The Canon EF 24-105mm ƒ/4L IS USM all-in-one professional zoom, for example, is available with a constant aperture of ƒ/4 through the whole range of the zoom. Lenses require very sophisticated designs and expensive optics to maintain a "constant" aperture throughout the range, so this lens is comparatively very heavy to other zoom lenses, and is also more expensive than most at $1,149.
35mm equivalence refers to the actual angle of view you see on an APS-C camera, since there's a crop factor of approximately 1.5x or 1.6x, depending on the sensor size of the camera. A 100mm lens on an APS-C camera would have the field of view equivalent to a 150mm lens on a full-frame camera, for instance, while a 200mm lens would translate to the same viewing angle as a 350mm lens. Manufacturers refer to this as a "magnification factor" since you're gaining zooming power, but in reality, it's a crop factor because an APS-C sensor is roughly two-thirds the size of a full-frame 35mm image plane. Micro Four Thirds sensors are even smaller than APS-C, with a 2x equivalence. The Olympus M.Zuiko 14-150mm ƒ/4-5.6 is a Micro Four Thirds lens equivalent to 28-300mm in 35mm. Because Micro Four Thirds sensors are so small, the lenses (and cameras) can be designed to be quite compact, and the 14-150mm is only 3.3 inches long with a light weight of 0.62 pounds. Estimated Street Price: $599.