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I agree that good zoom lenses can easily compete in overall picture sharpness with prime lenses. Especially the 70-200/4 IS is a super sharp lens at all aperture stops; it is definitely my sharpest zoom lens which I own. But isn't it true that best lens sharpness (not talking about DoF!) is achieved a few stops beyond the fastest aperture? For a 50/1.4 lens this would be between f/2.8 and f/4. For a zoom lens at f/2.8, this position would be moved to smaller apertures. I think it simply matters what sort of composition you want to achieve - especially...Show more →
Again, the main issue isn't "best sharpness" when all of the options are very sharp. I understand how counterintuitive it is for many folks, perhaps especially those in photography forums, but in cases like this "sharpness" differences are pretty much the least important considerations if they matter at all.
As I wrote, if I need maximum aperture (and a few other potential characteristics) I may choose to use a prime. If I need focal length flexibility (and a few other characteristics) I may choose to use a zoom. But I can produce a very, very sharp photograph with either.
Sometimes the obsession with The Very, Very Sharpest Lens brings to mind an imaginary automobile shopper. This shopper decides that one must get The Very, Very Most Powerful Engine. (Or whatever other thing you want the imaginary shopper to obsess over.) One vehicle does, indeed, have an engine that is .5% more powerful than the nearest competitor, so our shopper buys that vehicle - after all, it is "The Most Powerful Vehicle" available. However, it turns out that it is also bigger and heavier. It only seats two and our buyer has a family with two adults and two kids. It has uncomfortable seats. The sound system isn't as good.
Yes, it is .5% More Powerful, but the other options were within fractions of a percentage of being as powerful, all of the options were more than powerful enough, and getting the Most Powerful Among The Powerful had a very real cost in terms of other important considerations.
With lenses, getting the Very, Very Sharpest Lens at a cost flexibility, size, bulk, ability to fit filters, usage issues, and more does not always make sense, and it usually won't make a difference in the perceived sharpness of the resulting photographs. There are some cases in which it can make sense, but typically not for sharpness alone, but rather when the functional characteristics of the lens match up better with the functional requirements of the shooting.