Ratio between maximum and minimum focal length in a zoom lens — 24–70mm yields 3:1, limiting versatility vs. fixed prime lenses. Higher ratio trades optical quality for range.
A zoom lens thrives on its flexibility—and this is measured by the zoom ratio. You divide the longest focal length by the shortest focal length to get a ratio that immediately tells you how much leeway you have without changing lenses. A 24-70mm offers a ratio of 2.9:1, while a 70-200mm offers only 2.8:1. It sounds similar, but in practice, you'll notice the difference immediately.
The higher the zoom ratio, the more range you pack into a lens. This is worth its weight in gold on set—fewer lens changes, less dust on the sensor, faster transitions from close-up to wide shot. An extreme zoom ratio like 10:1 (e.g., 24-240mm) makes you mobile and saves time. But—and this is the catch—the wider this range, the more optical compromises you have to accept. Sharpness at the edges suffers, vignetting becomes more pronounced, and the maximum aperture shrinks. A 24-70mm F2.8 with a 2.9 ratio will always satisfy you optically; a 24-240mm with a 10:1 ratio, on the other hand, is often slower (F3.5-6.3) and exhibits noticeable aberrations in its extreme ranges.
In practice, this means: for documentaries or reportage shoots, you'll opt for a higher zoom ratio and accept optical compromises because flexibility and speed are paramount. For controlled productions—feature films, high-end commercials—you're better off with multiple lenses with smaller zoom ratios. A 16-35mm F2.8, a 24-70mm F2.8, and a 70-200mm F2.8 give you a similar focal length range to an extreme 10:1 zoom, but with significantly better optical quality and constant aperture. Changing lenses costs time, of course—but not image quality.
Therefore, the zoom ratio is not an absolute mark of quality but an indicator of compromise. The closer you stay to 3:1 or 2.8:1, the better the optics—and the higher you go, the more flexible it becomes, but the more critically you must examine aperture and edge sharpness. On set, you'll particularly notice this difference in lighting situations: a constant F2.8 zoom will forgive more in low light than a variable F3.5-6.3 lens with a high zoom ratio.