 mb each. I do not want mongodb caching these images in ram, as there are 100s of thousands of these which may kill performance.
Is there a way to specifically disable caching of documents in ram for specific collections? I couldnt find it in the documentation sorry.

Do not be sorry. There is no such feature. If a document is not recently in use, then it will not take any space in RAM. When it is needed and not in RAM it will be read from disk. If your 100s of thousands of 1 to 5 mb documents are constantly needed, then you want them in RAM. Reading them from disk will kill performance an order of magnitude compared to have them in RAM.