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because sometimes a photo matters so much more than sharpness 
When you say the optimal f/ stop, I take it you mean the sharpest for the lens in question? There is no f/ stop that gives front to back sharpness in all situations.
At f/22 you might have a large depth of field, but not all the image is "in focus", only a particular zone of it, and the closer the focussing point is to the camera, the smaller the depth of field is.
Some lenses go as small as f/64!
Some of mine go to f/90
Then there are the pinholes, f stops in the hundreds
Quote: what situation would you need a smaller F stop than the one that gives the sharpest image?
where depth is more important than sharpness. e.g. for some macro work, there's a trade off really.
sharpness isn't everything anyway. here's one of my pinholes, some people like it, http://therationalacademy.bandcamp.com/album/pacific-rope-2
f/2.8 to allow as much light in as possible or a fast shutter speed or shallow DOF. f/32 to achieve slowest shutter speed without using filters, for example.
You are given choice.
The combination of ISO, aperture and shutter speed produces an appropriate exposure.
The choice of aperture and shutter speed are chosen on your creative intent, sometimes outside of the sweetest apertures of the particular lens.
Quote: Why don't he manufacturers just stop at the optimal F number?
A few reasons like above, also say you wanted to do a long exposure effect like smokey water or traffic trails and your lens stopped at f11 you wouldn't be able to do as long an exposure as you would if the lens went to f22
As a rule of thumb around 80% of lenses are about 20% less sharp either wide open or at minimum aperture than at f8 or f11.
As a rule of thumb around 80% of sensors loose up to 50% of their resolution by their highest ISO setting.
As rule of thumb sensors/lens loose up to 50% of resolution in very low contrast light.
All this aside I see some first class images at f1.4, some at f16, some in low contrast light, and some at a very high ISO 
Quote: As a matter of interest, how do you measure '50% resolution'?
50% resolution is 'resol' or 'ution' or 'rslto' or 'eouin' or.......... well, you get my drift..........
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