lens - Why are large zoom ratio constant apertures lenses not generally available?

I know about 17-55 f2.8 and the likes of it, but I am talking about something say 18-135 f2.8. There exists 70-200mm f2.8 and the range (200-70 = 130mm) is much bigger than in 18-135.

I am aware of the meaning of f numbers being a ratio between focal length and the effective size of the aperture. The key point being "effective size of aperture". E.g. 600mm f4 does not mean that the blades open up to 600/4 = 150mm.

1. Are they impossible to make due to laws of physics?



2. Would they be too heavy/long making them unusable?


3. The range is not the only factor. I am hoping that this is correct, but why?

P.S. I use a Canon 60D and my questions are for the 1.6 crop factor sensors.

Answer

It is more about ratios than addition/subtraction. 70-200mm is less than 3x from the shortest to longest focal length. That makes it possible to place all of the moving elements that enable the lens to change focal length in front of the aperture diaphragm. 18-135mm is 7.5x. Placing all of the zooming elements in such a lens in front of the diaphragm would make the lens much longer than designs that also include some zooming elements behind the diaphragm. The downside is that the entrance pupil receives no benefit from that magnification done behind the diaphragm. The higher the ratio between the shortest and longest focal length, the more some of the magnification happens behind the diaphragm and the greater the difference between the "effective aperture", more properly called entrance pupil, for the shortest and longest length at the constant aperture.

Another consideration is optical quality in terms of things such as distortion, chromatic aberration, and vignetting. The wider the ratio between the shortest and longest focal length of a zoom lens, the more compromises must be made when designing the lens. Correcting these optical flaws becomes increasingly difficult as the size of the entrance pupil expands due to the higher angles that light from the edges of the lens strike the sensor (or film). It is not just more difficult, it is more expensive and adds size and weight to the design. At some point you reach a practical balance between the flexibility of a wide ratio zoom lens and the image quality of a prime lens. For most buyers of premium lenses, they would prefer sacrificing an "all in one" lens" rather than sacrificing image quality and size/weight.