I want to reverse mount my 28mm EF 1.8 on 100-300mm telephoto zoom. Is there a bayonet adapter which can be used with a filter to protect the contacts and exposed glass on the 28mm? The zoom has a 58mm filter connector so a 58mm coupler will be perfect. The zoom itself is around 9 inches long. Will a coupler be sturdy enough to hold the weight of the 28mm safely?
When focusing this setup should I use the zoom autofocus, the zoom manual (in either case, should the 28mm be pre-focused before mounting?) or the 28mm lens' manual focus? Might be best to focus from a distance on a point manually.
The zoom is an old L with really great(!) optics but somewhat clunky autofocus which has three focus modes: M, 2m to infinity, macro to infinity. I don't know what the working distance is for the macro setting. Aperture range is f/5.6 to 32.
I should be able to get huge magnification with this rig. Any insights on what working distance and best possible DOF might be? I'm thinking it might be great on a tripod for butterflies and such.