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stevem1928 May 27, 2006 11:01 AM

Why is it that the large apature / low f stop lenses are so much more expensive? From a lay persons perspective, it doesn't seem like it would be that hard to make the apature open up a little more. Or, is this a marketing deal?

Why does the apature size have to go down (increase f stop) as I zoom on my 18-200mm lens?

Boily May 27, 2006 11:04 AM

Because the zoom lens will be faster and sharper!! (I believe)

amazingthailand May 27, 2006 3:53 PM

A larger aperature requires the lens to be physically larger. More glass, more precision grinding, more coatings, more... Well, you get the idea. And all that means more expensive to produce and hence to buy. So no, it is not all marketing.

To maintain a constant aperature on your zoom lens would result in a MUCH larger and therefor, more expensive, lens. That 18-200 is a consumer lens and most consumers are just not going to pay the price. Take a look at the price, and size, of a constant aperature zoom and you will see what I mean.

peripatetic May 28, 2006 8:14 AM

The f-stop is quite literally the ratio between the aperture and the focal length.

Think about this - a f1.4 50mm lens needs an aperture of 36mm. That means you need a hole 36mm wide at the inside of the lens to let light through. Add the glass and plastic/metal surround and it has to be physically quite large.

The longer the focal length the worse it gets.

That's why the long lenses have to have such wide front elements.

They are more expensive because you need lots more glass and metal. The manufacturers would love to make their lenses smaller and lighter, but they physically can't.

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