The glass never fails. The clamp does, and the clamp is what keeps it in the door
Three facts about the object. A viewer is a tube with a lens at each end: a concave one on the street side to gather a wide view, and a convex one on your side to magnify it enough to recognise a face. It is not screwed into a thread in the timber — it is clamped, by the decorative ring you can see on your side, which pulls the tube so that both ends press on the two faces of the door.
That clamp is the whole of its security. A viewer that turns is a viewer that unscrews, and it unscrews from the outside, because the barrel is a thread all the way through. Wrap a rubber band or a loop of tape round the outdoor bezel for grip, turn, and the whole assembly comes out, leaving a 16 mm hole. A rod bent to a right angle goes through it and reaches the inside handle — which is why this is a known method rather than a theory. So the first test is not about clarity at all: grip the ring on your side and turn it. If the whole tube rotates, stop and fix that first.
Only then does blur matter, and blur has one usual home. The outdoor lens is the one that ages. It lives in rain, grit, sun and window cleaner, and on cheaper viewers it is a polycarbonate moulding whose surface hazes and whose coating goes cloudy. The indoor lens has never been outdoors in its life and is almost always clean and clear, which is why taking the viewer out and looking at both in the light is the decisive test — and why polishing the inside is wasted work.
Three faults, three answers
The tube turns is a clamp problem. Tighten the indoor ring by hand — firmly, not with pliers, because the ring is usually plastic or thin brass and the door skin dents long before the ring complains. If it still turns, the door is thinner than the viewer is made for, so the tube is bottoming out before the ring can clamp: take it out and fit a spacer ring or a shorter barrel. Then check it properly by trying to turn it from the outside with a rubber band. A viewer that cannot be turned from outside by hand is doing its job.
Milky, and worse every year is the outdoor lens. Unscrew the indoor ring, take the barrel out, and hold it up: one end will be hazy, dull or finely scratched, the other clear. If the haze is on the surface, a little cerium oxide or fine plastic polish will bring it back enough to see faces; if it is inside the plastic — a yellow or milky cast that does not wipe off — it is finished, and a replacement viewer costs less than the polish. Refit with the ring hand-tight and test the clamp.
Sharp in the middle, black at the edges, or only a slice of corridor is the wrong viewer for the door. Viewers are sold by thickness band — roughly 35–55 mm, 45–75 mm and 60–90 mm — and the barrel has to be long enough to cross the door while keeping both lenses at their designed distance. Too short a barrel fits but cannot focus, so you get a small clear patch and darkness. Measure the door with the viewer out, read the band on the old barrel, and buy for the thickness rather than for the widest angle on the packet.