contents contents
full screenprintcontentsfull screenprintTECHNOLOGY
54 VIDEOMAKER >>> MAY 2019
New video shooters may notice
that fellow camera geeks like to
talk a lot about “glass.” While
there are a lot of things made of
glass in the video world, there’s
only one thing called glass: Your
camera lens. Here’s how it pro-
duces a usable, in-focus image.
image — far beyond the rest of the
camera hardware you’ll ever own.
Still, how does it actually work to pro-
duce a focused image? Plus, what do
all the numbers and letters in a lens
name mean? Let’s take a look.Focus, please!
To start, imagine a magnifying glass,
which is a simple lens. If your mag-
nifying glass is the right distance
from whatever you’re looking at, theSimply defi ned, a lens is something
that bends (or refracts) the light that
passes through it. As you can imagine,
lenses are everywhere in our world
— from a drop of water on a car’s
windshield, to your eyeglasses, right
down to your actual eyeballs.
Thus, a single object that bends
light is called a simple lens. Similarly,
a set of two or more lenses working
together is called a compound lens.
In general, camera lenses are
almost always compound lenses. To
reduce confusion, when we talk about
the simple lenses inside a compound
lens, we refer to them as elements.
Your lens will arguably have the
most important impact on your fi nalLens es: How does focus work?
by Mike VanHelderrefraction will make your subject
look bigger. This is because the lens
gathers light from a wide area, bends
it at the right angle, then shoots it into
your eye. This functions similarly to
the way a funnel might gather and
redirect rainwater.
Comparatively, if you’ve ever tried
to light a leaf on fi re with a magnify-
ing glass on a sunny day, you un-
derstand exactly how bending light
works. You collect and bend the light
from the sun onto a specifi c point,
which is the point of focus on the leaf.
A camera lens works the same way,
collecting and bending the light onto
your digital sensor (or fi lm, if you’re a
time traveler from 1995). This is why
you never want to point your camera
directly into the sun. It can fry your
sensor — and possibly your eye, de-
pending on the camera.
An object viewed through a magni-
fying glass looks bigger, but it’s usu-
ally distorted and fuzzy as well. How
do you reduce the distortion of the
image and make it sharper?
First, you use another lens — one
that straightens the light a little. In
fact, that’s how simple telescopes
work — they have one lens to bendLike a funnel collecting and redirecting water into a more focused stream, lenses collect
and redirect light. A magnifying glass can be used to condense light into a single point,
while the aim of a camera lens is to direct light onto a camera sensor or film strip.384 C15 Technology.indd 54 3/19/19 2:33 PMTECHNOLOGY
TECHNOLOGY MAY 2019
VIDEOMAKER >>> MAY 2019 55
Lenses: How does focus work?
by Mike VanHelderIn short, the farther away you need to be
from your subject, the higher the focal length
should be. A 300mm lens is great for shooting
wildlife because you can be far away from your
subject. It takes a narrow shot, so your subject
takes up more of the frame.
On the other hand, if you want to get up close
to your subject, a 20mm lens might be what you
need. In brief, this will take a wide shot so your
subject takes up less space in the frame. Zoom
lenses, which have a movable sweet spot, will
have two focal lengths listed. They’ll have mini-
mum and a maximum like “80mm-200mm.”
Lenses that can’t zoom are called prime lenses.
Focal distance, by contrast, is the closest you
can be to your subject and still have it in focus.
In other words, focal distance is your minimum
focus distance.
In general, it’s important to note that focal
length and focal distance are not always related.
Two 20mm lenses from different manufactur-
ers may have very different focal distances.
Focal distance is not usually in the name of the
lens, so you will have to check the tech specs to
find out what it is for the specific lens you are
considering.)oFal SlanH GHSth oI fiHlG
Additionally, you might hear about fo-
cal plane. Focal plane (also called depth of
field) is how deep your area of focus is. It can
be hard to explain, but imagine you’re taking
a portrait of a person. If you have a very nar-
row focal plane, if your subject’s eyes are in
focus but their nose won’t be, and neither will
their ears.
Similarly, if you have a wide focal plane, their
entire face — nose, eyes, ears — is in focus at
the same time. In general, you don’t usuallythe light to make things look larger, then another
to bend it again to make it look clearer. In fact,
this is how two simple lenses become elements
of a compound lens.
By varying the distance between the two
lenses, you can change how much the light gets
straightened. This changes the focus of the light
and changes how blurry the picture is. That’s
why, when something is blurry, we say that it is
“out of focus”.
Likewise, to get even clearer and less distorted
images, we add even more elements to unbend
and rebend the light exactly right. Plus, we add
a mechanism to move those elements around to
adjust the bending, and therefore your focus and/
or magnification.
The math necessary to figure out how many
elements you need and their precise shape is
extremely complicated.
An average lens might have as many as six
or seven elements in it. Thus, more expensive
and complicated lenses may have as many as
15 to 20. Elements with different shapes canmake for different effects, which is how you get
specialized glass like macro, wide-angle, fisheye,
apochromatic (“APO”) or telephoto lenses.Technical details
Now that we have a basic idea of how a lens
works, let’s get into some of the technical nitty-
gritty. When you buy a lens, it usually comes
with a bunch of numbers and letters in the name,
like “Nikkor 20mm f/1.8 AF”.
The first part of that name is simple: Nikkor is
the brand name.Focal length & distance
The first number after the brand name is the
focal length, measured in millimeters. This is
not to be confused with focal distance. The focal
length refers to the distance between your cam-
era sensor (or film) and the “rear nodal point” of
your lens. Defining a “rear nodal point” usually
takes a master’s degree in optics and a couple of
textbooks, so for our purposes, just think of it as
your lens’s sweet spot.Modern camera
lenses combine
multiple elements
with various shapes
and positioning to
create a compound
lens that can project
a clear, undistorted
image onto your
camera’s sensor.
TO GET EVEN CLEARER IMAGES,
WE ADD EVEN MORE ELEMENTS
TO UNBEND AND REBEND THE
LIGHT EXACTLY RIGHT.384 C15 Technology.indd 55 3/19/19 2:33 PM