There are many myths and legends as to what a lens sweet-spot is or isn't, but in reality, unless you check manufacture's specifications or have an accurate bench-test method, the sweet spot will vary from lens to lens and from manufacturer to manufacturer.
First, let's look at the legend and a fact--Leica. Leica invented the 35mm format, not Kodak. Kodak invented film. According to Leica, all Leica lenses have a sweet-spot throughout the entire aperture range. Leica even makes an F/1.0 lens--they have the best glass, they also are appropriately priced. With that said, the entire range of F/stops (apertures) on a Leica are sweet.
Now let's look at the myths--
1. Many say that the sweet-spot on a lens is two to three stops up from the widest aperture of the lens. While this could be, it's not scientifically proven, only myth.
2. Many say the sweet-spot on a lens is F/8 to F/11, again, this is not scientifically proven, only myth.
3. Many say it's the middle F/stop of the lens, again, this is not scientifically proven, only myth.
4. Zooom lenses have a better sweet spot at the middle focal-length, middle aperture, i.e., a 70-200mm zoom has the best sweet spot at 140mm at the middle aperture, again, this is not scientifically proven, only myth.
Now, with all the three myths, let's look at a lens with the maximum aperture of F/2.8 and a minimum aperture of F/22, focal length of 70-200mm (non-Leica). If we apply myth #1, then the sweet-spot will be between F/5.6 to F/8. If we apply myth #2, the sweet-spot will be F/8 to F/11. If we apply myth #3, the sweet-spot is F/8. If we apply myth #4, the sweet spot for the lens is at F/8 at 140mm.
What all three myths have in common is F/8. Perhaps that's why photojournalists always say, "F/8 and be there?" Nope, that's not why, so don't go there, that has more to do with exposure and the Sunny Sixteen rule and the old days of Tri-X pan film--another thread!
The whole concept of the sweet-spot is the characteristics and physics of the lens as all lenses have a sharper center with the edges becoming softer and because of this, their sharpest point is not wide-open, and at smaller apertures the lenses suffer from diffraction.
There are so many variables that determine the sweet-spot, focal length (magnification), zoom vs prime, glass quality, resolving power, contrast of the lens glass, longitudinal and lateral chromatic aberration, refractive index, low-dispersion glass, volume of fluorite in a lens, diffraction and accutance.
Bottom line, know your equipment. You won't go wrong with the sweet-spot at F/8 on any lens--now the real question, do you want the sharpest image you can get with every subject? Of course not. You can use the sweet-spot knowledge to your advantage, on a subject with not so porcelain skin you'd shoot with a wider-aperture to not only avoid the sweet-spot, but to reduce your depth-of-field (DOF) for a more soft image of your subject. On the other hand, shooting products or a model with superb, porcelain skin, shoot at the sweet-spot of your lens.
In the end, the manufacturers can tell you for each lens the specific "optimum aperture" is for their specific lens, also know as the sweet-spot, or the compromise between chromatic abberation and lens diffraction. Trust me, it's in their engineering notes and probably filed with their patents.
Thanks, wishing you the best, and thanks for being a part of the Glamour 1™ community and family, rg sends!
(Image of the Spurs shot with Leica 70-180mm zoom lens at F/2.8)
(Images below shot w/Leica 100mm prime lens, somewhere between F/4-F/8.0)