"the images will look very similar on d60 and d7000, when downsized to lets say 7 megapixels"
Assuming you are seeking the "best" f-stop to produce the most sharpness (and of course you many not want the most sharpness because other things, such as depth of field, my be more important to the image you are intending to create) at what point will anyone be able to tell your f8 image is sharper than the same image you took at f22 or at f2.8 (assuming of course all items in the image were at infinity or equal distance from the lens so all are in focus no matter which f-stop is used)?
I suggest that many (most?) people are looking at their full images downsized to around 2 megapixels because that it what most computer monitors will display. You cannot get a complete image displayed on your monitor unless that image is completely contained in about 2 megapixels. How "downsized" in terms of megapixels are the prints most people view? How many megapixels in an 8x10, 11x14, 16x20 or 20x30 when printed at 300dpi? http://www.unlikelymoose.com/more/cameras/megapixel_converter.html If this table is accurate a 300dpi 8x10 image uses 8 megapixels and a 300dpi 11x14 image would use less than 16 megapixels. Most people won't be viewing prints larger than these sizes so any larger megapixel image will have to be downsized in the process of printing and that downsizing my hide sharpness differences.
Here is a converter. http://auctionrepair.com/pixels.html If this converter is accurate my 16mp D7000 image printed at 300dpi would be 16x10 and a D800 36mp file printed at 300 dpi would be 24x16 inches. Since the flat panel computer monitor displays at 96 dpi a full D800 image would be 76 inches wide and 51 inches tall. My 16mp D7000 image displayed at 96dpi would be 51 inches wide by 34 inches tall. We don't have monitors that large so we cannot see the full image on a monitor unless it is first downsized which my hide any sharpness differences between f2.8, f8 and f22.
Now we can all compare two images at 100% and detect slight sharpness differences. You see this done commonly by reviewers with those tiny 100% excerpts published so we can see which is the sharpest lens when lenses are compared. That is interesting but what is the practical reality? When can this "sharpness" actually be seen in printed work product and not be disguised by either the downsizing done when viewing on a monitor or by downsizing performed when printing less than full size at 300dpi? For example, we can take a certain lens and prove f8 is sharper than f2.8 or f22 but when will that sharpness difference be visible and when will it be hidden by the downsizing needed to view our image on our monitor or to print that image at 8x10? We could call this a "practical sharpness" limitation inherent in how we normally view the image. Just as a 36mp and a 16mp image will look equally "sharp" in a 300dpi 8x10 because the 8x10 does not contain enough enough dpi to display the additional sharpness contained in all 36 megapixels, there must be some limitation on how much sharpness can be displayed in how we normally view a photo full size in prints. I don't know what it is.
Of course, you must use a shutter speed sufficient to stop camera and subject movement and this may limit you to certain f-stops because of the light available. But lets say you can freeze all movement and still be able to select among the full range of f-stops because you are shooting off a tripod and a brightly lit calm day with the subject not in motion (such as a distant landscape). Your camera has sufficient clean ISO latitude that you are able to use any f-stop from f2.8 to f22 and you know your lens is sharpest at f8.
Is is possible, or likely, that "best" (sharpest) f8 cannot even be seen in how we normally view images because it is hidden in the downsizing of the image? If so, lenses may have a range of "practical sharpness" (such as from f5.6 to f11) in which the image produced will be "equally sharp" when viewed or printed as we normally view and print our images no matter which of those three f-stops was used to produce the image. Perhaps seeking absolutely the best sharpness is a bit of a practical illusion unless we are going to print really big. If this is correct than "practical sharpness" just becomes a matter of avoiding the extremes when possible.
