Originally posted by Rondec As to whether or not your 60-250 could actually be a variable f1.2 - f4 zoom, it seems unlikely, regardless of the size of the front element. Seems like the image quality would suck mightily if you let the aperture go that wide.
But , my point was, if it were a prime with that size of front element it would be 1.2. At 60 mm only a fraction of that front element is being used.
It's all about weight efficiency for me.
Another thing you seem to have not noticed, DoF is wider at the same distance with wider angle glass, so really having a smaller ƒ-stop in the long end works to keep your DoF the same.
The quality issue, I'm not even sure where that comes from. At many focal lengths at their available apertures, both the DA 18-135 and DA 16-85 out perform the DA*16-50, so I'd suggest the opposite might be true. There would seem to be more compromises in producing a fixed aperture zoom than a variable. The fact that many variable aperture zooms are cheap just means there are no bargains, well there are, the 18-55 is an incredible bargain). But look at reasonably priced variable soon like the 16-85, and the cost advantage favours variable aperture. Often, better IQ for less money.
I have no problem anyone preferring fixed aperture zooms. It's unfortunate that the reasons you use to justify that are so inaccurate. It gives the impression you are going t get better IQ from a fixed aperture zoom, and that isn't always the case. IN fact using the above lenses, you might ay if you aren't ing narrow DOF, you'll get better IQ for half the price by going variable aperture.
As for the getting caught of guard by the aperture change... that's just a function of using the lens and getting used to what it does. You make it sound like you never used a variable aperture zoom enough to get used to shooting with it. That's just part of learning your lens.
Sure it's probably easier with a fixed aperture lens. Like you might have to take the lens out 15 times instead of 10 to get used to it. But, this little thing you describe, not watching your aperture change in the viewfinder and being caught off guard or using an inappropriate setting, I can't remember it ever happening, like even once. Usually when I have an issue with aperture it's because my finger adjusted the scroll wheel while I was doing something else. I think I'm shooting ƒ8, but I'm shooting ƒ7.1. Now that's an aperture issue that's hard to deal with because I can't anticipate it. The variable aperture change when you zoom out a lens is predictable. It happens exactly the same way every time, If you're capable of learning, you're capable of dealing with it unconsciously and it becomes second nature.
It's just not something I ever think about, and I doubt anyone who has used a variable aperture lens as their main lens for a few hours does.
On the weight scale, primes are more weight efficient than zooms, variable aperture zooms are more weight efficient than fixed aperture zooms. Fixed aperture zooms are the absolute worst, if you have to carry multiple lenses in a camera bag. You can't shoot with what you can't carry. Carry a few fixed aperture zooms and you need a sherpa and a really big camera bag. Where I go, I have seen photgraphers with both. No one is ever in as bad a mood as a photographer on a portage trying to muscle a huge pelican case over a km. And I've seen photographers miss great photo opportunities because their 15-30 type zoom was so heavy they left it in camp when they went for a hike. Meanwhile I was happily snapping away with my Sigma 8-16 variable aperture lens. You can go fixed aperture, but, there are consequences.