Page 30 - Security Today, March 2017

Generally, the more lens elements you have the better the resolu- tion 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, surveil- lance 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 open- ing. 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 con- trol 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 cam- era in the same way as P-iris.
Determining Aberrations
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’
center.
• Astigmatism: Off-axis points are blurred in the radial or tangen-
tial 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 differ-
ent 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.
UNDERSTANDING CAMERA LENSES
• Lateral chromatic aberration: Slightly different magnifications for different wavelengths cause color fringing at the edges of the frame.
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0317 | SECURITY TODAY
Understanding camera lenses can help add more creative control to digital photography. While creativity isn’t a requisite for a security system, the idea that the lens you are choosing will make a difference in clarity and quality. Choosing the right lens for the task can become a complex trade- off between cost, size, weight, lens speed and image quality. This tutorial aims to improve understanding by providing an introductory overview of concepts relating to image quality, focal length, perspective, prime vs. zoom lenses and aperture or f-number.
LENS ELEMENTS AND IMAGE QUALITY
All but the simplest cameras contain lenses that are actually comprised of several “lens elements.” Each of these elements directs the path of light rays to recreate the image as accurately as possible on the digital sensor. The goal is to minimize aberrations, while still using the fewest and least expensive elements.
Optical aberrations occur when points in the image do not translate back onto single points after passing through the lens, causing image blur- ring, reduced contrast or misalignment of colors (chromatic aberration). Lenses may also suffer from uneven, radially decreasing image brightness (vignetting) or distortion.
INFLUENCE OF LENS FOCAL LENGTH
The focal length of a lens determines its angle of view, and thus also how much the subject will be magnified for a given photographic position. Wide angle lenses have short focal lengths, while telephoto lenses have longer corresponding focal lengths.
CAMERA LENSES