The lenses: Zoom, Focus and Aperture

This is the 3rd part of a six-part tutorial about photography basics. So far we’ve seen

• Some physics background related to photography
• The pinhole camera and the basics of camera building

We have seen in the previous post, that by isolating rays of light through a tiny hole, we can separate and project images of objects. The smaller the whole, the sharper and darker the image, and vice-versa.

It would be really great if we could find a way to widen the aperture of the camera and correct the rays of light and project all diverging rays of light the same point, isn’t it? That would provide a sharp image that is also very illuminated.

In this post, I’ll show you the lens basics and how to use focus and aperture to your advantage in pictures.

Magnifying glass on a sunny day

Thanks to this properties, we have the magnifying glass. If you put a sheet of paper under the sun, infinite sun rays are projected on the paper. These are images of the sun, casted on the paper and they cover all the extension of it.

By using a magnifying glass, it is possible to change direction of the sun rays and make them converge to one single point. This point is called focus. By doing this on a sunny day, the amount of energy in the focus is so great, that you can, literally, burn the paper.

Well, that is the way to go, by using these properties to build lenses we can put in the black box we can increase the size of the hole and keep the focus at the same time.

As you might have guessed already, the focus of the lens must be exactly over the projection surface (translucent paper, photographic paper, film, CCD). So the longer the box is, the greater the focal length should be, and vice-versa.

A correction lens in the aperture of the camera to correct divergent rays of light.

A single lens like this would only be able to focus objects at a certain distance, just like a magnifying glass. By combining multiple lenses you can have a focusing ring that allows you to chose the range in which objects will be focused. Objects positioned closer or farther that this value will be blurred.

How much the objects off-distance will be blurred largely depends on the size of the opening in the lens, the smaller the diameter the less blurred, but the darker the whole scene becomes. The greater the diameter the lighter the picture, but the more blurred the off-distance images become.

The range that will be focused at a certain aperture is called Depth-of-Field.

Shallow depth of field at aperture f/4.8

Deep depth of field at aperture f/32

This is obviously a trade-off and that is why almost all lenses for professional cameras and even a lot of the amateur cameras as well come with an aperture device that, roughly speaking, allows you to control the diameter of the inner hole of the lens.

The aperture of a lens is measured in f/ stops like f/2.8, f/4.6 or f/32. What these numbers represent is too technical and is outside the scope of this tutorial, but you should keep in mind that the smaller the number, more luminous the lens and better… and more expensive.

On the inner side of the camera, using a similar idea, we can also lengthen the focal distance of the lens and keep the focus over the projecting film or CCD while putting the lenses closer or farther to it, changing the approximation and perspective accentuation of the image. This is called a zoom lens.

Another trade-off, of course, is that all this technology is more expensive and requires more knowledge to handle, so you end up having a huge range of cameras to buy, from cheap to obscenely expensive and from very simple to very complex ones.

The more you understand about this and the more the camera lets you explore your knowledge, the more you are able to take gorgeous, fantastic photos and hopefully, this is why you are reading this text.