Saw this on http://www.engadget.com/2013/04/18/sigma-announces-18-35mm-f-1-8-dc-hsm/.  Sigma is coming out with an 18-35mm zoom for APS-C sized sensors with a constant aperture of f/1.8.  

If the glass is quality, this should be a really nice lens!

http://www.engadget.com/2013/04/18/sigma-announces-18-35mm-f-1-8-dc-hsm/

Woops, put the first thingie in the second thingie's place, and vice versa!

Too bad it will probably be a $2k+ lens.  I hope other manufacturers figure out how to get crazy large apertures into their zooms.  It's about time.  


I'm still waiting for Olympus to come out with something to compete with Panasonic's constant f/2.8 zoom lenses... because I'm gonna be buying one soon!

That's a maximum aperture through the zoom range, right?  Otherwise, I don't understand what kind of benefit there is to a lens with a fixed aperture.

Correct.  You can still go all the way down to f/22 (or f/32 depending on the lens).  A fixed aperture just means that the aperture functions independently of the focal length of the lens, so it doesn't change as you zoom in.  This allows you to keep a constant exposure through the full length of the zoom range on your lens.

Yep, when someone describes a zoom lens as "constant aperture", it means that actually the aperture physically opens bigger while you zoom in, so that the f-stop value is the same.  The f-stop value is a ratio of the focal length divided by the aperture size... It ends up in a number expressed as f/x.  Aperture size is f (focal length) divided by the f-stop value.  If you keep that value the same while you increase the focal length, the aperture has to get bigger.  

It's called 'constant' though, because the amount of light getting to the sensor will remain the same.  In this lens, at 18mm f/1.8, you can zoom to 35mm at f/1.8 and you don't have to change your shutter speed or ISO values.  

You'll see on the kit lenses, like the Canon 18-55mm lens, the aperture value is listed as f/3.5-5.6.  That is the maximum aperture at both ends of the zoom range.  So with a lens like that, if you set up your exposure at 18mm wide open, then zoom in, you're going to lose something like 2 full stops of light.  You'll have to increase ISO or slow your shutter down to compensate for it.

Oh I think it's really neat also but I won't have one cause like you said it's going to be expensive

Sigmas new stuff is looking good, the 30 1.4 and the new 120-300 2.8...of course money is the main issue. But Canon/Nikon need some solid third party competition.

I knew that's what you meant, but the article describes the aperture as fixed.  I just thought it seemed strange.

Thanks for that explanation as that has puzzled me for a while.



 

Great for portrait photography, but the fixed aperture would wreak havoc on wide-angle landscape work.
Or don't I understand how 'constant aperture' works with depth-of-field?


 

It's not fixed, you can still stop down.

What he said.  The "constant" part is as I described.  If you open the aperture all the way up at the 18mm end, as you zoom to the 35mm end, it opens up more keeping the f-stop at 1.8.  You can still stop it down.  It probably goes to f/22 or so.  Should work fine for landscape stuff.

So it's more properly described as a 'constant exposure' lens.  The aperture is linked to the zoom, so that one can vary the shutter speed alone to control exposure.



 

And for the majority of people that set their DSLR to the "P" setting, or use auto exposure modes, it won't make a difference except in aperture priority. There you'll be able to keep the lens wide open in order to blur out the background for portraits. Low noise sensors and auto exposure in cameras that can bump the ISO to 6400 and faster without producing excessively grainy photos sort of moves a lens like this into a spot where it will only appeal to a few types of consumers: the kind that will understand how to use it, the kind that will be happy to just buy anything expensive because it will "make their photos better," and the kind that will just buy whatever the dealer recommends (and has the highest profit margin on).
 

I'd argue that they'd all see good/better low light focusing, regardless of user skill level or settings used. But worth it for that if you leave it in dummy modes? nope.

No, "constant aperture" is the correct term.  

Apertures are always expressed as an f/ number, and the "f" refers to focal length.  A 100mm lens with the aperture set to f/2 has an entrance pupil that is 100/2, or 50mm, in diameter.  If you stop down to f/4 then the entrance pupil is 100/4, or 25mm, in diameter.  

On a simple zoom lens, if the aperture is 25mm in diameter and you zoom to 50mm then the aperture is f/2 (50 / 25) but if you zoom to 100mm then the aperture would be f/4 (100 / 25).  On a constant aperture lens the position of the aperture blades moves within the lens as you zoom so that the 'effective' aperture stays the same as you change the focal length.  

It's a remarkable bit of engineering "magic" but other than maintaining the same aperture value as you change the focal length, it performs just like any other aperture assembly and allows you to select from a range of aperture values.  Constant aperture designs are much more difficult to implement and the lenses that have this feature tend to be more expensive than other lenses, if it is available at all.  Using Canon's lenses as an example, the kit lenses with their various APS-C cameras are all variable such as the EF-S 18-55mm f/3.5-5.6 lenses, which cost a few hundred dollars of you buy one new.  These lenses have a max aperture of 3.5 when they are set to 18mm but when you zoom in to 55mm the max aperture is only f/5.6.  The kit lens with the more expensive 5D is Canon's 24-105mm f/4 lens, which has a maximum aperture of f/4 no matter what focal length you use, and the lens runs about $1000 new.

Or the very nice 17-55mm f/2.8 EF-S lens.  I had one when I had my 7D.  Very nice glass, but it was a big unit, and still goes for around $1k new.  Used ones are going for around $700 lately.  It's well worth that amount.

The bum thing about getting in to u4/3 now is that the lens economy is still kinda small.  It's a young platform, so variety isn't there yet.  It's also mostly a compact platform, not a professional one like DSLRs... so that might end up hindering the production of really stellar glass.

I'm still a bit confused, but thanks for the detailed explanation.  I will ponder.



 

Short and simple example:

If a lens says for example "70-200 F/2.8", that means at any zoom length you can open the aperture up to f/2.8.  If a lens say "70-200 f/3.5-5.6" that means throughout the zoom range, your maximum aperture will vary.  At 70mm, the maximum available will be f/3.5, at 200mm the maximum available will be f/5.6.

It can get a little confusing, but try to divorce the technical aspects from the practical ones.

Any lens at any focal length with a maximum aperture of f/2.8 will allow in the same amount of light (at that aperture setting).  You'll generally get to learn that low f-stop numbers mean big apertures and lots of light.  Large aperture lenses are called 'fast' because you can use faster shutter speeds since the large apertures allow so much light in.  Generally f/2.8 is where 'fast' begins.  f/3.5 and down are not as fast.  

Another affect of a large aperture is depth of field.  You get more shallow depth of field with larger apertures, which is another reason 'fast' glass is desirable.  


I'm to the point where I'll think:
- use the open end of the aperture selection for indoors, low light, or where I want to blur the background  (f/1.4 up to f/2.8 ish)
- use the midrange for general photos outside in decent light  (f/4 up to f/8)
- use the closed end of the aperture for scenery or where I want as much in focus as possible (f/11-f/22)

You can use these values across all lenses, since the same aperture value across all focal lengths lets in the same amount of light.

I get the basics of aperture and f-stop.  I understand how aperture affects depth-of-field.  In fact, I use aperture-priority a lot, using ISO settings and ND filters to keep control of shutter speed.



 

Cinema zoom lenses have been able to do this for quite a while now....but at that price point?  Now that is nice.

Forget that limited zoom range. I want a 14-400ƒ1.4 on a full frame.

The problem there is that on the wide end, the aperture would have to be physically 285mm in diameter.   That's 11.22 inches.  The glass would be amazingly huge, as would the housing!  

There's a reason that large aperture zooms are limited in focal length.