Wide-Angle Zoom Lens
If you are frustrated when trying to take photos indoors without flash with the 18-55mm f/3.5-5.6 lens that came with your camera, consider Sigma's 17-50mm f/2.8 lens or Tamron's 17-50mm f/2.8 lens, both with image stablization features. These constant aperture lenses are one and a half stops faster at the wide end and two full stops faster at the telephoto end than a kit lens. This enhances the camera's ability to let in enough light for motion-stopping shutter speeds inside. The quality of the glass in the lenses is better too, so you can expect sharper images (as long as your focusing techniques are good.) Note that the Sigma and Tamron lenses fit only APS-C (cropped) sensor cameras, not full-frame bodies.
|Canon 17-35mm f/4|
Fast Telephoto Lens
Many people develop a desire for a longer telephoto lens when their children start participating in sports or because they enjoy seeing wildlife. But if you are frustrated by not being able to get fast enough shutter speeds to stop action in dim light with your current long lens, consider a third-party lens such as Sigma's 70-200mm f/2.8 lens or Tamron's 70-200mm f/2.8 lens. These lenses provide one- to two-stop larger apertures than telephoto zooms like a 55-200mm f/4-5.6 lens. This larger aperture lets in more light in dim situations, allowing the camera to use a faster shutter speed to freeze movement. The Sigma has image stabilization, while the Tamron does not.
|Canon 70-200mm f/2.8|
Disclaimer: I have not used any of the third-party lenses listed here; my suggestions are pulled from the DPreview articles. Please do your own research to see which lens is best suited to your type of photography.