What is a camera?

We’ll start this class with a rather gentle introduction, by asking ourselves what a camera really is, and what its different components are. Chances are that you will already know some of this, but going through it anyway will at least ensure that we have defined a common vocabulary.


In the strictest sense, it is simply a device which can record light. It does so by focusing light on a photosensitive surface. From this simple sentence, we can see the three main parts of any camera.
The photosensitive surface reacts to light through either a chemical process (film) or an electric one (digital sensor). There are fundamental differences between these two, which we will cover in a subsequent lesson, but for now we can consider both of them to be identical: they are a grid of several million tiny dots (pixels) and each can remember how much light it received in a given period of time. There are three important qualities to each sensor: resolution, size and what we can call “quality”.

• Resolution is simply the number of pixels (it is slightly more complicated with film, let’s forget about it for now). The more pixels you have, the more fine grained details you can theoretically record. Any resolution above 2 or 3 megapixels (i.e. millions of pixels) will be enough for displaying on a screen, but higher resolutions come into play for two important applications: printing and cropping.
  • In order to have a good reproduction quality, it is generally estimated that between 240 and 300 pixels should be used for every inch of paper (dots per inch, or dpi), which will give a natural limitation to the biggest size one can print. For instance, a 6MP image of dimensions 2000×3000 pixels can be printed at a maximum size of 12.5×8.3″ at 240dpi (2000/240 = 8.3, 3000/240 = 12.5). It is possible to print bigger by either lowering the dpi or artificially increasing the resolution, but this will come at a serious loss of image quality. Having a higher resolution allows you to print bigger.
  • Cropping means reducing the size of an image by discarding pixels on the sides. It is a very useful tool and can often improve composition or remove unwanted elements from an image. However, it will also decrease resolution (since you lose pixels), so how much cropping you allow yourself will depend on the initial resolution, which you want to be as high as possible. This is also what some cheaper cameras call “digital zoom”, which use should be avoided as the plague, as the same effect can very easily be reproduced in post-processing, and the loss of image quality is often enormous.

• The physical size of the sensor is very important and will have an impact on many other parameters, most of which we will see in subsequent lessons: crop factor, depth of field, high ISO noise, dynamic range are some of them. Bigger sensors will also allow to have more widely spaced pixels (increasing image quality) or more of them (increasing resolution). Bigger is almost always better, and this is one of the main reasons that DSLRs (and medium format cameras) produce much better images than compact cameras. In tomorrow’s lesson, we will cover the different types of cameras in more details.

• Finally, sensor quality is harder to quantify, but it refers to how well the sensor reacts to difficult light conditions: either low light which will require to increase ISO and for which we want the sensor to have as little noise as possible, or high contrast, which will require a good dynamic range to be recorded adequately.

The lens is the second component of any camera. It is an optical device which takes scattered light rays and focuses them neatly on the sensor. Lenses are often complex, with up to 15 different optical elements serving different roles. The quality of the glass and the precision of the lens will be extremely important in determining how good the final image is.
Lenses must make compromises, and a perfect all around lens is physically impossible to build. For this reason, good lenses tend to be specialized and having the ability to switch them on your camera will prove extremely useful.
Lenses usually come with cryptic sequences of symbols and numbers which describe their specifications. Without going too much into details, let’s review some of their characteristic:

• Focal length refers roughly to the “zoom level”, or angle of view, of the lens. It will have its own lesson in a few days, as it can be a surprisingly tricky subject. A focal length is usually expressed in millimeters, and you should be aware that the resulting angle of view actually depends on the size of the sensor of the camera on which the lens is used (this is called the crop factor). For this reason, we often give “35mm equivalent” focal lengths, which is the focal length that would offer the same view on a 35mm camera (the historic film SLR format) and allows us to make meaningful comparisons. If there is a single length (e.g. 24mm), then the lens doesn’t zoom, and it is often called a prime lens. If there are two numbers (e.g. 18-55mm), then you can use the lens at any focal in that range. Compact cameras often don’t give focal lengths but simply the range, for instance 8x. This means that the long end is 8 times longer than the wide one, so the lens could for instance be a 18-144mm, or a 35-280mm, etc.

• The aperture is a very important concept which we will talk about in much detail later on. The aperture is an iris in the centre of the lens which can close to increasingly small sizes, limiting the amount of light which gets on the sensor. It is refered to as a f-number, for instance f/2.8. To make things worse, it is quite counter-intuitive, as the smaller the number, the bigger the aperture! For now, we don’t have to worry about this too much. The important number on a lens is the maximal aperture, the lower the better. Professional zoom lenses often have f/2.8 maximal apertures, and cheaper consumer lenses have ranges such as f/3.5-5.6, meaning that at the wide end, the maximum aperture is f/3.5 and at the long end, it is f/5.6. Aperture can be closed to tiny levels, usually at least f/22.

• Lenses also need a focusing system. Nowadays, most lenses have an internal motor which can be piloted by the camera: the autofocus. They also have a ring to allow the photographer to focus manually. There are plenty of options for autofocus motors as well, for instance hypersonic or silent ones.

• Lenses are increasingly equiped with stabilisation systems (called VR by Nikon, IS by Canon). They detect small movements, usually handshake, and compensate for them by moving internally the optical elements in the opposite direction. Though no magic pills, those systems tend to work very well and allow to take sharp images at quite slow shutter speeds.

• Finally, lenses can have all sorts of fancy options: apochromatic glass, nano-coating, etc, designed to increase the quality of the final image. You probably shouldn’t worry too much about those.

Finally, the body is the light tight box connecting the lens to the sensor, and ordering everyone around. Though some film cameras are just that, black boxes, most digital cameras are now small computers, sporting all sorts of features, often of dubious usefulness. Let’s review some of the components found in most bodies:

• The most important is probably the shutter. Think of it as a curtain in front of the sensor. When you press the trigger, the curtain opens, exposes the sensor to light from the lens, then closes again after a very precise amount of time, often a tiny fraction of a second. Most shutters operate between 30 seconds and 1/4000s of a second. That duration (the shutter speed) is one of the three very important exposure factors, along with aperture and ISO.

• A light meter. As the name suggests, it measures the quantity of light and sets the exposure accordingly. How much manual control you keep at this stage is one of the most important questions in photography. There are different metering modes, but except in very specific cases, using the most advanced, most automated one (matrix metering on Nikon cameras) will provide the best results.

• A focus detector, used to drive the autofocus motor in the lens. There are two competing technologies, contrast detection and phase detection, with at the moment an edge for the latter, which explains why DSLRs tend to focus faster than compact cameras. These systems tend to vary greatly between basic and advanced bodies, but it should be noted that they all need reasonable amounts of light to work properly.

• A way to store the image just created. Back in the days of film, this was just a lever to advance the roll to the next unexposed frame. Now, it is a pipeline which ends up in the memory card that the camera is using. If you are shooting jpg instead of raw (more on this in another lesson), there is an additional stage where the internal computer performs all sort of black magic on the image to output a ready-to-view jpg file.

• A way to frame. It can be a multitude of things, optical or electronic viewfinder, LCD screen or even ground glass. Here too, DSLRs have an edge as an optical viewfinder allows “through-the-lens” viewing and immediate feedback, while electronic viewfinders (really, a LCD screen inside a viewfinder) and LCDs often have limited resolution and slight updating delays.

Assignment

Take a good look at your camera, whatever its type, and try to identify each component we have discussed here. It might be a good opportunity to dig out the manual or to look up its exact specifications online. Now look up a different camera online (for instance at dpreview) and compare their specifications. Try doing this for both a less advanced and a more advanced body, and for different lenses. Report here if you find any interesting difference, or if some parts of the specifications are unclear.