Monday, May 23, 2011

Lenses: Introduction to Focal Lengths

The focal length of a lens refers to the actual length of the lens from a point inside its front lens element to the point where it comes to focus -- in our case, that is on the camera sensor -- when the lens is focused at infinity.  Modern optical systems and zooms can make this calculation more exacting, but this definition is sufficient to understanding the elementary difference between lenses and how to use them creatively.  Focal length is typically expressed in millimeters for modern DSLR lenses. 

What we call a "normal" lens is one where the focal length is approximately equal to the diagonal of the sensor plane.  On a full-frame (35mm) sensor that would be a 50mm lens.  On the smaller APS sized sensor that would be a 35mm lens.  All other focal length categories are given relative to what is "normal".  For instance, a wide angle lens is one that is wider that a normal lens, while a telephoto lens is longer than a normal lens.  

Wide angle lenses take in a larger visual area that makes them most suitable for wide landscapes, while telephotos act more like telescopes for close-up viewing of distant subjects.  These are the obvious, practical reasons for selecting one focal length over another.  More importantly, however, are the creative characteristics each focal length has.  

This introduction will give a brief overview of the most common focal length categories and their characteristics.  Later posts will cover each focal length separately and in more detail.

These photos illustrate the difference between four common focal lengths. All were taken at the same distance from the subject.  Upper left is a 28mm wide angle (18mm on APS sensor).  Upper right is a 50mm normal lens (35mm on APS sensor). Bottom left is a 100mm portrait lens (65mm on APS sensor). Bottom right is a 200mm telephoto (135mm on APS sensor).  You can notice from the relationship of the lions head to the background columns that when different focal lengths are used from the exact same distance all they do is crop or take in more of the scene.  To gain a creative benefit from the different focal lengths you need to take alter the distance of the lens to the subject.  This is illustrated in the samples below.
The four broad categories of focal lengths are: wide angle, normal, portrait, and telephoto.  The main creative difference between focal length categories is the sense of perspective they give.  Wide angle lenses tend to expand space, while telephoto lenses compress space.

Take a look at the sample photos. The 400mm telephoto lens used for the photo on the left produces a compression of space between the jumps and the jumper.  The 24mm wide angle lens used on the right produces a more "rounding" effect on the jumper and puts the viewer right into the action.

The long 200mm telephoto lens on the left compresses the scene by bringing the Independence Hall building in Philadelphia right up to the statue in front.  A 35mm wide angle lens used on the right relates the two in a very different way.
For any given aperture a wide angle lens will have more depth of field, that is it shows more of the background in focus when the lens is focused on the foreground.  Depth of field will be covered on its own in a later blog entry.

Wide angle lenses:
A wide angle lens is one with a focal range from around 14mm to 35mm on a full frame sensor, or 10mm to 28mm on an APS sensor.  The smaller the focal length, the more area the lens covers.  A 14mm lens (10mm on APS) would cover a viewing angle of 114° (81° with APS).  On the other, longer end of the wide angle spectrum, a 35mm lens would have an angle of view of 63°  (44° with APS).

The most obvious use of a wide angle lens is to gain a wide view of the subject.
In this photo of a departing storm and rainbow over the Grand Canyon, a 24mm lens on a full frame camera provided plenty of surrounding detail by including the blue sky in contrast to the departing stormy sky.  Relating the central subject of the storm and rainbow to its larger environment told more of the story of what this scene was all about, and gave a better sense of the immense scale of the Grand Canyon.
Wide angle lenses could be used creatively by placing them very close to the foreground area of a scene.  This establishes a relationship of the foreground detail to the background scene.  When doing this it is often best to place the focus in the front, and by stopping the lens down to around f/8-f/16 to provide a very deep area of focus, called depth-of-field (more on this topic in a later blog).
In this photograph of the giant sequoias a 24mm lens on a full frame camera was place right next to the tree trunk on the left and the camera pointed upwards.  This enhanced the sense of scale and provided a better idea of how large the trees are.
In this cityscape a 35mm wide angle lens was used to create an interesting sculptural frame of the Chicago city skyline

Normal lens:

The normal lens of 50-60mm on a full frame camera (35mm on an APS sensor)  is most closely related to the natural perspective the human eye has of the environment.  It is best used when you want to recreate a scene that feels natural, as if the viewer is actually part of it.  In a creative sense, a normal lens places the viewer in the scene without drawing attention to the fact that the camera was even present.  It downplays the role of the photographer in the scene.

Use a normal lens when you want to achieve a realistic perspective between the foreground subject and the surrounding scene as in the samples below.

Many normal lenses come with a very "fast", or wide open, aperture.  This enables them to be used in very dark situations without a flash.  The photo above was taken with available light in a dark barn using a 50mm lens set to f/1.4.  A normal lens is very good at relating a subject to its background without distorting the perspective.
In portraiture the normal lens provides story-telling detail by relating the subject to the surrounding area

Portrait Lens:

A "portrait" lens is so-named because it causes the most natural perspective on the human face.  In addition, it has a slight telephoto effect that throws the background gently out of focus.  This serves to put more attention on the main subject.
This illustration shows the difference in the perspective properties of a wide angle lens versus a portrait lens.  The photo on the left was taken with a 35mm lens on a full frame camera, while the one on the right was taken with a portrait lens of 85mm.  You can easily see how the wider 35mm lens distorts the face, making it unnaturally rounder, while the 85mm lens produces a flattering, natural look.
 Portrait focal lengths range from 70-110mm on a full frame camera (50-75mm with an APC sized sensor)

Here an 80mm lens kept the background scene of the mountains slightly out of focus so that they tell a story of where we are but do not compete with the main image of the cowboy.
Two examples of the 100mm lens delivering a natural perspective to the faces.  Placing the mother and baby on the same plane in the top photo kept both of them in focus and at the same time kept the background softly out of focus.
Using the same lens for the bottom photo but placing the mother and the baby on different planes allowed the mother's face to be sharp and the baby's to be soft -- a result of the longer focal length of the portrait lens.

Telephoto Lens:

Telephoto lenses range from 135mm on up.  The upper practical limit of the telephoto range is 200-400mm (135-300mm in APS), but even longer  lenses can be had at 600-800mm.  In addition to magnifying distant objects, telephoto lenses have the drawback of also magnifying camera movement.  A shutter speed that might be sufficient to stop camera motion with a 50mm lens needs to by three times greater to stop the same motion with a 200mm lens.  In other words, every time you double the focal length, you will need to increase the shutter speed the same amount.  For instance, 1/250 second with a 100mm lens is equivalent to 1/500 second with a 200mm lens.

Fortunately, most modern telephoto lenses are equipped with a vibration reduction technology that can effectively freeze the action for you and allow lower shutter speeds to be used hand-held with long lenses.  Still, it is best to use very long focal length lenses on a tripod to minimize motion blur by increasing steadiness.  With extremely long telephotos it is almost impossible to steady them without a tripod or monopod.

Compressed perspective and shallow depth of field are typical characteristics of the telephoto lens.

This photo of a football shot with a 200mm lens compresses the lines on the field and makes them very out of focus due to the shallow depth of field.
Notice how in this picture of Prague all the buildings and statues are compressed together in a "telephoto effect".

A telephoto lens is a great way to relate a foreground shape to a background as in this example of the pine trees echoing the shape of the Grand Teton mountain range.

Conclusion:

The real reason for choosing one focal length over another is not necessarily the practical one of simply including more or less of a scene, or putting you closer to your subject.  A more important point to consider when choosing a focal length is how it can contribute creatively to your image.  When you want to bring the viewer right into the action, a wide angle lens used up close would be the best choice. The compression effect of a telephoto lens will relate a foreground to background in a very different way than the expanse created by a very wide angle lens. You might want to switch to a normal or portrait lens in situations where you want to illustrate the subject in a more natural way.

In later blogs I will be providing an explanation of the important creative role the lens aperture of each focal length plays in the area of focus, or the "depth of field".

2 comments:

  1. Tom, with all due respect to your incredible experience in this industry, I beg to differ on the definition of focal length. Wouldn't it be, the distance from the rear nodal point of the lens, near the aperture plane, to the actual film or sensor plane?

    That said, your monochrome nature images are studied, elegant, and beautiful. You are one of the few photographers who have an eye for commercial and personal work both, and who can be true to each vision. Well done, Sir.

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  2. You are absolutely correct, Lawrence. I thank you for calling it to my attention. I was probably in too much of a hurry to complete this article before leaving on a long trip.

    You will note that I have changed the definition, but skipped the inclusion of "nodal point". This would require too much further explanation that might go beyond what people new to photography need at this stage of learning, particularly as this article is related to understanding the creative use of focal length as opposed to a full understanding of optics.

    I am trying to aim this blog at people who are starting out in photography and so don't want to over-complicate the explanations. Once I have this blog further along, I plan to add a glossary with links from the text. That will enable me to include deeper information for those who want it. I envision adding that sometime in the fall.

    This blog is a work in progress and I welcome any and all comments and corrections. A nice thing about the internet age is that information is malleable. My intention is to add to, expand, and correct the information as the blog grows with a view to compiling it into a cohesive whole somewhere down the line.

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