Originally posted by Michael Piziak I've been reading about the sweet spot or sharpest f stop of a lens. One general rule of thumb is 2 to 3 stops from wide open (with 1.4 being equal to one stop).
Like Steve said, most would say 1-2 stops from wide open is generally where a lens is sharpest. However, this will vary from lens to lens (also from copy to copy).
Originally posted by Michael Piziak I don't have these two, but other macro lenses I have are really very sharp already at its widest. I never bother with stopping down for increased sharpness, but I often stop down for increased depth of field.
Originally posted by Michael Piziak Copies are different, but testing my DA55-300 I have found very little difference in centre sharpness between f/5.6 and f/8. The corners will improve up to f/11, but beyond that diffraction will reduce sharpness (a bit at f/16, more from f/22). When shooting at 300mm I rarely care about the corners anyway, so I don't mind shooting at f/5.8 if I have to. And if the light is poor I'd say slow shutter speed (risking movement blur) or too high ISO will probably impact image quality more than what a very slight decrease in sharpness from the lens will.
In short, you have to get to know your lenses (as in your copies) and figure out what works and not - for you.
Technical comment on f-numbers:
An f-number is the relative diameter of the aperture to the focal length of the lens. If we look at a 50mm lens, f/1 would mean that the diameter of the aperture is 50mm. f/1.4 would mean a diameter of 35.34mm, f/2 is 25mm etc.
If the diameter of a circle is multiplied by the square root of 2 the area of the circle will double - and thus the amount of light let through the aperture will also double. So for each "stop" you will double (or halve) the amount of light let through the lens.
The reason for "stops" being the values they are (1 - 1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22 - 32 - etc) is simply that they are all square roots (of 1 - 2 - 4 - 8 - 16 - etc).