Introduction

I think it is important for every photographer to understand what exactly they are dealing with when entering the digital photography arena. This is important because a better understanding in the basics of what digital photography entails can vastly improve your images! It is a tool to change your thinking into “digital” and thus opening a whole new world to photography. Yes, you can continue being a point and shoot digital photographer and you never need to know how it all “works” and you might even end up with some nice images, but I like to know how I achieved what I did and even more important, how do I improve even further. So if you are a newbie or just want to check out my understaning  of digital, then follow my posts on “What is Digital Photography” and maybe, just maybe you learn something worth your while! I will try and steer clear of fancy words and too much detail.

Film, the basis 

In short, to understand better what Digital Photography is, you need at least a bit of background on film photography. This is a quick explanation of black and white film photography.

In film photography, a “film” is put behind the lens and “exposed” for a short while to light entering the lens. This film has a light sensitive emulsion layer on it. This emulsion will burn away (yes it is an over simplification) in different amounts depending on the amount of light that hits it.  So if the subject you are photographing has dark areas in it, these dark areas will cause the film emulsion to be burnt away less than the bright areas, which will cause the emulsion to burn away more (see Figure 1).  In the film development process (at the photo lab) your film is “cured” (that is fixing it so it is not sensitive to light anymore) and the burned emulsion is washed away. What remains is your negative! On the negative the light areas represent the dark areas in the image where the emulsion was not burned that much. The dark areas on the negative represents the light areas in the image where the emulsion was burned a lot.

Light is now passed through your negative onto light sensitive paper to “burn” the final positive of your image. You can see from the above figure that where a lot of emulsion is left, little light will reach the paper and where little emulsion is left, a lot more light will reach the paper. The paper works on the same principal as the film but the inverse. Where bright light hits the paper the paper will burn darker and where little light reaches the paper it will be lighter. Colour film photography uses the same principals, just a little bit more complicated.

The digital Camera

Digital cameras do not have any film to expose to light and to physically “burn” a light sensitive emulsion layer. So how do we create images on digital cameras?

Your digital camera is in part an optical camera as well as a pretty sofisticated computer. The computer part of your camera is responsible for “seeing” and capturing the image you are photographing in order to give it to your home computer or printer or other device for display at a later time. We understand how film was “burnt” in the days of film photography but how does a computer “see” an image coming in through the lens if it has no film? Well, computers can only process numbers. They think in numbers and they store numbers and they talk in numbers and in the camera’s case, they also see in numbers. That is where the sensor in your digital camera comes into play. Instead of film, we put a digital camera sensor behind the lens of the camera in place of the film. Instead of film this sensor is exposed to the image through the lens for a short time in order to “capture” the image. It is the purpose of the sensor to convert the light falling on it into numbers that the camera’s computer can understand. Once the sensor has converted the light image into lots and lots of numbers, these numbers are stored in the camera for later use.

In order to understand in more detail how the sensor goes about it’s job a bit of background of how a digital images are represented on a computer is needed.

The Digital Image

In the computer’s world (the digital world) a digital image is an image composed of what is called pixels. Each pixel in the digital image is an element of the image that represents a very small part of the image. Each pixel has associated with it a colour. In Figure 2, if we assume that every little square in the grid represents a pixel (as if we zoomed in quite a bit), then this image is made up of 10 pixels by 15 pixels.

To recreate this image we need to store each and every pixel that makes up the image as well as the colour that it represents. For this image then we will have to store 15 x 10 = 150 pixels each with it’s colour value. But how do we represent the colour “yellow” for a specific pixel? We know what yellow looks like but does the computer? Colour can best be described by it’s colour components Red, Green and Blue. With these 3 colours you can create every other colour ( these are called additive primary colours ) as shown in Figure 3.

Figure 3: Additive Colours

• Red: 255
• Green: 255
• Blue: 0

So for each of our 150 pixels we store the 3 colour values to describe the colour of the pixel.

The Sensor

he eye of the camera, or the sensor, consists of many many small light sensitive elements. Each element represents a pixel in the image you will be capturing. As the light falls on the snsor each little element converts the light it “sees” into a pixel with its colour information ( red, green and blue components ). So a sensor with 10×15 elements could take a picture like the questionmark in figure 2. We would call this a 150 pixel sensor (or camera). You will agree that taking a picture of grandma will not look good in 10 x 15 pixels! That is why camera sensors use millions of pixels or mega pixels (mega = million). So my sensor in say a 2 mega pixel camera is made up of  around 2000 x 1000 pixels and so on. Remember that each of these pixiels actually consists of 3 colour values or numbers so the camera’s computer at the end of the day has to store 2000 x 1000 x 3 = 6000 000 (six million) values for each picture you take with a 2 mega pixel camera! 

Conclusion

This article basically explains how a digital camera takes a picture and what the picture looks like to a computer. Once you have your image in “computer format” or numbers you can easily manipulate this bunch of numbers with all kinds of fancy algorithms to change colours, enhance, sharpen etc. your picture. Because in the computer your beautifull image is just a bunch of numbers and computers love math

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