Fresnels are the workhorses of stage
lighting and the most common luminaire (light) used on stage.
They are called a ‘Fresnel’ because they use a ‘Fresnel’ lens,
recognised by the characteristic ‘stepped’ moulding
on one face and the texturing on the other, named after its designer,
Augustin Fresnel.
This Fresnel lens produces a very even light that is soft at
the edges and tends to project a soft shadow. Because the edge
of the light is soft, it is not absolutely precise and will blend
easily with the edges of other Fresnels to give smooth stage coverage.
SOFT EDGE = the centre of the beam of light
is bright and gradually darkens towards the edges.
The beam is a cone of light so the size of your lit area increases
as the throw distance from the Fresnel to that area increases.
However, the beam angle of this light cone can be altered by a
focus knob that changes the distance between the lamp and the lens.
As the lamp (with its reflector) is moved towards the lens, the
beam becomes wider, and as it moves away from the les, the beam
becomes narrower.
This adjustment is accomplished on Selecon Fresnels using a unique
posi-slide lamp and lens focus system located on the left hand
side (looking from the front) of the luminaire using a focus
knob which simply slides back and forth.
A detailed knowledge of light’s behaviour and 
measurement
is not required to light a production however you will need to
know about the
INVERSE SQUARE LAW:
When a beam of light leaves a spotlight,
the area illuminated by the beam increases as it goes further
away from the spotlight. The same quantity of light must illuminate
larger areas. If the distance from the light source to the surface
(otherwise known as ‘the throw distance’) is doubled, the intensity
drops to ¼ of the original. This happens because the illuminated
surface is now four times bigger.
What does this mean when lighting a stage?
Take two actors (A and B) on a stage, both lit by the same spotlight.
If the distance from the spotlight to B is twice the distance
from the spotlight to A, then B will only have a quarter of the
light illuminating A.
This
is one of the reasons why a number of lighting positions are
needed. If every spotlight were hung in front of the stage, the
back of the stage would appear quite dark. To avoid this, lighting
positions are needed over the stage as well as in front of it.
With the wide beam range Fresnels are very versatile, often used
for stage colour washes as well as selective highlighting and
modelling.
In addition to giving a soft edge to the beam, a Fresnel lens has
a tendency to cast some low-intensity light outside the main
beam; such spill light can be a nuisance if the luminaire is
close to a scenery border or wing.
The spill of scatter light can be controlled an accessory called
a barndoor which slips onto the front of the Fresnel. The barndoor
has four rotatable shutters that can be used to introduce up
to four straight soft edges into the beam.
HANDY HINT: Barndoors should
be used sparingly on the smaller stage – they take up a
lot of extra space and may catch on curtains or gauzes causing
rips and damage.
To summarise: the Fresnel gives you control of the
beam size and the barndoor gives you control of the beam shape
however, a Fresnel has no adjustment for beam quality; by choosing
a Fresnel luminaire you are choosing a soft edge.
