The most popular ND filter for waterfalls is a 3-stop (0.9) filter. However, you can use higher versions, up to 10-stop (3.0) filters, to shoot exposures of over 30 seconds in the midday sun. The darker the filter, the slower the shutter speed you can use. 

Sorry, the fram is solid, you can’t pop out the lens. 

9.8 "Lx5.5 "Wx10.69 "H (25x14x27cm) 

Made of glass, equipped with 5 blank specimen slides and 5 biological specimen slides. 

You can attach any Nikon F mount lens to it but there is no electrical contact so an AF-S lens will be manual focus only. Also there will be no metering so exposure will be trial and error. The aperture will also be stuck to its smallest setting, likely f/22 so the screen will be dark. If you have a "D" lens with an aperture ring then you can set it at it's widest setting to focus and then stop down if necessary. Similarly boosting the ISO to aid focus may work. I have a 90mm macro lens with an aperture ring and I set it to f/2.8 to focus and then rotate the ring back to my desired aperture if necessary. One workaround to help with the aperture would be to use either tape to hold open the aperture before mounting the lens or blue tack. Even with all the added problems to use an F mount lens with this it is still cheap and kind of fun.