Newbie Question on aperture

[Jimbo73]

If a lense is labelled to have fixed aperture say f2.8, why can we still vary the f value? So what's the different between a f2.8 compared to f2.8-4 lense. Both can achieve 2.8. Does a f2.8-4 means that the maximum f is 4? appreciate if someone can help to clarify my doubts.:dunno:

 

[jbma]

That value is the minimum aperture. It is not a constant aperture.

 

[jbma]

Actually depends on how you look at it. Minimum aperture or maximum opening of the lens aperture.

 

[drumma]

if a lens is F2.8-4, it means that u can get F2.8 on the wide end and F4 on the tele end.

 

[+evenstar]

If a lense is labelled to have fixed aperture say f2.8, why can we still vary the f value? So what's the different between a f2.8 compared to f2.8-4 lense. Both can achieve 2.8. Does a f2.8-4 means that the maximum f is 4? appreciate if someone can help to clarify my doubts.:dunno:

For the Sigma 17-35 or 28-70mm or 28-105mm, at the 17mm end (for 17-35) or 28mm end (for the other 2), you can shoot at f2.8. At the most tele end (35/70/105mm) however, you can only shoot at f4 widest...

 

[LordAeRo]

Hello,

Let's assume your zoom lens is from 70-200mm ya?
A constant f2.8 means you can use an constant aperture value of f2.8 throughout 70mm to 200mm. This doesn't mean that it's the only aperture value you can use. Depending on the lens, you can use it from f2.8 to f22.

For a f2.8-4 lens, it means that the minimum fstop you can use at 70mm is f2.8. However, at 200mm, the minimum will be f4 and not f2.8. So in summary,

for a f2.8 lens,

at 70mm : 2.8 - 22
at 200mm : 2.8 - 22

and

for a f2.8-4 lens,
at 70mm : f2.8 - f22
at 200mm : f4 - f22

Yeah?

If it's wrong, please be gentle with me, I'm a book. :sweatsm:

 

[Jimbo73]

i c............ but at what focal length will you not be able to use 2.8 say for 70-200 f2.8-4. at 71mm? Is f2.8 strictly for the lowest focal length for the above example?

 

[espn]

That depends glass to glass.

Yes, once you leave 70, perhaps to 80, it would hit f/3.2 already.