Yes the other lens are slower and can be more difficult to focus. But they are certainly usable. On the plus side the slower lens are smaller and lighter than f5.6 in equivalent focal lengths with is an advantage for field work. I use an 90mm F8 Super Angulon and a 150mm F9 G-Claron and this includes night photography. In most cases there is something bright enough to focus on and if not I carry two Mini Maglites which I use for focusing aids. The darkcloth you use also make a difference, it is important that the darkcloth fit well around the camera and blocks out all the light.
Originally Posted by Alois
You own the lenses so I would encourage you to try them out, you may be pleasantly surprised.