Lense speed

[carusoswi]

Ok, so, I've been taking pictures for more years than I wish I had to claim. My question: I understand that a 1.4 is a faster lens (it can shoot better in low light situations than a slower lens (say a 3.5)).

What is it about the construction of lenses that makes one a super light gatherer and another not so? Why do variable zooms chance "speed" as you zoom out, and what is it about non-variables that allow them to maintain a constant "speed" as you zoom out?

I understand that practical side of these variables - want to shoot in low light situations, a 1.4 will give you an edge, etc.

What I don't understand is the design variables that cause one lense to be stellar and another to be less so.

Caruso

 

[jon_k]

Why do variable zooms chance "speed" as you zoom out,

For them to work properly and have a sharper image I believe the aperture has to have a smaller limit. There are probably also physical design limitations that go in to play as well. A prime (fixed mm "non variable") lens on the other hand does not have these limitations.

I'm not sure if it has to do with the number of lens elements, or the size of the elements in the back of the lens. It could be on a zoom lens there are more elements and the elements in the back of the lens are smaller, thus limiting the widest aperture available. Whereas the lens elements on a prime "non variable" lens are larger and thus allow for a larger aperture.

The larger your lens elements are, the larger the aperture you could obtain, I think in theory.

Take this with a grain of salt. I don't know /anything/ about lens construction -- but this explanation is what I came up with just by thinking about it and it seems to be a pretty good explanation.

 

[astrostu]

A larger aperture (f/number) pretty much requires the front of the lens to be larger. Look at Canon's 70-200 mm f/4L vs. its 70-200 mm f/2.8L. The f/4 has a filter size of 67 mm wheras the f/2.8 has a filter size of 77 mm which gives it 32% more area from which to gather light. There are other aspects to get it to f/2.8 (which can gather 204% more light than f/4), but I believe that's one of the main ones.

The reason many zoom lenses change aperture when you change focal length is the physical process of making the lens longer. The f/number is proportional to the focal length and the diameter of the lens, so when you change focal length, you change the f/number. Fixed f/number lenses have a compensator built-in that (I believe) effectively stop down the lens when it's at a small focal length and open it up more when it's at a longer focal length to achieve the same f/number throughout.