[Pierre-Marie Baty]

@asp: yep, that's what I meant, when you apply Shanon, you get 30 fps as the optimal lowest value. But the actual sampling made by the eye will be roughly 15 (half).

@Exi: the 'flickering' you see when you put your monitor at 50fps is not the actual 50hz, but a combination its harmonics. With a game engine it's a completely different thing, because it's about the rate of the data that is sent out of the video card, not the rate at which the electron beam refreshes the screen. You can very well play CS:Source at 30fps and have your monitor refresh the image at 100 fps.
When your player turns around, all the walls need to be redrawn, that's for sure, but the difference is that with a non-LOD engine, the video card will also be asked to redraw ALL the polygons that make little model objects in the background. For example, if a player model is made of let's say 2000 polygons, be it 2 kms far or 3 feet away, the video card will have to cope with these 2000 polygons, which compared to the 200-300 polygons that make up the walls you can see, are quite a lot. Add this other entities and a lot of players, and you have an horrible feeling of lag, no matter how high your average fps claims to be. Remember that in HL1 the fps is an average, in fact most of the frames last no time, only some of them which last longer are cause of the lag. I think the HL2 way of computing the average display frame rate is much more representative of the reality than the HL1 way.