How Digital Cameras Work


I wrote this article on The Quillcards Blog and as a knol on Google. Now Google is stopping its service so I exported the knol here.

I am reproducing part of the article here and then there is a link to the Quillcards Blog for the complete article.


The Guide

You want to make the right decision when you buy a new digital camera, so what are the things to look out for?

1. The physical size and weight of the camera

Let’s do a quick review of the straightforward stuff.

In terms of sheer physical size we start with the smallest.

These are the super-compacts that fit in a shirt pocket and are about the size of a credit card. Next up are compact cameras, otherwise called point and shoot cameras, some of which will fit in a pants or jacket pocket and some of which are a bit too bulky for that, but are still small.

The next step up is to Bridge cameras. They get their name because they ‘bridge’ the gap between compact cameras and SLR cameras (SLR is short for single lens reflex and I’ll explain what that means that later in this article).

Lastly there are the Micro Four Thirds type cameras. I’ve put them last because they are new on the scene, but in terms of size, they are at the smallest end of the range of SLRs and around the size and weight of Bridge cameras. More about them later.

2. Seeing what you are shooting

SLRs are called ‘single lens reflex’ cameras because when you look through the viewfinder you are looking via a series of prisms and mirrors, right out through the lens. That is, you look into the viewfinder about half an inch or so above the lens but the light comes into your eye as though you were looking directly through the lens.

And the advantage of that is that there are no parallax errors. What you see is what the camera sees.

A way to think of looking through the viewfinder of an SLR is to imagine you are looking through a periscope in a submarine, though the periscope is only a half an inch tall.

SLRs are called ‘reflex’ cameras because the mirror that diverts the light that enters the lens flips up out of the way for a fraction of a second when you take a shot (to let light onto the sensor) and then back again.

And when the mirror flips out of the way, you can no longer see through the lens, so the scene goes black for a moment. How long that moment is depends on how fast the camera shutter fires.

With a rangefinder camera, bridge camera, or a compact camera that has a viewfinder (not all compacts have viewfinders), you are looking slightly to one side or above what the camera sees, so you are not capturing what you see but rather something seen from a slightly different vantage point.

Now you may think that having the viewfinder a half inch away from the line of the lens wouldn’t make much difference and its true that for something like a general landscape shot it doesn’t make much difference, but for subjects that are closer to the camera it makes a definite difference.

You can test that yourself by holding your finger in front of your eye and then moving your eye half an inch to one side. The view you see is quite different. The viewfinders in top of-the-range rangefinder cameras show a larger area than the lens sees.

That is, they show an area around what the camera lens sees and they have frame lines projected within that area that shows what the lens sees. And those frame lines are coordinated with the lens focusing mechanism and slide across the viewing area as the photographer focuses the camera and thus compensate for parallax error.

The fact that with rangefinder cameras the photographer sees beyond the frame lines is one of the reasons they are popular with some photographers.

Henri Cartier-Bresson
They claim that instead of, as it were, holding one’s eye to a keyhole and peering through, the photographer sees the scene in context and is able to compose the scene more effectively and more quickly. Henri Cartier-Bresson, voted the world’s foremost photographer of the 20th century, used a Leica rangefinder camera.

Not that everyone claims that all of his shots were world-beaters, but many feel that he did understand the way three dimensional reality translates into a two dimensional image, and being able to see around the subject may have aided him being able to frame his subjects so well. Henri Cartier-Bresson started his creative life as an artist, so that probably helped his sense of shape and balance between elements in a picture.

Looking through a book of his photographs (his book ‘The Europeans’ is a good example), one can see shot after shot where the elements of composition satisfy in a way that photographs from other photographers do not always do.

There is often a line or route of light and dark that traces through from foreground to background and unites the image. In that way he makes three dimensions come to life in the flat paper. And perhaps that is why some photographers are attracted to rangefinders.

But as Leica and Epson are the only manufacturers of digital rangefinders, and as both are very expensive – the Leica is horribly expensive – let’s move on.

The viewfinders on compacts or Bridge cameras are generally pretty bad compared to the view you get with an SLR. In fact, as we know, lots of compacts have no viewfinder at all. With a lot of compacts, instead of a viewfinder, you look at the LCD – the liquid crystal display – that displays the scene electronically on the back of the camera.

And that is how you compose your shot. Which is OK if it suits you and if you can see the subject in the LCD. One problem with LCDs is that in sunlight they can be very difficult to see.

Stand In A Dark Corner
And if you want to take a shot outside on a summer’s day, it’s no good if you have to find a dark corner to stand it to see the LCD properly.

Look through the viewfinder of an SLR at a general scene – nothing too close – a street scene is fine. Choose something in the scene that you want to be able to see clearly. Imagine it is your loved one standing across the street. Now try the same thing with a compact or a Bridge camera.

Now your loved one is a dot at the end of a long tunnel. How are you supposed to check that he/she is smiling or that he/she is in focus? You can’t. And it is no easier looking through the LCD with the camera held at arm’s length.

Which is not to say that compacts are no good; and they can take some very good shots. But you need to know their limitations before you put down your cash. Later in this article I will talk about how compact cameras with small sensors manage to get the subject in focus, and the advantages and disadvantages of this.

photo of religious man in Jerusalem
Man In Jerusalem

3. Image Quality

Digital cameras have an array of micro-lenses that capture the light coming onto them. An ‘array’ is a collection of lenses laid out in a pattern. Imagine a rectangle about as big as your thumbnail.

That is about the size of the chip that holds the microlenses in a compact camera. A six megapixel camera has six million tiny lenses laid out in a rectangular array on that chip. That fact ought to make anyone’s jaw drop to the floor.

Six million lenses cemented onto a little chip the size of your thumbnail, capturing light that comes into the camera. It’s amazing.

And it goes a long way to explaining why the image quality varies between compact cameras and SLRs. And why cameras with fewer megapixels can still produce good images, when most people’s first instinct would be to think that the more pixels the better. How micro-lenses work Here is how it works. Each microlens reacts to the light that falls on it.

The bigger the microlens, the better it is at capturing a good signal. Of course the manufacturers can boost the ability of the lens to receive a signal but, along with the boosted signal comes more ‘noise’. We see that noise as mushy, dirty, speckled, gritty-looking, yellow-black areas in the darker parts of the image and as little speckles all over the image.

Smeared Details Mean Bad Photos – Unless You Want A Painting
Manufacturers sometimes process the images in the camera in such a way as to hide that noise, but inevitably, they also hide the signal, and produce images that look like watercolor paintings rather than photographs. You are not necessarily going to see this in a 4 inch x 6 inch print, but make a bigger print and you will see it. So the best microlenses are big enough to allow a nice gentle rake to the angle at which they receive light through the camera lens. And they work best when they are not boosted to receive a stronger signal than is optimum for their design.

Now it is clear that if we have a chip that is as big as a thumbnail, the more lenses one cements onto it, the smaller each lens must be, and the more we have to boost the signal to get enough light to process an image. SLRs have sensor chips with an area that is about fifteen times the area of the chip in a compact camera.

Full-Frame Sensors
And there are some SLRs on the market that have chips that are bigger than that, and are known as FX or full-frame sensor cameras on account of the fact that the chip is the same size as a frame of 35mm film. And there are cameras with chips even bigger than that, but if you are considering one of those you need to know a lot more than is in this article, and you are talking about cameras that cost several tens of thousands of dollars.

Compact cameras with more micro-lenses (more mega-pixels) can look better for small prints than shots taken with cameras with fewer pixels.

And the reason is that at small print sizes the more micro-lenses, the more resolving power. That is, the more detail they can show. And as long as the print is not too big, that ability to resolve more detail can outweigh the propensity for more noise because of the fact that each individual micro-lens must be smaller to fit on the sensor chip.

And there is a second complication and that is that whichever way you cut it, lenses are at there best near the center. And if the micro-lenses are bigger, as they tend to be on bigger chips, then there are fewer ‘centers’ and more corners. But all of this only holds true for shots taken at the lowest sensitivity of the camera, and that’s the next thing I want to talk about.

4. Sensitivity

In the days of film, you went into the shop and bought a film suitable for daylight, or for shooting indoors. The manufacturers understood that there was likely to be less light for those indoor shots, so they made film for indoors that was more sensitive than film intended to be used indoors.

As with all things, there is a penalty to pay for making a film more sensitive, and that is that there is more grain.

The international standard for the sensitivity or ‘speed’ of a film is called ISO, and a typical low-sensitivity film would have an ISO of 100. And a ‘fast’ film with more sensitivity would be something like 800 ISO.

What does ISO 100 and ISO 800 mean? It means that to get enough light onto the film to expose it sufficiently for the image to be produced, you need eight times more light on the slow ISO 100 film than you do on the fast ISO 800 film. The ISO standard has carried over into digital cameras. Almost all digital cameras have a button or a dial to increase sensitivity, from 100 ISO (or perhaps 200 ISO) up to 400 or 800 or 1600 or even 3200 ISO or beyond.

And you may wonder how it is possible to just turn a dial and make a digital camera more sensitive to light. And the answer is that as in almost all things, there is a trade-off. More sensitivity means more noise.

And in an analagous way the noise problem is the same with digital cameras as it was with film cameras.

No matter how big or how small the chip used in the camera, The more we boost the sensitivity of the chip by turning up the dial, the more ‘noise’ we get.

The Ugly Sister
And noise is the digital ugly twin sister of grain.

In fact it is a lot worse than grain because it looks worse.

How sensitive? and how much noise? How does this translate into something practical we can say about compact cameras compared to SLRs?

The generally accepted word is that 800 ISO with an SLR produces the same amount of noise as 100 ISO with a compact camera. And that tells us a lot.

Because if I was thinking of buying a compact camera because it is supposed to give good exposures without too much noise, that isn’t such a big deal when I know that the noise on a compact camera is likely to be as bad at 100 ISO as it is with an SLR at 800 ISO.

Very few camera manufacturers make their own chips and the fact is that there are very few chip manufacturers. So the differences between the cameras made by the various camera manufacturers is often down to the processing technology they use to process the signal that the microlenses produce rather than differences in the chips.

Somewhere in all of this there is a trade-off but the bottom line is that bigger chips mean better image quality over a wider range of sensitivities. It’s as simple as that.

5. How images are processed – Jpeg and RAW

So what happens after the signal hits the microlenses? Well all cameras ‘process’ the signal that come from the array of microlenses, and the technology is a fine blend of science and art -that is between what ought to look good and what looks good.

But once that signal is processed, we come to a big division between those cameras that allow you to capture and download the original ‘digital negative’ and those that present you with an image that has been processed ‘in-camera’.

Jpegs and tiff files are ‘processed’, finished files. They can be read by computers and photographic printing machines. They are universal formats. RAW files on the other hand are ‘digital negatives’ and most camera manufacturers have their own proprietary RAW format.

And without being converted to a readable format they cannot be read by computers or the web or whatever. They have to be downloaded from the camera and then processed (or to use the jargon – ‘converted’) using a program designed for the job.

Proprietary Image Converters
Some camera manufacturers make their own RAW converters but there are many programs made by other companies that are widely regarded as being as good as the proprietory ones made by the camera manufacturers.

Adobe Photoshop is probably the most widely known of the RAW converter programs, apart from all the other things that Photoshop can do. Several cameras that produce RAW files also have in-built converters so that you can download a custom-tweaked jpeg right out of the camera as well as being able to download the RAW file.

As a side-note, all cameras produce jpegs in-camera, even if they are only tiny, low-resolution versions. That is because that is what you are looking at in the back of the camera when you review the shots you have taken.

6. Why you would want RAW files

One reason you would want a camera that allows you to download RAW files is that you can alter a RAW image more successfully than can be done with an image that has been processed in-camera.

You can alter the exposure (in case you or the camera messed it up when you took the shot) and you can alter the colors or the tonal distribution from dark to light areas. You can do that with images that have already been processed in-camera (jpegs and tiffs) but with less latitude and more likelihood of degrading the image.

This is particularly true with jpegs because of the limited range of colors of which they are composed. Change the exposure and color balance and the whole image suffers.

Which begs the question of whether you want to get involved with all this stuff, and if you don’t, then just know that to squeeze the best out of a digital camera you either want to be able to play around with RAW files, or you (or your camera) had better be pretty good at getting exposure and color balance correct when you take the shot.

And there are many brands and models of compact cameras that are good at good at getting exposure and colour balance right when the scene is bathed in diffuse, natural daylight and when the distribution of dark and light in the subject averages out at about halfway between white and black , or where there is a slightly lighter subject set against a slightly darker background.

And the reason for that is that there are limits to how well cameras are able to detect exactly what they are looking at.

Difficult lighting conditions, or ‘A Black Cat At Night’

For example, how can a camera tell whether it is pointed at a black cat against a white background, a white cat against a black background, a white cat against a white background –you get the idea.

And the answer is that it can’t and yet the camera has to know these things or it is going to get the exposure wrong. In fact all cameras are standardized to expose every scene as though the subject is a specific internationally agreed shade of grey. But it is only a starting point.

Many sophisticated cameras have certain standard scenes programmed into them and if they ‘recognize’ that the distribution of shapes in the scene is like one of those in their memory, they estimate that the scene they are looking at has a certain distribution of light and dark in the scene and expose accordingly.

But even the best of them are not always correct, and for certain scenes, ones that photographers learn to recognize, they are very often wrong. So some compact cameras or less ‘expert’ cameras go a different route. They ask the photographer to tell them what kind of scene the camera is pointed by turning the dial to the appropriate ‘scene’ mode.

In fact ‘scene’ modes are often used in compact cameras for another additional reason and I will talk about that when we talk about other settings – speed and aperture.

The bottom line is that cameras have a limited ability to detect how light or dark a scene is, and so they either expose all subjects as though they were grey, or they ‘estimate’ how light or dark a subject is by comparing it with a memory bank of subjects, or the camera asks you the photographer to tell it what the lighting is by flicking to the appropriate scene mode.

So with this knowledge you have got to expect that the exposure will probably be wrong in ‘difficult’ scenes.

7. The Quality of Light – White Balance

The next thing the camera has to know something about, is the quality of the light. Continue Reading

3 thoughts on “How Digital Cameras Work

    1. Thanks. He was sitting on a plastic chair near the entrance to the ‘tunnels’ at the side of the Western Wall in Jerusalem. When I motioned to him to ask whether he was OK with being photographed, he gave me the smallest of nods in agreement. 🙂


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: