The second major impact is how you can manage the background of the composition. A large maximum aperture, such as f/1.8, gives you more options for making the background detailed or soft. Large aperture settings allow you to soften the background more easily, while smaller settings (f/5.6 and smaller) tend to render more background detail.
Large aperture prime lenses, such as the Sigma 50mm f/1.4 and the Canon 50mm f/1.8, make it easier to soften the background. Set the lens to its largest aperture, such as f/1.4, and focus carefully on your subject. The background will render softer than your subject.
If your shots tend to have an even amount of detail from front to back, or if they haven't pushed the limits of existing-light photography in moody environments, then you may want to consider a lens with a larger maximum aperture. Reasonable affordable examples are a 50mm f/1.8 and a 30mm f/2.0. One of my favorite affordable "big aperture" lenses is the 85mm f/1.8. With any of these choices, you can shoot without flash indoors and have more control over background detail outdoors.
Size and weight
Maximum aperture also influences size and weight. The larger the aperture opening, the bigger the lens. A great example is to compare professional-quality 70-200mm zooms. A Canon f/2.8 model weighs 3.28 pounds, while the f/4 version is much lighter at 1.67 pounds, about half the weight.
The f/4 version of the Canon 70-200mm zoom weighs about half of the f/2.8 model. How much weight are you willing to carry all day?
This is also the case for wide-angle zooms and fixed-focal-length lenses. A 17-40mm f/4 wide-angle zoom weighs 1.05 pounds, versus 1.4 pounds for the 16-35mm f/2.8 model. And a Sigma 50mm f/1.4 is considerably heavier than the Canon 50mm f/1.8.
Of course, maximum aperture isn't the only influence on size and weight. Robust pro-model construction also plays a part. That Canon 50mm f/1.8 I mentioned is a consumer-grade lens with plastic construction, whereas the Sigma f/1.4 is a professional model with a metal housing.
How much weight are you willing to carry? That beautiful 70-200mm f/2.8 doesn't do you any good sitting on the shelf at home because it doesn't fit in the shoulder bag you want to carry while touring Paris. It's important to hold the lens you're considering and see how it fits in the bag you want to use.
If you shoot with a Canon, Nikon, or Panasonic camera, then you need an image-stabilized lens to take advantage of this important technology and compensate for camera shake. Those brands use optical stabilization that's built into the lens. Olympus and Pentax, on the other hand, build the stabilization feature into the body, giving you that technology regardless of the lens you've mounted on the camera.
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