Field of View (& Focal Length)

The field of view of an image is determined by the selected focal length of the lens. As discussed in my post on Aperture, the focal length is also detailed on the lens itself. Prime lenses have a fixed focal length, thus a fixed field of view. Zoom lenses- on the other hand- show the two focal lengths from the widest to the narrowest (often called the 'zoom' end). Perhaps the most common lens to discuss here is what is called our 'kit' lens, meaning the one we get as part of the kit when we first purchase our dSLRs. These days, the most common focal range of this lens is 18-55, which makes a lot of sense when we consider the types of shots the more amateur photographer will be taking as a starting point (landscapes at 18mm and portraits towards the 55mm end). I believe a couple of decades ago, most film SLRs came with a 50mm prime as standard. These have become popular once more over the past few years.

Crop Sensors:

One thing we must remember when thinking in terms of focal length is that the numbers we used are generally based on the old 35mm film negative. The dSLRs most of us buy these days have sensors roughly one and a half (Canon's are 1.6) smaller than 35mm, meaning the focal length has to be multiplied by this difference to arrive at what is known as the effective focal length. These crop sensors are called APS-C (Advanced Photo System- Type C) sensors.

What do the numbers mean?

This baffled me for years. Why does the smaller number represent a wider view through the lens? Well, this number refers to the distance between the camera sensor and the point within the lens where all the rays of light intersect at the point of focus. E.g. 18mm means the point where the rays cross are 18mm from the sensor. So the more the lens is 'zoomed' the further away this point is from the sensor. Below is a simple diagram of how this works:

The site I found this picture on offered a very comprehensive explanation of 'focal length': it imagines the eye (image sensor) looking through a magnifying glass (lens): "you have to hold it a certain distance from your eye for it to be in focus. This is because the magnifying glass has a set focal length. It must be a certain distance from your eye to focus in on your subject. This distance is referred to as the focal length". So magnifying glasses are- in effect- Prime Lenses, for there is a set distance at which the 'subject' can be seen (1)

How does this translate into Field of View?

As with everything in photography, there is a point where the scientific/technical must be translated into the aesthetic, for this is only a means to an end (the end being the taking of photographs pleasing to the eye). We use the focal length of our lenses to determine a field of view to create based upon what we aim to show in the image, and the elements we want to show to represent our interpretation of the real world (the field of view of the human eye is meant to be 50mm). For example, for landscapes we tend to use a very wide plain of view in order to include as much as the scene as possible, whereas for images of people and still life shots we narrow this view to focus/highlight on our chosen subject. However, there is nothing to say a portrait cannot be taken with a wider focal length (I'm thinking here of brides stood in fields).

Needless to say, the more accomplished we become as photographers the more it becomes second nature to select the particular view we want to portray in any given situation. Field of view, perspective and composition are all linked in this respect, in that we are constantly evaluating these three things when shooting a subject:

*  How wide do I want the scene? (FoV).
*  From what angle do I want the subject to be viewed (P).
*  Where in the frame will I position the subject? (C).

In the next post I will show some of my own examples of how I have used different fields of view for creative effect.



1)  http://moodleshare.org/mod/page/view.php?id=630