Telephoto lens when to use?

Beginners group is next week looking at telephoto so off I went to take some telephoto images. Venue was Flint and I tried quite a few options and to be frank most were total failures stopped down to f11 lens will open to f6.3 and in hind sight that was rather silly on a wet dull day. Lens was a 400mm old 35mm lens which on the D-SLR is more like 600mm tripod was of course required so from the batch the two shown were about the best 28mm and 600mm with a red line drawn around the area taken with 600mm on the 28mm version. The aim was to show how the telephoto lens grouped things together giving a warped sense of prospective. It takes some time to change lens so traffic has changed between both photos but there is no cropping on either image comments welcome as I am not sure if any other members of the beginner group has access to such a long lens. OK I am sure club members can go well over 600mm but unlikely beginners will go this far.
I bought the lens cheap when a shop was closing in Ellesmere Port in around 1982 and to be frank in the main it has been a white elephant. In the main 600mm means the picture has too much haze to be used. My 95mm to 210mm was a far better lens and used far more. But since the subject is telephoto going to max seemed the right thing to do.