Consider the Lens
Video surveillance systems need the right glass to work properly
- By Andrea Iniguez
- Nov 01, 2015
This is a story of lens selection gone wrong,
and right. Often it seems lens selection is not
given much attention, and at worst is an afterthought.
Even when it is considered carefully, it
is not always fully understood. Due diligence is
important to avoid costly mistakes when designing
video surveillance systems.
The following two case studies will be used to illustrate some of the
most important considerations in lens selection. Both cases are from
the public schools sector. The two school had defined goals, both did
research, evaluation and testing of the selected lenses before installation.
Yet one project failed, while the other was a success. What went
wrong in the failed case? What made the difference in the successful
application? We’ll dig into the details to find out.
New Jersey High School
The school described their problem as a safety issue. They had unacceptable
aggressive student physical behavior they did not want to
escalate into violence, as well as limited theft. School officials thought
that an improved video surveillance system would help with deterrence
as well as post event response.
Their existing video surveillance system was composed of analog
cameras they considered “high resolution,” but not high enough resolution
for use with wide angle lenses that they felt would cost-effectively
meet their coverage needs. They did not have or wish to have PTZ
cameras as they had limited staff and would not have enough time to
control and monitor such equipment.
The goals they identified included having forensic quality identification
at entry and exit points, people recognition in parking lots, and forensic quality identification in an unusually shaped hexagonal
The school security staff conducted an extensive online search for
products to meet their needs. They felt that upgrading to megapixel
cameras would provide them higher image resolution and still allow
them to use wide angle lenses to cover wide areas. They looked for
megapixel cameras and did a comparison of numerous camera brands,
models and features. They discovered that not all megapixel cameras
could meet their needs.
Looking for Coverage
In addition, school officials researched wide angle lenses looking for
coverage of their wide areas; they consulted local integrators for advice.
They evaluated lens samples from four lens manufacturers. They found
most wide angle lenses they tested presented a severe fisheye effect. To
their surprise they found that some of the lenses tested resulted in a
fuzzy or blurry image even when using a 5 megapixel camera.
After the research and evaluation, they selected two,8-megapixel,
360 degree multi-sensor cameras, one each for the cafeteria, and
another for the computer lab. They also selected 34 5-megapixel cameras,
11 of which were outdoor domes, six were indoor domes, and
another 17 were 5-inch indoor domes.
They chose 18 to 25 degree horizontal field of view lenses, and eight
90-degree lenses. In addition, they selected 12 135-degree horizontal
field of view, low fisheye distortion lenses from Theia Technologies; six
to cover their hexagonal hallway, two to cover the parking lot and an
open field area, one covering the front entry, two covering the corners
of building wings, and one more covering two temporary buildings.
They achieved the results expected when the project came in 30
percent under budget; they were able to achieve what they considered
great resolution with the selected equipment in the designated placements,
with excellent image fluidity, and fast response time. The
administration was fully satisfied and planned to upgrade other
schools in the district as well.
Southern California High School
Similar to the problems outlined in the New Jersey high school, the
safety of students and staff was the primary concern for the administration
at this school. Their existing video surveillance system consisted
of 170 analog cameras.
The system did not provide enough resolution for wide angle lenses
and a legally acceptable description of individuals. They also suffered
from poor system reliability, with frequent breakdowns, and down time.
In addition, the system was complex and costly to maintain and monitor.
As to be expected, the administration’s goals were to achieve greater
spatial coverage than with their existing analog system, improved
image clarity, increased system uptime and a more manageable system
size. With a new system they expected to be able to zoom in to get
legally acceptable descriptions of individuals from 60 to 70 feet away,
and have wide and continuous coverage; they also did not want any
Their approach was a little different. The school district hired a
consultant to design a system to meet their needs. The consultant met
with the school to understand their needs, selected the equipment,
sought the advice of manufacturers, and conducted testing.
The consultant specified the system, reducing the camera count
from 170 analog cameras to 43 megapixel cameras, and 43, wide angle,
low distortion, 5 megapixel lenses, also from Theia Technologies.
The result: the cameras and lenses were purchased, installed, and
shortly thereafter the lenses were returned. What went wrong?
What went wrong? Enter the CSI effect.
As powerful as megapixel cameras combined with megapixel lenses
can be, there are physical limitations of the technologies that popular
shows such as CSI ignore, resulting in the creation of unrealistic expectations.
There is an undeniable temptation to believe that with a megapixel
camera and megapixel lens, anything is possible.
Image Resolution versus Field of View
One important, but often neglected consideration is the trade-off
between image resolution and field of view. With any given combination
of camera and lens, the native resolution from the camera is
spread over the entire field of view of the lens, affecting pixel density
and image resolution. The wider the resolution is spread, the lower the
pixel density. The images below, taken with the same camera from the
same distance, illustrate this trade-off. The widest field of view allows
you to cover the widest area, but does not allow you to see high detail,
while the narrowest field of view permits capture of high detail, but at
the expense of wide area coverage.
The next series of images, each taken with the same high resolution
camera from the same distance away, similarly illustrates the sacrifice
of high detail and ability to identify a person, with a wide angle of view
and the ability to cover a wide area.
One Size Does Not Fit All
Another important consideration in systems design is tailoring, understanding
the specific needs and using the right equipment to meet
those needs. Earlier in the story you may have noticed that both the
California and New Jersey schools recognized the need for different
resolution at different locations. And yet equipment selected for the
new California school system reflected a “one size fits all approach.”
This was a major fault of the system design which caused the return
of the lenses. After installation, it was discovered that the school administration
had expected not only to be able to cover the wide parking lot,
but also zoom in to see license plates and identify faces at a distance. In
this situation a combination of lenses, a wide angle lens for coverage,
and a narrower one covering entry and exit points with high detail,
could have been used in combination to accomplish both tasks.
A more tailored approach to equipment selection might take longer,
in terms of research and testing, and may also mean losing volume
discounts achieved through buying units in larger quantities. But the
annoyance, time, expense, downtime and safety risk of installing, then
ripping out the incorrect equipment and re-installing the correct
equipment will be saved.
Use Available Tools
There are many useful tools available to help support systems design
and correct equipment selection. One of those is Theia’s own free
image resolution and lens calculator that allows you to input some
important details of your application and potential equipment, then
quickly calculates image resolution given those assumptions. You
input the camera resolution and sensor size you intend to use, the lens
field of view you believe you need, then input your key project variables—
either how much resolution you think you need in terms of
pixel density, or how much coverage you need in distance (feet or
meters), or how far away the camera is from the subject. The tool then
calculates the remaining variables for you.
For example, you indicate you plan to use a 5 megapixel, 1/ 2.5” sensor
camera, and a 3mm lens, and you will be 50 feet away from your
subject. The tool will calculate your pixel density, in this case 27 pixels/
foot and coverage width, here 97 feet wide. The tool then provides an
image that represents the level of image resolution you would get for
your selected variables, allowing you to quickly see if you have enough
resolution to meet the needs of your application.
Do I Need a Megapixel lens?
In testing, the New Jersey high school determined that using a standard
resolution lens with a 5 megapixel camera produced a blurry image. Most often if you want to take advantage of all the pixels a multi-megapixel camera provides. A high quality lens with matching resolution
is needed otherwise the money spent on a high resolution camera
When using a megapixel camera it is important that the spot size of
the focal plane of the lens used be comparable in size or smaller than
the pixel size of the camera, otherwise the light falls off the pixel resulting
in a fuzzy image.
A megapixel lens requires more elements, more complex shaped and
higher precision elements such as aspheres, making them more difficult
to manufacture and more costly to purchase.
In megapixel cameras the pixel size and the required lens spot size
varies depending on the size of sensor and quantity of pixels it contains.
You can have 2 different cameras, both 1.3 megapixel, but if the
sensor size is not the same, the pixel size will be different. The figure
below illustrates this.
In selecting a lens it is important to choose one that is designed for
use with the same sensor size or smaller and designated to match or be
higher than the megapixel rating of the camera.
Beyond that, not all megapixel rated lenses are created equal and the
task of selecting a megapixel lens is complex.
Do I need a Day/Night Lens?
In the two school case studies, Theia’s SY125 lens was used. The SY125
lens is not a Day/Night or IR corrected. Was the lens selection correct?
The answer is: it depends.
A Day/Night, or IR corrected lens maintains focus when light from
the IR spectrum is present. Usually lenses are designed and focused in
the visible spectrum light range. If they are not IR corrected, when
light from the IR spectrum is introduced, there is a focus shift, or defocus
resulting in a fuzzy image.
Sunlight, streetlights (except fluorescents), and IR LED illuminators
for invisible illumination are sources of IR light. When using a lens in
naturally occurring IR light, when introducing IR illumination artificially
with illuminators, and when using a Day/Night cameras which
remove the IR blocking, or IR cut filter at night allowing IR light to hit
the sensor, an IR corrected, or Day/Night lens is required.
IR corrected lenses are specially designed to be able to focus visible
spectrum light and IR spectrum light to the same focal plane. This
requires a more complex lens design with often more lens elements,
more complex shaped elements such as aspheres, as well as special
The number of elements and glass coatings also affect light transmission,
again requiring more complex lens designs and often more
expensive lenses to achieve the desired F/# and light gathering ability.
No Substitute for Homework
As illustrated with these 2 cases, it is critical to not only research the
system equipment, its features and benefits, but it is just as important
to understand the needs of your application, keeping in mind that
these will vary by location, and may even be in conflict and require
trade-offs to meet those needs.
After understanding the needs and goals of the applications and
system, and researching the equipment for each location, then use
tools to calculate if the selected equipment will do
what you believe it should. Once this is done,
verify the equipment will actually work by evaluating
the products in the actual application.
This article originally appeared in the November 2015 issue of Security Today.