One number?

[Markw]

Ive been looking at different telephoto lenses lately and I see one confusing problem..as I usually do with just about everything. I was looking and most telephoto lenses have two maximum apertures, one at the lowest zoom distance and one at the longest zoom distance. Some, though, only have one aperture value. What does whis mean? Can the lens somehow maintain this value throughout all the zoom? Any help would be much appreciated.

Mark

 

[Mgw189]

Ive been looking at different telephoto lenses lately and I see one confusing problem..as I usually do with just about everything. I was looking and most telephoto lenses have two maximum apertures, one at the lowest zoom distance and one at the longest zoom distance. Some, though, only have one aperture value. What does whis mean? Can the lens somehow maintain this value throughout all the zoom? Any help would be much appreciated.

Mark

Yes they can they are usually larger lenses as well as considerably more expensive. If you are looking at the F2.8 range they are really handy when you want a lot of light.

 

[Samanax]

Some, though, only have one aperture value. What does whis mean?

It means the maximum aperture is the same at both ends of the zoom.

Can the lens somehow maintain this value throughout all the zoom?

Yes.

 

[AlexColeman]

Yes, they are called constant aperture lenses, and as a rule, they are more expensive, but are better, because they always have that aperture!

 

[Jaszek]

but also they are usually longer and heavier and the front of the lens is wide to fit the large aperture in there

 

[OldClicker]

Actually, they are constant 'relative aperture' or 'aperture ratio'. The aperture has to get larger at the longer end to maintain the same ratio of focal length/aperture (=f-stop). BTW - even with those that aren’t constant (such as an f/4-5.6), the aperture get larger - just not enough to maintain the constant f-stop. - TF