Learning the basics of lenses

Camera lenses cost from less than $100 to as much as $6,000 or more. Yet they all basically do the same job: create an image on the digital sensor. As a rule, however, better-quality lenses, such as the 70-200mm f/2.8L used to take the photo shown in 3-3, are more likely to produce higher-quality results. I love the texture and small features of the wall captured so well by this lens. They are only really visible close-up, such as the small plant growing out of it. The graffiti, however, is a bit of an optical illusion, and is best seen by not looking too closely to get the 3-D effect of the face.

This image taken in San Juan, Puerto Rico, shows the quality and depth a high-quality, professional lens can produce. Taken with an EOS-1D Mark IIn and a 70-200mm L-series lens, ISO 400, 1/400 second at f/6.3.

For many of you, your experience with lenses may be limited to film or digital cameras with fixed lenses, so the following sections explore some of the characteristics of photographic lenses. This will enable you to better understand how lenses work and which ones might be right for your style of photography.

FOCAL LENGTH AND ANGLE OF VIEW

Usually expressed in millimeters, the focal length of a lens is the distance from its optical center to the image plane (digital sensor), where theoretically it produces the sharpest image. The angle of view of a lens is how much of the scene, side to side and top to bottom, the lens includes in the image. The angle of view of a lens is determined by the size of the film or sensor and the focal length of the lens. So it stands to reason that for a lens projecting a photographic image, the size of the film format or image sensor determines the angle of view for a particular focal-length lens. This is why the angle of view of the same focal-length lens changes as sensor size gets larger or smaller, thus determining the crop factor (called lens focal length conversion factor by Canon) of your APS-size sensor.

The focal length of a lens is probably the major determining factor in deciding which lens to purchase. One key point is to understand the differing focal lengths and how this will affect the depth of field (DOF) and perspective, in turn controlling the perceived image.

The focal length is an easy way of referring to the angle of view of a lens. The angle of view is denoted in degrees (as in a 360° circle), while the focal length is in millimeters. The relationship between these two numbers is an inverse one: the smaller the focal length in millimeters, the larger the angle of view. An easy way to grasp the concept is to imagine yourself standing in the middle of a 360° circle and the angle of view of a lens as a slice of a pie. Lenses are usually referred to in terms that express their angle of view as listed:

  • Fisheye lenses. A specific type of wide-angle lens, which covers up to a 180° (or even wider in special cases) angle of view, often with significant distortion.
  • Wide-angle lenses. These lenses generally cover between 100° and 60°.
  • Normal, or standard, lenses. These lenses generally cover between 50° and 25°.
  • Telephoto lenses. These lenses generally cover between 18° and 10°.
  • Super telephoto lenses. These lenses generally cover from 8° to less than 1°.

Wider-angle lenses allow you to see more in your angle of view while allowing greater DOF between the viewer and the subject. Fisheye lenses have the most distortion, especially at the outer edges of the image.

PERSPECTIVE AND DEPTH OF FIELD

In photography, perspective is the way in which an object appears to the camera, based on its size and the position of the camera relative to the object. As objects become more distant, they appear smaller. For example, the Sun and Moon appear to be roughly the same size to our eye because the Sun, although much larger, is much farther away. The relationship between distance and apparent height of objects is not linear. If an object actually touches the eye, thus is no distance away, it appears infinitely tall. You can also think of the way railroad tracks appear to converge as they approach the horizon. Different lenses show the world with different perspectives, depending on whether they are magnifying distance, compressing a wide area onto a smaller one, or simply giving a clear, natural representation of a subject. And whether they’re magnifying or expanding, they will behave differently with different optical effects and distortion.

Perhaps the the most important consideration of how a lens will represent a subject is depth of field (DOF). The depth of field is the distance in front of and beyond the subject that appears to be in focus. There is only one distance at which a subject is precisely in focus, and focus falls off gradually on either side of that distance, but there is a region in which the blurring is imperceptible under normal viewing conditions.

Essentially, DOF literally refers to the area of a photograph that appears in focus. As you can see in 3-4 and 3-5, an image with a deep DOF has very few areas out of focus, while an image with a shallow DOF has areas out of focus in front of and behind the subject. As a photographer you can control this, and a good photographer knows how to use DOF to control the perception of an image.

Both 3-4 and 3-5 were taken with a Canon 30D set at ISO 100 and 1/125 second at 100mm focal length. However, 3-4 was taken with a deep DOF aperture setting of f/16, while 3-5 was taken with a shallow DOF setting of f/2.8.

The two elements that control depth of field are aperture setting and lens focal length. The wider the aperture (the lower the f/number) the more shallow the depth of field, the narrower the aperture (higher f/number) the deeper the depth of field. Longer lenses magnify the subject more while compressing distance and having a deeper DOF. Zoom lenses are different and a special case because the focal length, and hence the angle of view, of the lens can be altered mechanically without removing the lens from the camera.

F/NUMBER

For a given subject framing, the depth-of-field is controlled by the aperture diameter, the measurement of which is identified by the lens f/number. Each f/number represents one stop. Increasing the f/number (reducing the aperture diameter) increases the DOF, while decreasing the f/number (increasing the aperture size) reduces DOF.

CROP FACTOR

If you’ve already studied some about digital camera technology, you may have run into the much-discussed crop factor. You might be wondering exactly what it is and what it means. You need to know the sensor size of your camera to begin to figure this out and understand it because it not only affects the size of the image, but also affects the angle of view of the lens you use. For example, both of the Digital Rebels and the 40D use an APS-C-size sensor, which is approximately two-thirds the size of a full-frame, 35mm sensor.

With an APS-C sensor, the angle of view is approximately 1.6 times narrower than the actual focal length of the lens. That means that, for example, a 100mm lens on a Digital Rebel XTi (which uses an APS-C sensor) will have the field of view of a 160mm lens and a 50mm lens will appear to have the field of view of an 80mm lens. On a full-frame sensor camera, such as the EOS 5D, the lenses will behave at their specified focal length (meaning a 100mm lens will appear as a 100mm lens). That said, while most people say when you mount a 100mm lens on an XTi it becomes a 160mm lens, it’s important to realize that the lens’s focal length doesn’t actually change by mounting it onto cameras with different sensor sizes — it’s the angle of view that changes.

Figures 3-6 and 3-7 demonstrate how a full-frame image sensor helps you get a wider field of view without having to use a wider-angle lens, which is especially helpful with architectural shots. If you have a camera without a full-frame sensor, to include the full scene you’d have to move farther away from the subject (which may not be possible) or use a wider-angle lens.

 

3-6 and 3-7 illustrate where a wide-angle lens would have potentially caused some distortion. Using a camera with a full-frame sensor, in this case an EOS 5D, allowed the photographer to get more of the scene into a normal-range image. Both taken with an EF 24-70mm f/2.8L lens. 3-6 taken at ISO 100, 1/50 second, f/8; 3-7 taken at ISO 250, 1/100 second, f/13.

BOKEH

Bokeh (from the Japanese word, boke, meaning blur) refers to the out-of-focus areas in a photograph. Bokeh is directly related to depth-of-field, but the nature and limits of this direct relationship are not necessarily obvious and are somewhat hard to measure or quantify scientifically. Photography enthusiasts, particularly in Internet chat groups and blogs, discuss the quality of bokeh, but without any well-established, formal system of scientific assessment. A given person may view a certain image as having pleasing depth-of-field, and often this assessment is attributed to “good lens bokeh.” The terms good lens bokeh and bad lens bokeh are frequently employed, and sometimes it is not made clear in the context that these are to some degree subjective assessments. Any assertion that the lens is the reason why the out-of-focus parts of an image are good or bad is subjective and can be argued given many factors — including the angle and framing of the shot, which is largely in the control of the photographer.

For lenses with differing focal lengths that are used to photograph the same scene at the same aperture, if the subject is made the same apparent size, technically the depth-of-field will be the same. However, assuming the camera is all that moves to alter the apparent subject size, the longer lens produces less distracting detail in the background. It provides a subjectively greater degree of bokeh because the long lens has a narrow field of view, meaning that a smaller portion of the background will be in view behind the subject.

A shorter lens shows a wider angle of view of the background, so even though the details individually have the same degree of blurring, there are typically many more such details filling the same area of the image, and the eye perceives this as more detail. If the background has almost no detailed features, it would be hard to notice the difference in either regard.

So science attempts to prove what makes good lens bokeh a part of optical engineering, but the fact remains that I know what my eyes tell me. The image in 3-8 is an example of a narrow DOF silhouette image using the EF 24-70mm f/2.8L lens, which in this case shows, in my opinion, very good lens bokeh.

Depth-of-fieid is an important consideration in shooting silhouette images, and this L-series Canon lens has the bokeh to handle the job such as in this image that was taken in the Casbah of Algiers, Algeria. (ISO 100, 1/250 second, f/2.8, taken with a 1D Mark II and an EF24-70mm f/2.8L lens.)

Category: Photography System

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