The Basics of Digital Cameras Part 1: Capturing Light, Lenses and Image Sensors

In the first part of this series we took a general overview of the types of digital cameras that are available. Before we get to the practical tips in the next lesson, let's take a look at some of the key features and characteristics of digital cameras. This section will help you make sense of all the tips that follow and give you some important knowledge to use next time you're buying a camera. So let's get started...

The fundamentals of capturing light

Photography has always been about capturing light, and digital photography is no different. But what actually is light? Well, as photographers we only need to know a few simple things about the nature of light to begin with:

At its simplest, photography is a very basic process: A small amount of light is admitted into a light proof box, via a little hole, which projects on to a light sensitive surface at the back of the box to form an image. This, in a nutshell, is how a pinhole camera works. It's so simple that you or I could make one!

The image that is projected to the back of the box appears inverted, upside down and left to right. Why? Because, as we've seen, light travels in straight lines. Make a fist with your hand, but leave a tiny hole for you to see through. Now look through it, and try to see further to the left then further up above. You had to move right to see left and down to see up didn't you? Images are inverted in photography for the same reason.

Continuing with our example of the pinhole camera, let's say we have created an image but it's too dim:

What should we do to make it brighter? Increasing the size of the hole is the obvious thing. So we do that and, sure enough, we create a brighter picture. Except, this time the picture has become blurry, with fuzzy edges:

So a small entry hole gave us a dim but sharp image, whilst a big entry hole gave us bright but blurry image. This is a perennial trade off in photography, brightness v sharpness, so bear it in mind as you work through this series. Luckily, the problem is largely solved when we add a lens.


DSLR lenses

Lenses make photography a lot easier. They enable us to do several key things: gather light from a specific area (angle of view) and focus it to an image); choose a precise focal point that will be the clearest part of the picture; magnify the image to a chosen amount; use a particular angle of view and perspective; alter the entry pupil/aperture size to influence the brightness and sharpness of the image.


Focusing the light:

The basics of how a lens works relies upon the fact that light can be bent (or refracted). Have you ever seen a torch-light shone into water, or the stems of flowers sitting in water in a glass vase? Well, I'm sure you've noticed how the position of the torchlight or flower stems suddenly deviates when it comes into contact with water.

This happens because the light is forced to travel slower through the water than through the air, effectively bending the light that we see. Glass, too has the effect of bending light, which is why lens manufacturers are able to use this to their advantage. The quality of the material (i.e. glass, water, plastic etc) and the angle at which light comes into contact with it, determines how much the light is bent.

Have you ever used a really wide angle lens with a camera - the type landscape photographers love because it gives them great foregrounds and a feeling of depth? Well, the surface of these lenses is really curved because they gather light from a very wide area - the angle of view - before focusing it to an image.


Focal length and magnification:

Focal length is the distance from the optical centre of the lens to the image sensor.

Different lenses require differing distances to focus the light that they gather. This is called focal distance/length, and it's measured in millimeters. The larger the focal distance, the more magnified an image will be. This is why compact cameras often refer to zoom/focal length in terms of 10x, 15x or 20x magnification, instead of millimeters.

Focal length is also determined (in effect) by the size of the surface on to which an image is projected. In digital photography this surface is the image sensor, and its impact on focal length is called 'crop factor'. Essentially, when an image covers a larger area than the sensor the edges are cropped out, slightly magnifying the central area that remains.

N.B. avoid compact cameras with so called 'digital zoom'. This is not really zoom at all, as it simply amounts to in-camera cropping of an image, discarding many of the pixels and reducing image quality. 'Optical zoom' refers to the normal kind of magnification, and does not harm image quality at all.

The higher the magnification the narrower the angle of view. This is common sense, because the larger one part of an image becomes the less we can see of everything else! Focal length also impacts perspective. In the same way to how we see things with our eyes, a small focal length makes close objects seem larger, whilst a large focal length makes everything seem rather flattened. The flattening effect of large focal lengths is often called 'telephoto compression'.


F ratio and lens speed:

There's a really important relationship between focal length and the size of the hole through which light enters the camera - the entry pupil. In digital cameras, the entry pupil can be adjusted using the aperture (more on this later). The relationship between aperture size and focal length is called the f ratio.

For example, a focal length of 100mm with an aperture of 50mm has an f ratio of 1:2 or f2. Meanwhile a focal length of 200mm with a 10mm entry pupil has an f ratio of f20. The lower the f ratio, or f number, the more light can be gathered through the entry pupil. This enables the speed of the camera's shutter to be quite fast, preventing camera shake. Hence, lenses with a low f ratio - large maximum aperture - are often referred to as fast lenses. The f number also determines 'depth of field', a concept we'll discover in a future lesson.

So, increasing the aperture size or decreasing focal length makes the f ratio lower. Not all lenses are zoom lenses (with variable focal lengths). Fixed focal length lenses are called prime lenses. Prime lenses have several advantages of zoom lenses: they often have large maximum apertures - so they are fast lenses; they do not contain the moving parts of zoom lenses, resulting in superior image quality; they encourage you to be more active and creative when shooting, because of the inability to zoom; they are cheaper!


Lens compatability:

Lenses are not designed to work with all cameras. The major camera manufacturers produce lenses to work with their cameras, and specialist lens manufacturers make different versions of a given lens to attach to different camera makes. In addition, lenses are made to function with cameras containing a particular size of sensor. The sensor size that a lens is intended for will be made clear on the lens itself.

If you tried to use a lens designed for a full frame camera on a model that actually had a smaller (crop) sensor, you would notice dark shadows around the edges of the picture (called vignetting). We'll see exactly why this happens in the section on image sensors below.

When it comes to purchasing a lens, the greatest ranges are available with Canon and Nikon DSLR's. That is probably the biggest advantage of using a camera made by one of these two manufacturers. But specialist lens makers, like Tokina, Sigma and Tamron often fill gaps in the Canon/Nikon line-ups that can be very useful indeed.


Digital image sensors

Digital image sensors perform the same role as film, but electronically. On the surface of a digital image sensor are light sensitive photosites that convert light into electronic current, which is then saved to a memory card. These photosites broadly translate to the megapixels (MP's) in the final digital image.



what is a megapixel what is a megapixel

So, does an image sensor with more megapixels produce better quality photos. No, it simply enables you to print your pictures to a larger size before image quality decreases (due to pixiliation - where the individual block-like pixels become visible). The number of MP's a camera has is its resolution. Today's digital cameras all have a resolution that is more than sufficient for the size at which most of us view images.

It is not worth spending more on a camera that has loads of MP's, unless you specifically intend to produce poster sized prints on a regular basis. In any case, it's true to say that the need for lots of MP's for large prints is offset by the fact that these prints are viewed from further away than hand-held prints, making slight deficiencies hard to spot. Try not to become preoccupied by the resolution of printed images - something known as 'pixel peeping'!


Sensor size and crop factor:

Much more important than resolution for picture quality, is the size of an image sensor. Larger sensors have larger megapixels, and vice versa. The smaller sensors produce less clear images, that are also more prone to 'noise' (a grainy texture that appears in dark areas of photos, in the blue toned areas and when a fast ISO speed is used).

The different camera manufacturers produce image sensors of various sizes. Those cameras with a sensor the same size as a frame of 35mm film (about 36mm x 24mm) are called 'full frame' cameras. These produce images of full size. Smaller sensors 'crop' the image by varying degrees - the 'crop factor' that we touched on above.

Crop factor is where the image projected by the lens is too large to be contained on the sensor, so the edges are effectively cropped out. This, in turn, effectively increases the focal length beyond that stated on the lens you are using. Fortunately, the image you see through the viewfinder is the same as what the final cropped image will be. Crop factor is part of life with digital photography. Most us, who cannot afford a full-frame camera, just need to get used to thinking in terms of whatever increase in focal length our camera's sensor produces. It's not a big deal once you get used to it.

Most sensors that are not full-frame produce a crop factor of about 1.5x. In other words, focal length is 1.5x that stated on the lens. Here's a chart and diagram to illustrate the effects of various crop factors:

Black = full frame
Purple = 1.3x
Green = 1.5x
Blue = 1.6x

Crop Factor Camera
1.3x Canon 1D series
1.5x Nikon DX cameras
1.6x Canon consumer cameras
2.0x Olympus four thirds system


That's the end of The Basics of Digital Cameras Part 1. You now understand the basics of how light is captured by a camera and the trade off between brightness and sharpness; the key characterisitcs of lenses - how they gather light, magnify images and control depth of field; the importance of sensor size, resolution and crop factor. In the next lesson of this series we'll get to grips with autofocus, exposure modes, Live view, viewfinders and saving images.

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