Hi guys! I have a question.. Why is it that when i use a zoom lens [Nikon AF-S 55-200 (4-5.6)], when i use the maximum aperture, most of my photos appear soft and not sharp? When i get to aperture 7.1 for example, the photos are much sharper. I'm focusing on a single subject which isn't moving much, yet at maximum aperture the images are soft.
That is pretty much how lenses are.... It is not that they are so less sharp at f/4 (they are a little less sharp when wide open), but that the zone of sharpness is narrow. Just for an example... There are depth of field calculators, (
Online Depth of Field Calculator is one),
and say at 75mm zoom, and
focused at 20 feet, then
f/4 - the depth of field (zone of sharpness, acceptable maybe) is 18.4 feet to 21.9 feet.
f/8 - the depth of field is 17.1 to 24.1 feet - a little wider range (about double) of perhaps acceptable sharpness. Called Depth of Field.
These are approximations, not biblical numbers. There is no sharp dividing line at these numbers.
The subject exactly at 20 feet is probably OK either way, but the background and foreground changes.