Question on sharpness-aperture

[Black_Dog]

The best explanation I've read of sharpness as it relates to aperture and depth-of-field is How To Improve Sharpness by Lars Kjellberg at photodo.com. It's an oldie and a goodie. In short:

Large apertures =

• Shallow DOF, which is good if that's what you want
• Less sharp because the large hole allows light from the edges of the lens, which is more prone to distortion

Small apertures =

• Deep DOF, which is good if that's what you want
• Sharpness compromised due to proportionately large percentage of diffracted light caused by such a small aperture hole (see Lars' article if you want a detailed explanation)

Mid-range apertures =

• Mid DOF, which is okay if that's what you want
• Best sharpness because the smaller hole means you are only using the best part of the glass (the center) but the hole isn't too small, which would allowing diffracted light to dominate what hit's the sensor

[Scott Stephen]

Of course, the average photographer often judges "sharpness" in terms of "is it blurry or is it clear."

It is harder to get properly focused shots with a wide aperature/shallow depth of field.
On the other hand, in dimmer light, a wide aperature allows you to use a faster shutter which prevents motion blur.
Motion blur and OOF shots can be easily mixed up when one is just getting into photography.

For average shooters, there is a lot to be said for the high-percentage safe shot, in the mid range of aperatures. Not too spectacular or artsy, but not as easy to screw up as long as it is not too dim and the subject is not too fast.