[Sudhir Shivaram]

Dear Deepak, f6.3 is probably the lowest f value supported by the lens. The upper limit can be adjusted in the camera or lens depending on the model of the lens. You have just mentioned 18-150 lens, I am not sure if it is Canon/Nikon/Sony/Tamron/Sigma. etc. In aperture priority mode of the camera, you can change the aperture value to a good upper limit of at least f22 in almost all lenses.

On a different note, an aperture of f8 or max f11 is good enough for majority of landscape images. Anything beyond that, you may lose the image quality.