It's the reason I keep the 150/2.8 AND the 75-150
I use the 75-150 primarily for convenience as a landscape lens. As such, I am shooting it mostly at f11 or 16, and occasionally at f8 or f22 depending on the situation. I have owned TWO copies of this lens and can confirm there is some sample variation. My first one performed very similarly to the example above so I returned it. On a whim, a year later I tried another copy after hearing from others how satisfied they were. To my delight, it was a good stop better than its predecessor and very usable at f8 through f22 at all focals. The other note on this lens is that the performance varies with focal -- mine is better at 75 than it is at 150 by one stop: at 75 my current copy is excellent wide open, but at 150 it is slightly soft wide open, better at f5.6, and shows loss of contrast until stopped down to f8. I would go so far as to say my current copy is essentially as good as the primes it replaces after f8.
Ironically however, I find the lack of performance at the long end and more open apertures on my current one a benefit for portraits if/when I want to exploit the softer, lower contrast rendering -- it's a sort of variable effect from about 120mm wide open on, disappearing almost instantly as you stop down at 120, then gets stronger as you move up to 150 gradually disappearing as you stop down through to around 150 f8. If I need razor sharp, I have the 150/2.8 D, and as Guy said, that lens is a laser even wide open and basically you only gain DoF as you stop down...
One other advantage to the zoom for landscape, is with the #1 and/or #2 extension tubes, it makes a very nice variable macro lens :thumbs:
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FWIW Sidebar: I also own both other AF zooms. I have tested 3 copies of the 55-110 and most are quite good if stopped down to f8 or more. On the 105-210 I've looked at 2. There is some variability, they seem to be weak at the short end, but very good at 210. Mine is best from 150 up (I don't use for anything under that) and excellent at 210 after f5.6. It is only slightly larger than the 210 ULD and nearly as good after f5.6 (the ULD prime is excellent wide open) and they're about the same price.