When you have a variable aperture zoom, like for example the fine, underrated 35-70/3.5-4.5 lens, when you are at 35mm the lens is a f3.5 lens and when going to 70mm the lens is a f4.5 lens. Since metering is through the lens (TTL), the amount of light that the meter will sense at 70mm is less than with the lens at 35mm. Thus, you simply use the aperture that the meter indicates, with no correction; as if the lens was at 35mm.
This explains what cuthbert experienced.
BTW I claim that Canon is the best manufacturer of zoom lenses of all the japanese brands at that era. Some of their zooms are second to none, like the 35-105/3.5 or even the 35-70/4.0 which was ranked by a german magazine in 1979 or 1980 above similar lenses by Angenieux and Leitz, and of course Vivitar and Nikon; not to mention Pentax and Minolta. And the Canon was one of the cheapest ones on the list (!)
Canon invented the modern two-group standard-zoom configuration which then Nikon directly copied years later, in the seventies. Very possibly there were many optical engineers at Nikon with nightmares and perpetuous headache after the Canon FD 35-70/2.8-3.5 was introduced. That lens was groundbreaking, a milestone in optics. It took Nikon years to do release their "copy" (ha!!).
Marco Cavina's website explains this with solid proof.
For zoom lenses, Canon is the king. Accept no substitutes. Canon is where you want to be. Don't go for the brand with those focus rings that rotate in the wrong direction and lenses that mount in the wrong direction too. Go Canon FD, which stands for "Faithful Depiction", "Fairly Decent", "Free from Defects", or "Fair Dinkum".