Lenses (Part 2) – Focal Lengths

In Part 1, I introduced the theme of “lenses” by providing a definition of what a lens is, and presenting two “concepts” relating to lenses – the “zoom factor” and “focusing“. Today’s entry deals with the former.

For starters, let’s clarify one thing. In photographic terms, the preferred term to refer to the “zoom factor” of a lens is focal length. In other words, a lens’ focal length represents that lens’ ability to make objects appear bigger, smaller or life-size on the camera’s film or sensor. In other words, focal length can also be said to represent the “magnification” properties of the lens.

Focal lengths are denoted in millimeters, “mm“, format, for instance 24mm, 35mm, 50mm, 70mm, etc. The “mm” reflects the “angle of view” of the lens, in other words, how wide or narrow a view can be seen by the lens. The rule is as follows: the smaller the “mm” number, the wider the angle of view is. So for example, a 24mm lens is wider than a 70mm lens. Let me explain…

Recall the “funnel” metaphor used in my previous post… A funnel with a very wide (top) opening will of course “catch” more water if left out in the rain than one with a narrow opening, since it would have a larger surface area. The same is the case with lenses trying to “catch” light. The wider the lens is, the bigger its angle of view (AoV) would be. In practice, it doesn’t mean that wider lenses are in fact physically wider, rather, it means that they are able to modify light that is coming from a wider area in front of them than would be possible with another “narrower” lens. The diagram below should help explain this pictorially.

A comparison of the effect of having a wider/narrow AoV.

A comparison of the effect of having a wider/narrow AoV.

In the diagram, the dotted, red lines represent two light rays entering into the lenses at their widest extremities. You can notice the difference between the first lens, with a 30-degrees AoV and the second one, with a 90-degrees AoV. Of course, the image from the first lens (30) would show a more “zoomed in” image, at the expense of “breadth” in the final image. In fact, only three of the subjects show in the final photo. On the other hand, the second lens (90) would show a more wider view, at the expense of size – there are in fact more subjects showing, but these appear smaller in the photo when compared with the first photo.

As seen in the diagram, the lens focal length will heavily influence what photos you can or cannot take. Suppose for instance you want to shoot a picture of a flying aircraft during an airshow. The higher the plane goes, the smaller it will appear to the naked eye. This implies that we would need to “zoom in” further to get a picture whereby the aircraft is sufficiently large to make the image useable. For such a scenario, a lens with a large focal length would come in handy, for instance a 200mm lens. By way of a practical example, such lenses are used in binoculars to enable you to make far away objects appear larger.

On the other hand, if you want to take a picture of a large group in a small room, you would typically need to be “zoomed out” and as far back from the group as possible, in order not to leave out anyone from the final picture. What you need, here, is a wider angle of view, that is, one with a small focal length, such as 16mm. Real world examples of wide-angle lenses are “peep-holes” in doors.

Since lenses’ focal lengths determine what they are best used for, in photography, lenses are generally “categorised” according to their focal length. Wide-angle Lenses are lenses that offer a wide (duh!) angle of view and typically include anything less than 35mm. Standard or normal lenses offer angles of view that are neither very wide nor very narrow, and typically these include lenses that go from 35mm to around 60mm. Finally, telephoto lenses are those offering a very narrow angle of view, and would include all lenses above 60mm. Their “zoom factor” is inversely proportional to the angle of view, i.e. wide-angle lenses make objects look smaller than they look like with the naked eye, standard lenses approximate the same size as seen, whilst telephoto lenses make objects look bigger than what we can see unaided. Note: In truth, the effective angle of view, i.e. what will actually be seen by the camera’s sensor/film, is also affected by the size of the sensor/film in your camera. This theme will be addressed later – for now, it suffices to know where to “classify” a lens if you only have its focal length to go by.

Finally, you might ask whether all lenses fit in one of the above categories. Incidentally, if a lens can vary its effective focal length (zooms in or out), what does that make it? For that, you’ll have to check back again as that’s the theme of the next post: Lenses (Part 3) – Prime or Zoom?


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