Why don't they make all camera lenses with large maximum apertures (less than f/2.0)?

I'm assuming it's to keep cost down. But exactly why is it more expensive to engineer a larger aperture? Thanks!

It helps to actually understand what aperture means. It is a ratio between the focal length of the lens and the size of the opening that admits light. If you have a 50 mm lens with an f/2.0 maximum aperture, the opening has to be 25 mm in diameter. This is the definition of f/stop. If you had a 100 mm lens, f/2.0 would mean the maximum opening would be 50 mm - about 2 inches. If you had a 300 mm lens, f/2.0 would mean the maximum opening would be 150 mm - about 6 inches. That's a pretty huge chunk of glass. In order to make it optically correct requires an amazing amout of precision, both in making the glass in the first place and then in shaping it properly to be an accurate lens.

I see that Nikon has a 600 mm f/4.0 lens. Do the math and you will see that the maximum opening has to be 150 mm in diameter. This is NOT the measurement of the front piece of glass, but it is actually a measurement of the lens at the smallest diameter. The front element on this lens if over 6 inches in diameter! This is why it costs $8,000. What I don't really understand is why there is such demand that the lens is backordered...

Okay, to be techincal, the f/stop refers to the "effective" aperture of a lens, but I don't understand the optics well enough to explain that, so I'll just let the general principles as explained above stand as my answer.

In other words, you and everyone above me are correct. They don't do it because it is just too expensive.

You're welcome. I'd say I just added the detail in a footnote to 3 correct answers above me. Report It

You're right, it's to keep lenses affordable. A large aperture requires a lot of light to hit the sensor. With zoom lenses, this requires huge lens diameters (77mm is the standard for f/2.8 Canon and Nikon zooms). That means lots of huge lens elements, all of them top notch quality, coated, etc, which in turn means lots of cash. And even if you could AFFORD an f/2 zoom lens, you'd need a wheel barrel to cart it around ;-)
Professional zooms top out at f/2.8. If you need something faster than that, get one or more fixed focal length lenses. Most brands have a cheap 50mm f/1.8.

It would cost a fortune for SLR people to own one. $1000+ on a good lens. Plus the weight would be EXTREMELY heavy for zoom lenses. Look at the new Sigma thats coming out (its an extreme but it looks cool =) ) its an 100-500 I believe and its f/2.8 and wieghs over 20+ lbs. How would you want to carry that around?

There were a few, but they were mostly 50mm and all around f.1.8 or f/1.4. Nikon made a 85mm f/1.8 and I think Pentax had a 300mm f/2.0 about forty years ago, but the costs of manufacturing them and the limited number of buyers, made the design, manufacturing and selling of them unprofitable.

Think of it. The glass throughout the whole lens when designing a lens at f/2.0 vs. f/2.8, must have twice the surface area and still be critically sharp, edge to edge and produce the same amount of light, corner to corner at the film plane. Now all this heavy glass has to be mounted in a lens barrel that supports auto-focus, a fast auto aperture system and be able to withstand some rough handling, especially because most of these lenses are used by sports photographers getting hammered on the sidelines of games.

Here is a price comparison between two 400mm lenses. The one with a f/5.6 aperture is under $1,100 and weighs just under 3 lbs. The one which is two stops faster (f/2.8) is over $7,000 and weighs nearly 10 pounds.

Many pros rent these lenses if they need them for an assignment and others use the ones supplied by their clients who can afford them.

Fotoace it was Nikon had 300 f2, Pentax had a prototype but never made it.
http://www.cameraquest.com/nf3002.htm