The cloth is very thin and has enough space between the keyboard and the screen. It even protects the screen a little because the keys don't touch the screen. 

About 4m/13ft. 

That depends on the lens diameter. This is usually stated on the lens. The filter holder is then attached to the lens using the appropriate adapter ring 

Dear Customer, Thank you for your question. Please check the following steps to setting the focus: (Do not look at too close objects) Step 1. Cover the right objective lens with your hand or close your right eye, rotate the central independent focusing wheel until the image in the left eyepiece is sharp and clear. Step 2. Now cover the left objective lens or close your left eye, rotate the right eyepiece focusing wheel to adjust the diopter of the right eye until the captured image is sharp. 

Our adapters support to focus to infinity. Since it's a manual adapter, if you come with this issue, please try to set the camera as MF (manual foucus) mode, then turn the focus ring to the infinity (∞), and try to turn back a little to achieve focus to infinity in case the infinity mark of some lenses is not accurate enough. Besides, the position of the infinity mark [∞] of the telephoto lens or super telephoto lens is shifted slightly to the positive side to enable focus adjustment even when focus shift is caused due to temperature changes. Please be sure to check the focus through the viewfinder even when you shoot at infinity.