|
Why can't dust be seen a few
centimetres in front of the lens?
What area are we talking
about?
Have a look at the 'orb
zone' diagram again. There is a triangle shape in
between the lens and a line marked 'between this line
and the lens, particles appear invisible'.
The question is why does
dust in this area appear invisible? In other words when
we take a photograph, why do no orbs appear just in
front of the lens or on the lens?
Why is this dust invisible?
When the camera is
'focussed' on a far away point everything closer than
that point becomes less 'in focus'. The camera
focuses on a 'far away' subject because cameras 'focus'
on the nearest 'big' thing they can pick up. Dust,
being absolutely tiny, is much too small for a camera to
focus on.
When you get really close to
lens the dust is so extremely out-of-focus that it
actually looks invisible! The dust is still there, but
it's just so big that it's larger than the camera lens
so you cannot see it.
Why does this happen?
This is fairly complicated.
So as not to interrupt the flow of text we will talk
about this in the 'depth of field' section.
Show me an example!
The following two
photographs are copyright of
Vanwalree.
In these two photographs the
photographer takes the photographs from exactly the same
place: behind a wire mesh.
In the first photographs the
mesh, being close to the lens, appears out of focus
(just like orbs):
In the second photograph the
photographer increases the size of the 'invisible area'
so that it includes the wire mesh. And the wire
mesh is still there, but it's just so out-of-focus that
it appears invisible!
The photographer did this by
increasing the depth of field, but, again, we'll go into
that later!
Summary:
Dust appears to be invisible
in the first few centremetres in front of the lens.
The reason for this is because the dust is so out of
focus that it becomes large enough to become invisible.
Thinking about orbs, this means any dust just in front
of, or on, the lens will not show up as orbs; it will be
so out of focus and large that it becomes invisible
Next Questions:
- Why can't dust be
seen further than a few
centimetres
from the lens?
- Why is dust so big and
out-of-focus between these points?
- Why doesn't all dust
appear on camera between these points? |
