Lens focusing and bokeh
There are a few things going on when a lens refocuses.
symmetrical and asymmetrical
When a lens defocuses, the appearance of the defocus is called
bokeh. If the iris blades of the lens are hexagonal, then the
bokeh will also be hexagonal. If the iris blades are round,
then the bokeh will be round. If the iris blades are shaped
like a heart, then the bokeh will be heart shaped.
Long lenses, like astronomical telescopes, are often mirror
lenses or cadiotropic. These have a doughnut shaped
bokeh, because the optical path is doubled back and there is a
small mirror in the center that occludes the bokeh.
higher quality the lens, the less lens flaring there is. Lens
flaring is when bright light hits the lens elements and it is
an optical aberration. Some people use lens flare for creative
effect. Anamorphic lenses have an especially distinctive lens
flare. This can be simulated with Knoll
Lens Flare tools.
When you focus a lens, there is a slight change in zooming or
magnification. This is called lens breathing or bellows
In a spherical lens the breathing is symmetrical, but in an
anamorphic lens the zooming will be quite different
horizontally and vertically. In 3DE, this is called X-stretch
In a simple lens like a spherical Primo, the lens breathing is
a simple formula. At infinity, the focal length will be the
nominal value, e.g. 100mm. As the lens close focuses, the
focal length will increase. So when close focused, a 100m lens
might zoom (breathe) to 110mm.
On more complicated lenses, like zooms and floating element
designs, the lens breathing might be completely different,
since there are complex lens groups inside the lens counter
animating one another's motion. In a simple lens all of the
glass is moved in unison towards and away from the sensor and
the basic bellows extension formula is followed.
displacement for focus closer than infinity
Click And Save As:
spreadsheet for lens breathing
This calculates lens "breathing" for a simple lens
like a Panavision spherical Primo
D = displacement of lens
f = focal length of lens
a = distance of focus
D = f2/(a-f)
effective focal length of lens = nominal lens focal
length (at infinity) + D