Just for a laugh I tried with some of my Canon DSLR lenses. As expected, not good.

The EF lenses were bad - just could not focus but gave a circular picture the diameter of which was approximately the screen height with the N900 in portrait.

The EF-S lenses worked better for focussing but gave a tiny circular picture the diameter of which was approximately the width of the NOKIA logo on the front case of the N900. They would only focus close-up, not at a distance.

The resultant pictures were much worse than the stock camera could do, so I would suggest that the only solution here is to use the door viewer style attachment for fun value only.

Indeed, I'd expect the focus plane to be 2cm behind the lens, so holding the n900 against the lens won't work.

It did get me wondering if you could create some sort of case mod where by using some mirrors (periscope style) you could move the camera lens to the middle of the n900 and then have a EF lens mount at the correct distance away



Then all we would need to do is wire the EF contacts to some unused IO lines in the N900 and voila! lmao

Some time ago I bought the case with fish-eye lens for iPhone 4 and this provides satisfactory results for me. Though I still didn't sorted out the mounting to N900. I just hold the cutted piece of iPhone case with lens mounted on it close to N900 body. Focus works fine as the lens was designed for mobile phone cameras. Only the center area of the picture is usually sharp, margins are more blurred but this is normal for such a cheap lens.