Originally Posted by
GrumpyDiver
Binnur - as photographers we are always making tradeoffs, so we have to determine which ones are right for an image.
The "sweet spot"; i.e. maximum resolution of a lens occurs when we stop down a stop or two from full aperture. When we do this, we also have to either increase ISO or decrease shutter speed to compensate for the smaller aperture. Depth of field is shallower at wide apertures, so if we decide that we want a very soft background, we may choose to shoot wide open or perhaps one stop slower, because that compositional choice may be what we want in our image.
If we are taking a picture of a moving object; for instance a bird flying; we may decide to shoot wide open, so that we can shoot at a higher shutter speed to reduce the risk of motion blur. Similarly, we might decide to raise the ISO value and sacrifice a bit of exposure latitude and colour depth for a broader depth of field and no motion blur. In that case we may also decide the the additional image noise is an acceptable tradeoff.
We can have a similar choice when we shoot a landscape and opt to shoot at f/16 or f/22, and will live with a slightly less sharp image due to diffraction, but end up with everything in focus.
Every photographer makes some choices when selecting the camera settings, even when shooting in program mode.