Despite all of my traveling, I am due for a technical
discussion and I refuse to miss a scheduled blog. This week I would like to shed some light on exactly
how lenses work (don’t worry I won’t use that pun more than once).
Figure 1
A camera lens is actually a series of lens elements, or
individual sets of glass lenses. At its
most basic, a lens is simply anything that bends light. Water droplets, for instance, act as lenses
and distort the light reflecting off of an object (Figure 1) and back to the
front of your eye (also a lens) before reaching the photoreceptors at the back
of your eye (an analog to film or a camera sensor). Why do we need lenses to create sensible
images? Because when light reflects off
of a surface it scatters in all directions and we need to refocus the scattered
light into a crisp point so that the image is not blurred.
Imagine that we want to take a picture of the star in Figure
2. On the far left we have a simple
pinhole camera where we have put a piece of photographic film on the inside wall
of a box and cut a hole on the opposite side to let light in. When light hits
each of the individual blue, green, and red points on the star it will scatter
and hit the film at all points between the similarly colored lines. In other words, the reflected light from the
blue point will hit the film at every spot between the two blue lines that
extend to the film and so on for the other colors. You can see how this might make a very blurry
and incomprehensible image. In the top
center panel we have made our pinhole smaller, which has improved our image
resolution by decreasing the number of angles of scattered light that reaches
the film. Unfortunately this also
drastically reduces the amount of light that reaches the film, so we may have a
very underexposed slightly blurry image as where before we had a very bright
but extremely blurry image. Let’s take
this same concept to the absolute extreme:
on the far right panel we have found a way to make a hole that is
exactly one photon (or light ray) wide.
In this scenario we will have perfect resolution in our image because
there is only one angle at which light inters the pinhole for each unit of surface
area on the object. But our image will
probably be completely black because such low light will not be enough to
activate the film. Enter the lens. With the lens, the amount of light gathered
is equivalent to the far left panel with the large pinhole, but produces an
image with a resolution much closer to that of the far right panel.
Figure 2
Website viewer: “Ok
Patrick, that is all well and good but why exactly does a lens bend light in
the first place?”
Patrick: “Light
passes through different materials at different speeds”
Website Viewer: “I
missed that day in high school physics”
Patrick: “Don’t
worry, I’m super nerdy and wrote detailed notes that you can benefit from now
that you care about the physics of light.”
Imagine that you have a birds-eye view of a marching band
practicing on the football field after it has rained, but there is really poor
drainage on part of it which makes it very muddy (Figure 3). These band members are highly trained and
will not break a perfect front facing line (all band members in a row will
always face the exact same direction and remain shoulder to shoulder). As they march, the left side of the marching
band encounters the mud first and is slowed down. As a result, the rows must rotate to remain
in perfect front facing formation. The
end effect is that the marching band’s direction of travel was “bent” from its
original angle of incidence to the new angle of refraction when it encountered
the new material (the mud versus the dry ground). If, however, the marching band approaches the
mud head on such that every band member in a row arrives at the mud at the same
time (Figure 3D,E, & F), there is no need for the rows to rotate. The whole formation is slowed down equally,
thus the angle of incidence and the angle of refraction are equal and there is
no change in direction. In this analogy
the marching band members represent light waves, the dry ground represents air
(the first material), and the mud represents a convex lens (new material) in
which light travels at a slower speed.
Figure 3
The further away from the center of the lens a light wave
strikes, the more it will be bent. This
is why all of the light gathered by a convex lens will eventually converge at
the focal point and then be reflected beyond that point (Figure 1D; the image
of the star is upside down from the real object). The degree of curvature
determines the angle of refraction, and thus also the focal length. More curvature = more bend = shorter focal
length.
Figure 4
A camera lens is actually a series of lens elements, which is a series of individual lenses with varying shapes (Figure 4). The basic elements often include: a front element, one or more lens groups, the lens diaphragm, and a rear element (Figure 5).
Figure 5
The number of lens elements in a camera lens can vary based on the number
or amount of corrections that must be made so that image appears “correctly” on
the camera sensor or film. The Diaphragm
(aperture) is used to adjust the amount of light that reaches the film or
sensor and effects the depth of field (the amount of lateral space that is in
focus at any one time). Tune in next
time for a continued discussion of aperture, depth of field, and blurriness.
Ciao
-Patrick
P.S. – for further reading on topics covered above, see the
following links…
Good job! Very interesting, I learned a lot!
ReplyDeleteThank you! Hopefully everything made some sort of sense.
ReplyDelete