Camera Lens

A light-tight box with film in it doesn’t require a lens to expose film—even a pinhole camera can capture an image. But a lens can gather more light than a pinhole. A lens can focus and clarify. It’s the tool we use to define our image.

Scientists and photographers have studied the lens since the early Twentieth Century. Few will describe any lens as technically perfect, although a lens’ unique optical characteristics make it the best choice for a particular shoot.

Lens function is complex. We know that since all visible objects reflect light rays in all directions, we must gather as many rays as possible and get them to our film without distortion. Simple lenses use a single convex glass element positioned so that the light rays from the subject are bent towards and converge at the film. By carefully placing the lens relative to the film, we successfully record an image.

An iris is an aperture of variable size used to control the intensity of light falling on film. The iris control is usually calibrated in f-stops or T-stops. A change of one f-stop or T-stop is equivalent to doubling or halving the intensity of light falling on the film. T-stops are more accurate because they factor for light loss through the lens glasses.

Simple lenses have a limited ability to focus light. Optical distortions can result when light rays enter from the lens perimeter; those rays have to travel farther to reach the film and will be less focused. Toa limited degree we can solve this problem by narrowing the glass width; the resulting loss in light, however,makes the lens “slower.” For each additional stop of light passage in a given lens, the design becomes more complex—the geometric correction becomes more extreme.

Standard, wide-angle, telephoto, and zoom lenses include the ability to adjust focus and iris. Some lenses include a second element, positioned between the first element and the film. Its concave surface compensates for the distortion of the first element. Each element introduces distortion, typically resulting in internal reflection, or flare. The distance between these elements and the precise grind of the glass is critical, and these factors add significant cost to the lens.

An aperture controls the quantity of light passing through a lens. The best lenses perform well at every aperture setting. As such, the precision of the lens must be consistent at every point within each glass element.

Dr. Max Berek of Leitz established image quality standards before 1914 by capturing “miniature” still photographs on 35 mm film. His “circle of confusion” defined the measure of permissible out–of–focus quality in a ten-inch paper photograph. Though modified, this concept endures.

Color is probably the most complex factor in lens design. Because each color has a specific wavelength measured in nanometers, a particular shade has a unique wavelength. Blue objects and red objects will focus in diJerent places on a film frame, whether the film is black-and-white or color. Getting all colors to converge on a single plane despite their diJerent lengths is fundamental to lens design. In 1938 Kodak pioneered the concept of making glass lenses with exotic types of rare-earth elements and cementing them together in each element to correct the aberration.

Zoom lenses were developed later. A good zoom lens must address each of the potential pitfalls while oJering the utility of variable focal length. In motion picture applications, the light transmission, sharpness and individual color focus must remain unchanged despite the focal length change within a shot.

Focal Length and Focus

Lenses are identified by their focal length in millimeters and maximum aperture in f-stops (e.g., 50 mm/f1.4 lens). The focal length is defined as the distance from the optical center of the lens to the film plane. The f-stop is calculated from the dimensions of the lens.

Focal Length and Angle of View

The focal length of a lens determines the angle of view, or perspective, seen through the lens. Normal lenses provide a perspective that approximates human vision.

Lenses that are shorter than normal provide a wider angle of view—they are wide-angle lenses. Lenses that are longer than normal provide a narrower point of view and magnify the subject—they are telephoto lenses. Wide-angle lenses make background objects appear further away; telephoto lenses compress distance and make the background appear closer. Thus, moving the camera toward a subject (as in a dolly move) results in a look that is very diJerent from a scene captured by zooming the lens from a stationary camera position. The apparent separation from the background, making objects relatively smaller, makes camera movement less noticeable. Thus, using wider lenses for hand-held scenes is preferable.

Advantage of the CMOS Sensor

Sony - April 2011

1. Introduction

Existing mainstream analog security surveillance cameras have being using analog signals with a video format that can then be sent through a single BNC cable using a TV format (NTSC, PAL). Today’s typical imager is the CCD sensor in analog security surveillance cameras. Until a few years ago, the other type of imager, the CMOS sensor (mainly VGA) had been used in web cameras and some imaging devices. 

 

2. Comparing CCD & CMOS

Although both image sensors act as capture devices for a camera, the latest CCD and CMOS sensors are very different in structure. Figure 1 describes these structures. 

CMOS sensors structure

CCD sensors structure

CMOS sensor - Advantage power consumption and speed

To use CCDs in video applications, it is necessary for all the vertical and horizontal shift registers to constantly relay received image data as electronic signals. As a result, there is a limit to achieving high resolutions and increasing speed. Additionally, power consumption becomes comparatively high.

On the other hand, CMOS sensors only need to move one readout column of circuitry, so power consumption is low and it’s easy to increase speed.

3. CMOS Sensor “Exmor”

In recent years, with growing interest in small HD-resolution camcorders, there has been significant development of CMOS sensors which are low power consumption devices with high-speed image readout capabilities. In the field of security surveillance, this development is accompanied by the increasing prevalence of IP networking, which in turn builds demand for HD resolution, as the digital of the network surveillance camera signal does not depend on a conventional TV format.

Because of these growing needs, Sony has amassed its image quality knowledge accumulated in CCDs, and dedicated this to creating new, more advantageous high- speed, high-resolution CMOS sensors. The result is a CMOS sensor with an entirely new structure: the “Exmor”.

Both CCD and CMOS sensors have the same part that converts light into electricity (a key element of image quality). With the “Exmor”, however, Sony uses high image quality pixel technology accumulated in CCD development to enlarge as much as possible the light-receiving section of the photodiode.

Tables 1 and 2 show specifications and Image Sensor Characteristics for the “Exmor” CMOS, CMOS and CCD sensors. 

 

(Table 1) Specifications

Item	IMX035	IMX012	ICX445
Sensor type	“Exmor” CMOS	CMOS	CCD
Image size	Diagonal 6.08 mm (1/3 type)	Diagonal 4.7 mm (1/3.8 type)	Diagonal 6.0 mm (1/3 type)
Transfer method	All-pixel	All-pixel	Interline
Total number of pixels	approx. 1.49M pixels 1384 (H) x 1076 (V)	approx. 1.33M pixels 1304 (H) x 1017 (V)	approx. 1.32M pixels 1348 (H) x 976 (V)
Number of effective pixels	approx. 1.39M pixels 1329 (H) x 1049 (V)	approx. 1.28M pixels 1296 (H) x 985 (V)	approx. 1.25M pixels 1296 (H) x 966 (V)
Chip size	7.64 mm (H) x 7.64 mm (V)	6.452 mm (H) x 6.402 mm (V)	6.26 mm (H) x 5.01 mm (V)
Unit cell size	3.63 μm (H) x 3.63 μm (V)	2.925 μm (H) x 2.925 μm (V)	3.75 μm (H) x 3.75 μm (V)

(Table 2) Image Sensor Characteristics

Item		IMX035	IMX012	ICX445	Remarks
Sensitivity (F5.6)	Typical value	460 mV	290 mV	460 mV	3200K, 706 cd/m2 (Exposure time: 1/30 s)
Saturation signal	Minimum value	830 mV	550 mV	350 mV	Tj = 60 °C

Another major element that determines image quality is noise reduction. In “Exmor” CMOS sensors, noise on the analog part is eliminated by the built-in Correlated Double Sampling (CDS) circuit. Other new structural elements drastically also decrease the noise-contamination level.
Figure 2 and Figure 3 describe these structures.

• The A/D conversion conventionally done just before signal readout is now performed immediately after the light-to-electricity conversion, and is performed for each column. This helps to reduce noise because the analog circuit is made shorter, and the frequency lower.

• Noise-elimination circuits (CDS circuit) are equipped in the digital domain in addition to in the analog domain.

Conventional CMOS sensor structure vs “Exmor” CMOS sensor structure

Through these methods, rapid improvements have been made in CMOS sensor image quality, achieving the same performance level as CCD sensors.

Taking the example of recent, typical Sony IP cameras, the minimum object luminance specifications for cameras equipped with either a CCD or an “Exmor” CMOS sensor are as follows:

Sony CCD network surveillance camera SNC-CM120: 1.3M CCD equipped 0.8 lx Sony CMOS network surveillance camera SNC-CH140: 1.3M CMOS equipped 0.2 lx .

How to Buy a Digital Camera

Buying a digital camera can be disorienting. There are hundreds of cameras available at many different types of retail outlets (online and in traditional stores), with prices ranging from $75 to several thousand dollars. In this digital camera guide, we aim to help you overcome some of this confusion.

Buying a digital camera can be disorienting. There are hundreds of cameras available at many different types of retail outlets (online and in traditional stores), with prices ranging from $75 to several thousand dollars. Some cameras are small enough to fit in a shirt pocket. Others are large and can weigh up to two pounds. Some are easy to use. Others look like you need an engineering degree to operate them. And almost all are advertised with abbreviations that can be cryptic and confusing for the novice. In this digital camera guide, we aim to help you overcome some of this confusion.

What is a digital camera?
The first step is to understand what a digital camera is. With a film camera, an image is formed by collecting light from a particular scene or subject and focusing on film, which reacts chemically when struck by light and is said to "capture" the image. What makes a camera "digital" is that, instead of film, it has an image sensor that reacts to light by sending out electrical signals.
The camera takes the information from the image sensor and processes and stores it as a collection of pixels in a digital file, usually on a memory card inside the camera. Although the actual process is more complex than that, in essence it is how a digital photo image is made. It's essentially made up of thousands and thousands of tiny dots, or pixels.

What are megapixels?
When you collect a million pixels, you have a megapixel. The number of megapixels tells you how many pixels the image file has. A camera that captures 8 million pixels, for example, is called an 8-megapixel camera. The number of megapixels a camera features can also help to determine the size photos you can print or the amount of cropping you can do. For example, a 4-megapixel camera may be enough for snapshots, but if you want to print poster-size images or crop heavily, 8 megapixels (or greater) is more suitable.
A 6-megapixel camera might be all you'll need because higher resolution doesn't necessarily produce better prints. Lenses and other factors affect quality too. But most cameras today have at least 10-megapixel sensors. The size of the sensor, and the size of each individual image sensor element, which corresponds to pixels, can affect photo quality. But remember, the number of megapixels alone doesn't determine the quality of a digital camera's images.

Types of digital cameras
Our Ratings are divided into two main categories: Basic cameras, are simple point-and-shoots with just the features needed for routine shots, and advanced cameras, which are feature-laden cameras that include sophisticated point-and-shoot and models that let you chance lenses. Note that all point-and-shoots, whether basic or advanced, include cameras with lenses built into the camera (that is, non-removable).
Our basic camera category is divided into three subcategories: subcompacts, compacts and superzooms.
Subcompacts fit in a pocket, are lightweight but generally have few manual controls. A few include nontelescoping zoom lenses, and others have zooms as high as 14x. Compacts are a bit larger, and often have more manual controls than subcompacts. They can also be among the most inexpensive cameras available.
Superzooms offer 15x or greater zoom, with some recent models including optical zooms as great as 30x. Like compacts, superzooms often, though not always, include manual controls. They're also among the more expensive basic cameras.
Our advanced camera category is also divided into three subcategories: advanced point-and-shoots, SLR-like and SLRs.
Advanced point-and-shoots have a nondetachable lens but differ from basic models because they have lots of manual controls, a hot shoe for an external flash, and support for RAW files. It's the lightest advanced type. SLR-like models have interchangeable lenses, but they lack a through-the-lens viewfinder. They're smaller and lighter than an SLR but usually larger than a point-and-shoot. SLRs have the most features, with interchangeable lenses and the largest sensors for the best image quality in low light, and a through-the lens viewfinder. Controls are extensive. They're also the heaviest, most expensive cameras.

Next steps
After you consider the type of camera you want and the number of megapixels you need, but before you dive into specific models, be sure to check out our brand profiles, which outline many of the most popular camera product lines and their respective character traits.
Next, look to our Ratings and Recommendations (available to subscribers) for the models that have the best performance and image quality, including scores for how models capture regular, low-life and flash photos. If you're interested in how well a camera captures video, consider the video quality score. And to see which models respond the quickest, consider the response time score, which is an overall speed judgment, including start-up time and the shutter delay for the first and later shots. In most cases, our Ratings found that point-and-shoot cameras take decent snapshots. So, look through our Ratings for specific features that are important to you. For example, if you want a point-and-shoot that has a better LCD than others, look for a model with a Very Good LCD quality score. Or, if you want a model that includes a touch-screen LCD, look for that in our Ratings. There are also scores for how well a camera handles shake, which can cause blurry photos, its controls, and versatility.

What you'll spend
For many, price is a major factor when buying a camera. In general, look to pay the following for the type of camera you're looking to buy:

• For basic point-and-shoots (subcompacts, compacts, and superzooms), expect to spend $100 to $400.
• For advanced point-and-shoots, expect to spend $350 to $600.
• For SLR-likes, expect to spend $450 to $1,200.
• For SLRs, expect to spend $500 to $2,000.
When you're ready to buy, consider where you will make your purchase. Although some walk-in stores, such as photo-specialty camera shops, might have knowledgeable salespeople, you can't rely entirely on the staff of walk-in stores to assist you in your purchase. Use the internet and our Ratings for information before buying. Also, if you decide to purchase at a traditional retail store, forgo the extended warranty because digital cameras have been among the most reliable products in our surveys.
Many respondents in our surveys found online shopping to be a more satisfying shopping experience than walk-in-store shopping. Most walk-in retailers offer either low prices or wide selection. But some online retailers offer both. But be cautious of very low prices and verify that the camera isn't refurbished or gray market (diverted from other retailers or not meant for sale in the U.S.).