A Line of Sight
The lens is a critical component on any camera
- By Fredrik Nilsson
- Mar 01, 2017
Video surveillance is both a science and an art. Success
often depends on whether you match the right
camera with the right lens. An optimum pairing
can give you a forensic record rich in visual detail.
A poor pairing could leave you clueless.
A Brief History
Twenty years ago most cameras sold were fixed box cameras with
exchangeable lenses. Since many cameras were sold without a lens,
aftermarket lenses became a big business. Even so, most cameras operated
on the same 4CIF resolution. For surveillance purposes, the
top selection criterion was the focal length of the lens. Fast forward
to today, and the selection process has become a lot more complex.
With the abundance of lenses and cameras and the variety of resolutions
and image enhancing technologies on the market, matching the
right camera with the right lens can be quite a challenge. Fortunately,
camera vendors today are simplifying the selection process by packaging
a majority of their cameras with complementing lenses.
As demands for ever-sharper image quality continues to grow, video
camera manufacturers have begun looking to technology from the
consumer electronics industry to meet the challenge. Now, we have
15-30 megapixels video cameras using Single-Lens Reflex (SLR), a
crossover from the professional photography market. Like their photographic
camera counterparts, new high-megapixel SLR video cameras
use exchangeable lenses further expanding user options. So what
should users know that can help with the selection process?
Understanding Lens Technology
A camera lens is an assembly of multiple individual lenses of different
shape and materials (glass/plastic) that are mounted at different
distances. The quality of the lens material, the coatings on a lens,
and how it’s assembled influence the quality and resolution a lens can
provide. Usually the most difficult task is to keep each glass element
in the exact correct position to get a perfect image.
The goal of a lens is to be able to distinguish small details in every
pixel of the sen sor. If the resolution of the lens is too low, a small
detail will be spread over many pixels of the sensor, creating an image
that isn’t sharp.
Generally, the more lens elements you have the better the resolution
you’ll get. If a lens is marketed as a 5MP lens it will mean that
it has a resolution matching a camera sensor of that resolution. In
standard definition surveillance cameras the typical size of a pixel on
the sensor is between 2 and 3 μm. In very high megapixel cameras,
a larger sensor with a 4 μm pixel size is often used. When compared
with typical SLR lenses used in professional photography, surveillance
lenses for 1080p or 5 megapixel video cameras generally have
higher resolving requirements. On the other hand, SLR lenses need a
large diameter which increases their cost.
Determining Light Sensitivity
Light sensitivity is determined by the size of the aperture or F-stop
of the lens. The lens comes with either a fixed or adjustable iris opening.
Fixed apertures are best suited for indoor environments with
consistent lighting. Set the iris opening once and leave it. Adjustable
lenses are best in situations with varying light levels. The opening can
be automatically adjusted by a control signal from either the camera
(DC-iris) or the lens itself (video-iris). In either case, the iris opens
and closes in response to the light level but is incapable of adjusting
its depth of field.
A third kind of automatic iris control is P-iris, a precise iris control
which optimizes the aperture opening in such a way as to achieve
better contrast, clarity, resolution and depth of field. Modern SLR
lenses usually have integrated electronic iris controlled from the camera
in the same way as P-iris.
It’s important to realize that no lens is perfect. All create some form
of image defects as a result of their limitations. These include:
- Spherical aberration: Light passing through the lens’ edges is
focused at a different distance than light striking near the lens’
- Astigmatism: Off-axis points are blurred in the radial or tangential
direction. Focusing can reduce one at the expense of the other,
but it cannot bring both into focus at the same time.
- Distortion (pincushion and barrel): The image of a square object
has sides that curve in or out.
- Chromatic aberration: Wavelengths of color are focused at different
positions on the focal plane. This causes color blurring and a
less sharp image.
- Stray light: Light objects at the borders or just outside create
ghost and flare phenomenon in the scene.
- Lateral chromatic aberration: Slightly different magnifications for
different wavelengths cause color fringing at the edges of the frame.
Professional Photography Lenses
in Video Surveillance
Designed for sport and bird photography the newest professional lenses
integrate high resolution and long zoom with excellent light sensitivity.
When used with a state-of-the-art DSLR they can easily outperform
most traditional security cameras for long-distance targets. This
opens up surveillance opportunities that we’ve not seen before.
Like PTZ cameras, most modern DSLR lenses come with a motorized
focus feature that automatically selects the best focus. But
DSLR lenses need to set focus much more accurately than lower resolution
camera lenses. And there are a few caveats. Though they can
be mounted on large pan/tilt heads the autofocus needs to be started
manually. It’s also important to avoid continuous operator use to
avoid motor wear out. Furthermore, image stabilization is available
in many lenses.
The advantage of longer DSLR lenses is that they provide a very
different perspective from a wide angle lens mounted close to the target.
Even if both lenses perfectly capture the target and fill the whole
image area the amount of captured background will differ greatly
and affect how the image is perceived.
The Eyes Have It
Pairing the right lens with the right camera begins with establishing a
goal. What do you want to monitor? What field of view do you want
to establish? How much detail do you need to capture? What environmental
challenges do you need to overcome?
Once you know the particulars, it’s a matter
of selecting the best camera and lens for the job.
Be sure to examine their strengths and limitations
with respect to your goals. Surveillance, after all,
is a matter of keeping your eyes on the prize.
This article originally appeared in the March 2017 issue of Security Today.