As the Intrepid Enlarger works as an attachment for a 4x5 camera, you have a choice of what sort of lens to use. You can either use a standard large format lens, or a traditional enlarging lens (mounted to the supplied adapter lens board for lenses with 39mm thread mounts). If you shoot 4x5 you'll likely be able to use a lens you already have, if not enlarger lenses are cheap and easy to get hold of. When enlarging 4x5 film you'll want to use a 150mm lens, which also happens to be the standard focal length for a 4x5 camera. If you are going to enlarge 120 and/or 35mm film, you should use an 80mm lens - in this case we recommend you use a traditional enlarger lens. Enlarger lenses are cheap and easy to find secondhand, and we are happy to help if you are unsure.