DIGITAL CAMERA NOTES

List of contents:

1. Introduction…………………………………………………………………………………………………….. 1

2. Working principle of a digital camera……………………………………………………………….. 1

3. Image description

3.1 Pixels………………………………………………………………………………………………………… 2

3.2 Pixel count……………………………………………………………………………………………….. 3

3.3 Aspect ratio…………………………………………………………………………………………...... 3

4. Digital camera

4.1 Focal length………………………………………………………………………………………………. 4

4.2 Angle of view…………………………………………………………………………………………….. 4

4.3 Digital and optical zoom…………………………………………………………………………….. 5

4.4 Shutter………………………………………………………………………………………………………. 5

4.5 Aperture……………………………………………………………………………………………………. 6

4.6 Depth of field…………………………………………………………………………………………….. 6

5. Exposure of camera………………………………………………………………………………………….. 7

6. Image sensors…………………………………………………………………………………………………… 7

7. Methods of image capturing…………………………………………………………………………….. 9

8. Types of camera……………………………………………………………………………………………….. 9

9. Appendix. …………………………………………………………………………………………….…………… 11

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1. Introduction:

A digital camera is a device that can capture and store images or videos with the help of an

electronic image sensor. The digital camera are in wide spread use in today’s world ranging

from mobile compact cameras to professional photographic and scientific cameras used in

applications like remote sensing, satellite imaging, etc. During the 1960s, NASA converted from

using analog to digital signals with their space probes to map the surface of the moon (sending

digital images back to earth). Computer technology was also advancing at this time and NASA

used computers to enhance the images that the space probes were sending. A digital camera

can sometimes hold hundreds of pictures. There are different sizes of memory cards and each

size can hold a different number of pictures. Most digital cameras can use a USB cable that

connects into a computer to take the pictures that are in the camera and send them to the

computer.

2. Working principle of a digital camera:

Capturing an image: Light reflected from the object enters the camera through a lens assembly

placed in front of the image sensor. This is done to control the path of the light entering. The

lens assembly also has a diaphragm or aperture to regulate the amount of light entering into

the camera. The light then passes through a shutter which when opened exposes the sensor to

the light. The image sensor consists of large number of “buckets” which converts the light

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coming from the shutter into an electrical voltage and this data is sent to the processor for

computing.

Viewing the object: The light from object is re-directed to the by using a rotating mirror that

reflects the light coming from the object on to a pentaprism (a five sided prism). Light travels

around in the prism in a complex manner and enters the eye piece for viewing by the user.

When the user has to capture the image he is looking at, the mirror flips upward and shutter

opens exposing the light to the image sensor.

Processing and compression: The voltage levels obtained while capturing an image are analog;

in order to convert them into a digital signal an ADC (analog to digital converter) is used. The

binary data from ADC is fed to a micro-processor that generates the image and compresses it to

be stored into memory.

Storage: The image generated by the processor is stored into a memory card placed in the

camera.

3. Image description

3.1 Pixels:

A pixel is a contraction of the term PIcture ELement. A digital image is made up of many

squared pixels, though a digital photograph looks smooth and continuous, it's actually

composed of millions of tiny squares as shown below.

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On the left the full image, on the right the area in the red square magnified to show individual

pixels

Each pixel in the image has a numerical value of between 0 and 255 and is made up of three

color channels. So for example a pixel could be 37-red, 76-green and 125-blue and it would

then look like this . If it was 162-red, 27-green and 12-blue, it would look like this .

There are over 16 million possible combinations using this scheme and each one represents a

different color. Each color in this scheme can be represented by an 8-bit number (byte), so the

color of each pixel is defined by three color bytes. This scheme can be expanded, for example to

use 16-bits (two 8-bit bytes) for each color. Images using three 8-bit values are sometimes

called 24-bit color images. Images using three 12-bit values for color definition are called 36-bit

color images, and those using three 16-bit values are called 48-bit color images.

3.2 Pixel Count:

One of the main ways that manufacturers categorize their digital cameras is in terms of pixel

count. It is the number of individual pixels that go into making each image. Sometimes two

numbers are given, total pixels and effective pixels. Total pixels count every pixel on the sensor

surface. Usually the very edge pixels aren't used in the final image. Effective pixels are the

number of pixels actually used in the image after the edge pixels have been dropped.

3MP 4MP 5MP

Largest Image

(typical) 2048 x 1536 2272 x 1712 2592 x 1944

Image resolutions for various pixel counts (fixed aspect ratio)

3.3 Aspect Ratio:

The aspect ratio of a camera is the ratio of the length of the sides of the images. For example, a

traditional 35mm film frame is approximately 36mm wide and 24mm HIGH. This has an aspect

ratio of 36:24, which can equally well be expressed as 3:2. Some digicams use the same aspect

ratio for their digital images. For example most digital SLR (single lens reflex) cameras have a

3:2 aspect ratio. However, video monitors typically use a 4:3 aspect ratio. For example a

monitor with an 800x600 display has a 4:3 aspect ratio.

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4. Digital camera

4.1 Focal length:

Focal length, usually represented in millimeters (mm), is the basic description of a photographic

lens. It is not a measurement of the actual length of a lens, but a calculation of an optical

distance from the point where light rays converge to form a sharp image of an object to the

digital sensor or 35mm film at the focal plane in the camera.

The focal length tells us the angle of view—how much of the scene will be captured—and the

magnification—how large individual elements will be. The longer the focal length, the narrower

the angle of view and the higher the magnification and vice versa.

4.2 Angle of view:

When shooting an object using a camera, there is a certain range that can be captured as an

image. Angle of view indicates the capturable range of the image (plane) measured by the angle

from the center of the lens to the width of the images in the horizontal, vertical and diagonal

directions. These are called, the horizontal angle of view, vertical angle of view and diagonal

angle of view respectively.

Angle of View becomes narrow when we use a lens of large focal length and the angle of view

becomes wider in case of a lower focal length lens.

Angle of view can be derived from the following equation.

W = 2tan-1 y/2f

(W: angle of view; y: width of the

image in horizontal, vertical or

diagonal direction; f: focal length)

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4.3 Digital Zoom and Optical Zoom:

Most cameras have both optical zoom and digital zoom. Optical zoom works just like a zoom

lens on a film camera. The lens changes focal length and magnification as it is zoomed. Image

quality stays high throughout the zoom range. Digital zoom simply crops the image to a smaller

size, and then enlarges the cropped portion to fill the frame again. Digital zoom results in a

significant loss of quality as is clear from the examples below.

Comparison of digital and optical zoom

4.4 Shutter:

A shutter is a device that allows light to pass for a determined period of time, for the purpose of exposing photographic film or a light-sensitive electronic sensor to light to capture a permanent image of a scene. A shutter can also be used to allow pulses of light to pass outwards, as in a movie projector.

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Shutter speed: shutter speed or exposure time is the effective length of time a camera's shutter

is open. The total exposure is proportional to this exposure time, or duration of light reaching

the film or image sensor. The shutter speed changes the way movement appears in the

picture.

4.5 Aperture:

It is referred to the lens diaphragm opening inside a photographic lens. The size of the

diaphragm opening in a camera lens REGULATES amount of light passes through onto the

sensor inside the camera the moment when the shutter curtain in camera opens during an

exposure process. The size of an aperture in a lens can

either be a fixed or the most popular form in an

adjustable type (like an SLR camera).

The size of the aperture is measured in f-numbers,

which is the ratio of focal length to the effective aperture

diameter.

Typical ranges of apertures used in photography are

about f/2.8–f/22 or f/2–f/16, covering 6 stops, which may

be divided into wide, middle, and narrow of 2 stops each,

roughly (using round numbers) f/2–f/4, f/4–f/8, and f/8–

f/16 or (for a slower lens) f/2.8–f/5.6, f/5.6–f/11,

and f/11–f/22. These are not sharp divisions, and ranges

for specific lenses vary.

4.6 Depth of field:

When focusing a lens on an object, there is a certain distance range in front of and behind that

also comes into focus. Depth of field indicates the distance between the closest and farthest

object that are in focus. When the distance is long, the depth of field is “deep” and when short,

the depth of field is “shallow”.

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5. Exposure of the camera:

The exposure of a camera to the light coming from the object depends on 3 major factors:

Shutter speed- when a person is trying to take the picture of a moving object, then a motion

effect can be created only when the shutter speed is low; when the shutter speed is high the

object appears still.

Aperture- the size of the aperture affects the depth of the field.

ISO speed- ISO is actually a common short name for the International Organization for

Standardization. The ISO speed is a measure of the sensitivity of the camera to the incoming

light. Similar to shutter speed, it also correlates with how much the exposure increases or

decreases. However, unlike aperture and shutter speed, a lower ISO speed is almost always

desirable, since higher ISO speeds dramatically increase image noise.

6. Image sensors:

An image sensor is a device that converts an optical image into an electrical signal. There are two types of image sensors being used in present day photography

- Digital charge-coupled device (CCD) - Complementary metal–oxide–semiconductor (CMOS) or active pixel sensors. Both CCD and CMOS are pixelated metal oxide semiconductors (photo-diodes) made from

silicon. They have basically the same sensitivity within the visible and near-IR spectrum. They

both convert the light that falls onto them into electrons by the same process and can be

considered basically similar in operation. Both CMOS and CCD imagers can only sense the

level/amount of light but not its color. Colored images can therefore only be created either by

coating each pixel with a colored filter (red, green or blue) and then interpolating the missing

information or making 3 exposures, each through a colored filter (again red, green and blue).

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A comparison of CCD and CMOS image sensors:

CCD CMOS

Long history of high quality performance

Lower performance in past, but now

providing comparable quality

High Dynamic range Moderate Dynamic range

Low noise Noisier, but getting better quickly

Well established technology Newer technology

High power consumption Relatively low power consumption

Moderately reliable

More reliable due to integration of

chip

Small pixel size (small sensors - best to develop

new cameras & lenses)

Larger pixel size (larger sensors - easier

to use within current camera

technology)

Needs lots of external circuitry All circuitry on chip

High Fill Factor (The fill factor indicates the size of

the light sensitive photodiode relative to the

surface of the pixel). Lower Fill Factor

CCD creates analogue signal that is digitised off the

chip CMOS creates a digital signal on chip

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7. METHODS OF IMAGE CAPTURING:

There are three main methods of capturing the image, each based on the hardware

configuration of the sensor and colour filters.

The first method is often called single-shot; pertaining to the number of times the camera's

sensor is exposed to the light passing through the camera lens. Single-shot capture systems use

either one CCD with a Bayer filter, or three separate image sensors (one each for the

colours red, green, and blue) which are exposed to the same image.

The second method is referred to as multi-shot because the sensor is exposed to the image in a

sequence of three or more openings of the lens aperture. There are several methods of

application of the multi-shot technique. The most common originally was to use a single image

sensor with three filters (once again red, green and blue) passed in front of the sensor in

sequence to obtain the additive colour information. Another multiple shot method is

called Micro scanning. This technique utilizes a single CCD with a Bayer filter but actually moved

the physical location of the sensor chip on the focus plane of the lens to "stitch" together a

higher resolution image than the CCD would allow otherwise. A third version combined the two

methods without a Bayer filter on the chip.

The third method is called scanning because the sensor moves across the focal plane much like

the sensor of a desktop scanner. Their linear or tri-linear sensors utilize only a single line of

photo sensors, or three lines for the three colours. In some cases, scanning is accomplished by

moving the sensor e.g. when using colour co-site scanning or rotate the whole camera; a

digital rotation line offers images of very high total resolution.

8. Types of cameras:

Digital cameras are made in a wide range of sizes, prices and capabilities. The majority are

camera phones, operated as a mobile application through the cellphone menu. Professional

photographers and many amateurs use larger, more expensive digital single-lens reflex cameras

(DSLR) for their greater versatility. Between these extremes lie digital compact cameras and

bridge digital cameras that "bridge" the gap between amateur and professional cameras.

Specialized cameras including multispectral imaging equipment and astrographs continue to

serve the scientific, military, medical and other special purposes for which digital photography

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was invented. Some hyperspectral cameras include HySpex VNIR-640, VNIR-160, SWIR-320i

ETC..

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9. APPENDIX

Image Sensor (CCD and CMOS): An image sensor is a device that converts an optical image into

an electrical signal.

Pixel: A pixel (picture element) is a physical point in a raster image, or the smallest addressable

element in a display device.

Pixel count: It is the number of individual pixels that go into making each image

Aspect ratio: The aspect ratio of a camera is the ratio of the length of the sides of the images

Sensitivity: The sensitivity of the sensor to the light coming from the object.

Digital zoom: Image is magnified and cropped to fit the size of the screen.

Optical zoom: The focal length of the lens is adjusted to get a magnified image.

Shutter: It is the device used as a window for the image sensor to stop it from being exposed to

the light always.

Shutter speed: shutter speed or exposure time is the effective length of time a camera's shutter

is open.

Aperture: It is referred to the lens diaphragm opening inside a photographic lens. The size of

the diaphragm opening in a camera lens REGULATES amount of light passes through onto the

sensor.

ISO speed: The ISO speed is a measure of the sensitivity of the camera to the incoming light.

Depth of the field: Depth of field indicates the distance between the closest and farthest object

that are in focus.