Higher Apertures
Typically a lens is most sharp around f/4 or f/5.6. Better lenses can
still resolve sharpness (despite the onset of diffraction) at f/8 and
even f/11. Beyond that, few lenses hold up. This does not mean
that we don’t use higher apertures but just that we have to consid-
er whether sharpness is absolutely important in any particular shot.
With my best lenses I typically push the aperture to f/8 and f/11 to
get greater sharpness and depth of field. The modern digital SLR
(DSLR) evolved from the 35mm-format film camera and that format
essentially covers a range from 35mm to 65mm, with 50mm being
the center of that range. The 35mm format was designed around
the fact that the 50mm lens is considered the “normal” lens
because the human eye sees at a focal length of about 50mm. Any
lens less than 35mm is considered wide-angle and any lens larger
than 65mm is considered a telephoto.
Sharpness
Sharpness is a topic that photographers endlessly discuss on
Internet forums. To understand sharpness we only need to consider
the term “acceptable sharpness,” as in: what degree of sharpness
is acceptable to you. Every analog (non-stacked) photo has one
and only one plane in the photograph where things are exactly
sharp. Every other plane in that photo (on either side) is gradually
relatively less sharp until it becomes blurred. Even a wide-angle
lens, where most everything may seem to be in focus, there still is
only one plane that actually is in exact focus. All other parts of the
photo are relatively blurred. It is a question of what you consider
acceptable sharpness, sharpness good enough for you. Only in
non-analog photos such as focus stacking do we find more than
one plane sharp.
The plane of focus is always at right angles to the plane of the
camera sensor unless we explore “view cameras” or tilt/shift lenses
for DSLRs that let us twist and angle that one focal plane this-way-
and-that to achieve very interesting effects.
So we have one plane of focus in every photo and the areas in front
of and behind that plane that are also in “acceptable focus” make
up our depth-of-field, which may be very shallow or very deep.
Obviously a lens set to infinity shooting a landscape has a very