Constant Apertures

[Orbster]

This one has been puzzling me for a while.

There are zoom lenses out there that have a constant aperture throughout the focal range so given that the f number is actually a ratio of the focal length how can this be?

Does the aperture close down as the focal length decreases? This would seem daft as why not have a superfast aperture at the widest focal length instead of a relatively slower one?

Confused

 

[Brachytron]

The f-number is the ratio of the focal length and the entrance pupil diameter (not the actual aperture diameter). Thus a 50mm lens with an entrance pupil of 50mm is f1. The entrance pupil is the size of the image of the aperture when viewed through the fromnt of the lens. This takes account of its magnification by lens elements in front of it. This is why fast tele lenses have large diameter front elements and slower tele lenses are narrower. The lens elements magnify the aperture to an apparent larger size than it actually is to give the wider aperture. In a zoom lens of fixed maximum aperture, as you zoom the lens elements will change the size of the apparent aperture diameter such that it stays in a constant ratio with the new focal length, thus the f-number doesn't actually change. With variable aperture lenses, this doesn't happen and the f number therefore changes. Fixed f-number zooms are harder to design which is why they are more expensive. Hope this answers your question.

 

[Orbster]

I'll have another read of that tomorrow - I find large blocks of text with no paragraphs hard to read,

Cheers though

 

[Canon Bob]

Part of the confusion is that we've become a little lazy with our terminology.
We freely use the simplified term "aperture" when it is more correct to say "apparent aperture". The first is physical and the second is practical.

Bob