Camera - how does it work?

This week the main theme will be photography.

I’m sure all of you know the topic somehow – probably you make photos by yourself, maybe you’re interested in a photography as an art, or you just pose for photos ;) In any case, I hope this weeks posts will be interesting for you.

Let’s begin with the basics: how does camera work?

It all started about year 900, when device called camera obscura (latin for „dark room”) was invented. Look at the scheme below: camera obscura is a box with interior painted black and with a tiny hole in one side. Light passes through the hole and produces smaller and upturned image on the opposite surface.

If you put a light-sensitive material (it can be photographic film or electronic image sensor like CCD or CMOS) where the image is reproduced, you will construct a simple camera :) Of course it’s not that easy, there are few other things that make this device work properly, like correct distance from the hole to the light-sensitive material, mat and transparent plate in front of the light-sensitive material, proper size of the hole, etc.

The last one should be small enough to make the light rays intersect in one point. If it’s not, it will affect the sharpness of the image. But making a small hole means that not much light will get inside the box and the light-sensitive material won’t be exposured to it enough, or time of exposure will be very long. To become indepent from the size of the hole, in 1550 Girolamo Cardano installed the converging lens at the front of the box.

Converging lens works like this:

As you see, the light rays intersect in focal point. Distance from the center of the lens to its focal point is called a focal length (f). The longer focal length is, the larger magnification and a narrower angle of view the lens have.

But drawing nr2 is only the theory. In practice the light rays run like this:

This phenomenon is called spherical aberration and it occurs due to the increased refraction of light rays when they strike a lens near its edge, in comparison with those that strike nearer the centre. To avoid blurring of the image, the focus regulation mechanism was installed. It simply moves the lens, so we can be sure that its focal point is situated exactly at the light-sensitive material.

There are also many other aberrations (deformations) that occur in lenses, like for example chromatic aberration, coma, astigmatism, flare, vignetting, etc. To avoid them, constructors started to use more than one lens. Last two optical defects I mentioned are not always undesirable – even Photoshop has filter which enables adding flare or vignette to your image, apart from filters that remove them ;)

Using more lenses allowed to construct zoom – lens with the ability to vary its focal length.

Below you can see how it works:

I haven’t mentioned about one very important thing- aperture. It’s a mechanism used to control the amount of light reaching the film or image sensor.

Larger value of aperture means smaller hole, what gives us large depth of field.

What is depth of field? It is the portion of a scene that is sharp in the image. In some cases we want to have the entire image sharp, in other cases we want the depth of field to be smaller, to emphasize the subject and de-emphasize the background and foreground.

In combination with variation of shutter speed, the aperture size will regulate the film's degree of exposure to light. So, for example, a slow shutter speed will require a smaller aperture to avoid excessive exposure. It will make the subject blured if it’s moving and will give us a large depth of field.




If combinations of shutter speed and aperture size don't give us the expected effect, we can help ourselves with additional light, tripod or by changing the sensitivity of light-sensitive material we use.

That's all for today :)

Write in comments some basic information- what do you think about photography, are you interested in it? Did you know how camera works or was that something new for you?

All images are from wikipedia.