I hope, anybody reading this already knows, how aperture controls the depth of field (DOF). That is the area around the focus, what is recognized as sharp:
One stop down.
Too less depth of field, the face of the locomotive is not completely sharp.
Three stops down.
Good compromise. The face is quite sharp and the loco is clearly separated from the soft background.
Six stops down.
This shot already has too much depth of field, the major object is distracted by the sharp background.
This effect is much more intense with larger focal length and the nearer you get to the object, so this is extremely noticeable in the macro-area. The images here were taken with the old manual Micro105/2.8 with maximum reproduction ratio (about 1:2 with the old lens).
Somewhat less well-known is the fact, that the maximum sharpnes of a lens is sometimes highly dependant on the aperture used. If you're interested to get maximum sharpness from your equipment, you will have to do a little test sequence.
I choosed to focus the loco's cylinder, the images following are full resolution crops from the images above, which were taken in RAW-format, to not loose anything. Of course the crops are not sharpened:
Wide open the depth of field is somewhat within a millimetre. Too little for most situations and only focusable on a tripod and static objects. The lens also produces some clearly visible chromatic errors wide open.
One stop down the results are not that much better.
With to stops down the lens begins to produce maximum sharpnes, at least with a 6MPixel body.
One more step just expands the depth of field here.
Maximum sharpness still avalilable with one more step down. But the available depth of field is still within one centimeter.
Beginning with five stops down some reduction of sharpness is already visible. until you don't need a maximum sharpness, this would be the best compromise between maximum depth of field and maximum sharpness.
With six stops down readability of the smaller writing gets lost.
With maximum aperture the result is less sharp than wide open. The sharpness at the rear bearing gained from the sixth step is already lost again.
What you see here is a normal, physical effect called diffraction, which can be corrected somewhat with modern lenses. You can clearly see, that this lens produces maximum sharpness only with apertures 5.6-11, this may be a problem for the macro-photographer.
When using on a camera with more megapixels this lens may not even reach maximum resolution.
And if we're already talking about sharpness, let's compare some quality-settings of the D70 here:
Since the overall image has little in-focus areas, you'll hardly see a difference between basic and fine setting (remember: D70 uses a size-oriented compression). The RAW-image reveals some more detail and brilliance in this case.
If you can see the difference in the final picture, highly depends on your motive. If the image contains contrasty areas and lots of fine structures a RAW-image is probably a superior base to work on. Images containing softer and less complex structures like flowers differences are hardly visible.
Just to mention, the following total view was taken from an image in basic-quality which obviously would be sufficient for this use: