American Cinematographer - April 2020 - 18

A photograph of a monitor looking at the bokeh from a Lomo Super Speed during a lens test.

of-focus highlights (bokeh) in your
image. (Actually, it defines the shape of
all the out-of-focus points in the image,
but we mostly notice it in the highlights.)
It's popular to have a rounded, multibladed iris because that creates more
organic ("pleasing" to some), circular,
out-of-focus highlights. If your iris has
sharp geometric shapes, then so, too,
will your bokeh balls.
A front-anamorphic lens will have
an elliptical/oval entrance pupil, as the
cylindrical elements in front of the iris
create a disproportionate magnification
that "squishes" the entrance pupil into
an oval shape. That's what gives frontanamorphic lenses that elliptical bokeh.
If you don't have an anamorphic
lens but want this look, you can actually
fake it by changing the shape of the
entrance pupil (or the shape of the iris).
You can do this with an oval cardboard
cutout in front of the lens. (There are
many DIY videos about this online.) You
can also use a cutout in pretty much any
other shape to turn those out-of-focus
highlights into little stars, hearts, Christmas trees or even Saturday Night Live
logos.
This look is so favored that
Vantage offers a set of "hybrid" spherical lenses called MiniHawks, which have
a special dual iris that forms an approximately elliptical shape and gives the
image a pseudo-anamorphic bokeh. The
MiniHawk lenses are not anamorphic at
all, but they do provide a somewhat
April 2020

anamorphic elliptical shape to the highlights.

A Peek Into the Optical Magic
What else can we learn from the
entrance pupil?
Well, as you're looking at it from
the front of the lens, try to figure out
where its position is inside the lens. Hold
your finger over the top of the lens
barrel and estimate where that iris sits.
Often, this won't necessarily align with
the actual position of the iris inside the
optomechanics, but don't worry about
that. We want to know its virtual position.
Take note of where that spot is
that you just estimated. Put a little piece
of tape on the lens in that position. Now
turn the lens around and look through
the back. See the iris in there? Now
we're looking at the exit pupil.
Yes, it's still really that simple: the
exit pupil is the image of the iris as seen
through the back of the lens.
You with me so far?
Put your finger on the lens barrel
where the exit pupil appears to be inside
the lens. Put a second little piece of tape
there. Are they in the same place?
If yes, then you're looking at a
symmetrical optical design. This is pretty
common in "standard" focal-length
primes that are often of a Double Gauss
or modified Double Gauss design.
Symmetrical means the optical elements
inside the lens are pretty much exact
American Cinematographer - 100 Years

mirrors of each other, with the iris in the
middle. This puts the entrance pupil and
exit pupil in the center of the lens at the
same position.
If the entrance pupil is significantly
smaller than the exit pupil, it's most likely
an inverse-telephoto (or retrofocus) optical design used on wide-angle lenses. This
is because inverse-telephoto designs
feature significantly powered negative
(diverging) front elements and lesspowered positive (converging) rear
elements in the lens group in front of the
iris, making the entrance pupil appear
very small and the exit pupil look larger
and closer to the back of the lens.
Inversely, if the entrance pupil is
significantly larger than the exit pupil, the
lens is most likely telephoto. This is
because the design is the exact opposite
of the retrofocus, with a significantly
powered positive (converging) front
element and a weaker negative (diverging) rear element in the lens group in
front of the iris. This positive element in
front of the pupil magnifies the entrance
pupil's perceived size.
If the exit pupil is deep into the
lens, then the lens has telecentric properties, meaning the light rays exit the lens in
a more parallel configuration that may
make the lens more suitable for use with
digital imagers.
If you look at the entrance pupil in
a zoom lens, you can see its size and position change as you alter focal length. In
some cases, you'll notice that the physical
aperture inside the zoom will actually
open up or close down as you zoom; this
is a mechanical compensation for the
optical change in entrance-pupil size to
make sure that the relative amount of
light you're shooting with (f-stop)
remains consistent even if you're magnifying or de-magnifying the pupil. Without
this compensation, you'll have a change
in light passing through the lens, and that
is called a "ramping" zoom.
So, while the definition of the
entrance pupil seems simple, the practical implications are not.
With gratitude for his consultation, I'd like to thank optical consultant
and ASC associate Iain Neil. Based in
Switzerland, Neil holds more than 100

American Cinematographer - April 2020

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