Variable Apertures... whats the deal?

[TinySpark]

This is probably a stupid question, but i need to ask, why are some lenses labelled with variable apertures? for example, on a lens I'm lusting after-
Sigma 28-70mm F/2.8-4 DG Asp AF Wide Angle Zoom Lens

Why can't I use f/2.8 anytime, or when can I use it?
Same goes with my kit lens!

Also what does the DG and Asp parts stand for too?

And slightly off topic, has anyone ever bought from 'microglobe' on the net? it's selling that lens i like very cheap!

 

[Martyn...]

It is a result of the design used to keep the cost of the lens down, you can have constant f2.8 however you would need to spend a lot more money.

 

[Harvey_nikon]

DG: Lenses for on both crop and full-frame cameras.

ASP: Aspherical. Compact lenses with a reduced number of internal elements.

 

[whitewash]

DG means the lens is designed for (but not exclusively usable on) digital SLRs, the glass is optimised for the requirements of digital sensors. Whereas DC signifies that a lens will only fit on a cropped sensor


the f2.8-4 means that at the wider end the aperture is f2.8 but at the tele end the aperture can only reach f4, in the middle it will vary dependant on the lens what you can use at what length. Its something to do with the lens design and that is why lenses with constant aperture across the range (i.e sigma 70-200f2.8 are in general more expensive and bulky.

 

[TinySpark]

aahhh right! that's been buggin me for ages, thanks for your speedy replies!
I also wanna ask, if Sigma fit Nikon, Canon and Sony, does that mean all sigma lenses fit each brand, or do they make sigma lenses specifically for Sonys and then the same lens except for Nikon etc.. ? Do the cameras have different mounts so this is necessary?

 

[nb62]

Have a look at http://www.sigmaphoto.com/lenses/lenses_all.asp it will give you all the info you need on sigma suffixes. Not too sure about Microglobe, they keep getting grey import warnings on the net. Have you tried SimplyElectronics? Don't know if they do Sony mounts though.

 

[whitewash]

aahhh right! that's been buggin me for ages, thanks for your speedy replies!
I also wanna ask, if Sigma fit Nikon, Canon and Sony, does that mean all sigma lenses fit each brand, or do they make sigma lenses specifically for Sonys and then the same lens except for Nikon etc.. ? Do the cameras have different mounts so this is necessary?

theres no compatability between different camera mounts a sigma lens for a sony will only fit a sony and a sigma lens for a canon will only fit a canon, the mount is different.
the likes of tokina and sigma make lenses with different mounts for different manufacturers

The bulk of the lens components on a sigma lens will be the same regardless of mount but the mount and its relevent electronics and architecture will be different from each mount.

 

[pearce_jj]

Sigma 28-70mm F/2.8-4 DG Asp AF Wide Angle Zoom Lens

Why can't I use f/2.8 anytime, or when can I use it?

It will be F2.8 at 28mm, reducing to F4 at 70mm.

 

[stan the man]

microglobe sell grey imports,so you won't get the sigma 3 year warranty as on the UK lens'.

 

[danbroad]

Here's an interesting thought. We like constant aperture lenses, and assume they're more 'professional'.

Would the Canon 24-105L have the same appeal, or the same buzz, if it was the Canon 24-105 f/3.0-4.0? Because it probably is able to gather more light at the shortest lens length, and it's probably throttled by the on-lens CPU to dictate constant maximum apertures to the camera body.

Would Nikon owners [who, let's face it, sorely miss an f/4 standard zoom to compete with the above lens, hence the ubiquity of forum posts] be all over the Sigma you mentioned if it was pitched as the Sigma 28-70 f/4 EX, using the same CPU-throttling as the Canon L?

 

[HoppyUK]

Here's an interesting thought. We like constant aperture lenses, and assume they're more 'professional'.

Would the Canon 24-105L have the same appeal, or the same buzz, if it was the Canon 24-105 f/3.0-4.0? Because it probably is able to gather more light at the shortest lens length, and it's probably throttled by the on-lens CPU to dictate constant maximum apertures to the camera body.

Would Nikon owners [who, let's face it, sorely miss an f/4 standard zoom to compete with the above lens, hence the ubiquity of forum posts] be all over the Sigma you mentioned if it was pitched as the Sigma 28-70 f/4 EX, using the same CPU-throttling as the Canon L?

Constant aperture zooms are 'throttled' as you say, although I can see why you might think so. Take a peek through one, and you'll see that it uses all the glass at lowest f/number, at all focal lengths.

If Canon was to turn the 24-105 f/4 into a variable f/3-4, it would be a lot bigger and heavier, and more expensive. And it would also suffer from bad vignetting and poor edge sharpness at the wide end. Not a good trade off for an extra fraction of a stop, half of the time.