Focal length and aperture effects

While some photography technologies relate only to your camera’s internal operation, and knowing them won’t necessarily make you a better photographer, understanding some others can have a direct impact on your compositional skills and your ability to control your camera more completely.

FOCAL LENGTH AND IMAGES

Knowing the relative behavior of lenses, how focal length works with them, and how they will work with and be effectively applied to various subjects is essential to producing good photography. The relative sizes and perceived distance between objects change as focal lengths change, whether in a zoom lens or by using different lenses. Of course, to achieve a similar photo with a wide-angle versus a telephoto lens — say, for example, an image of a house that shows the entire building — you need to physically change your position to get closer or farther depending on your lens’s focal length. A telephoto lens will make a row of racing cars, viewed head-on, look far closer to one another than they really are, while with a slightly wide-angle lens you might need to have a family pose closer together than might seem natural because the lens will spread them apart somewhat.

Other factors are affected by focal length, as well; telephoto lenses, for example, are susceptible to camera shake and can produce blurry images with minimal movement because their focal length is so long (which, once again, is why image stabilization is helpful). And aberrations at the edge of wide-angle lenses occur with such a wide field to be compressing into a small aperture and onto a focal plane.

APERTURE AND IMAGES

How wide or narrow your aperture is set for a given exposure affects your image in more ways than just providing more or less light. It is your aperture setting that controls depth-of-field and makes the difference in the artistic effect in a nature shot or whether a row of athletes is blurry in a bleacher-filled team photo. So understanding how apertures work is essential for the working photographer just as it is for the artist or the enthusiast.

It’s important to note that lenses with larger maximum apertures provide significantly brighter viewfinder images — possibly critical for night and low-light photography. Lenses with larger apertures also often give faster and more accurate autofocusing in low light. Manual focusing is also easier because the image in the viewfinder has a narrower depth of field, thus making it more visible when objects come into or out of focus.

A smaller aperture setting, such as f/22, creates an image with deep depth of field, such as is ideal for a large group of people or a landscape where you want to see foreground and distance both in focus. A larger aperture setting, such as f/2.8, produces a shallow depth of field, with much of the image out of focus, other than your subject, as in 4-13.

Here, the photographer used a shallow depth of field to place the emphasis on this beautiful little girl. Taken with an EOS 1Ds using an EF 70-200mm f/2.8L lens, 1/100 second at f/4.5.

Some zoom lenses are not capable of holding a maximum aperture width as they are zoomed out and as their focal length increases. If you are using such a lens, it’s good to know this if you have the aperture set wide and the lens at a minimum zoom and you decide to zoom in on something. The aperture will change automatically because the lens is incapable of holding that aperture width; however, you might have an underexposed image as a result. “Fast” zoom lenses are those that will hold the same aperture at any focal length; they are typically more expensive because of the complex optical engineering it takes to accomplish this feat.

Using deep depth of field is common among photographers shooting groups of people, crowds, architecture, and landscapes, where not having areas out of focus is very practical or helpful to the shot. A shallow depth of field has more artistic and dramatic value, such as in a macro photo of a bumblebee on a flower, or to isolate a quarterback about to pass the ball with a blurred, colorful crowd behind him. Obviously these are general examples, and it’s ultimately up to you, the photographer, to understand how the aperture you choose will make your images winning shots.


Tip

Here’s an old photographer’s trick that will help you determine your shutter speed to avoid a blurry image, depending on your focal length: Your shutter speed should be at least as fast as the numerical value of your focal length. So, if you’re shooting a lens with a 100mm focal length, your shutter speed should be set to at least 1/100 second. While this isn’t an absolute law, it’s a good rule of thumb from which you can fine-tune your shot. Plus, the rule is generally applied to a full-frame sensor or 35mm film frame; for smaller sensors, you’ll need to up your shutter speed a little to account for the crop factor.

Category: Science of Lenses

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