constant f2.8 - why is it important?

DocFrankenstein wrote:
I can only imagine the chromatic aberrations at the wide end.

What about the price? Somehow CA can be handled, but the price ?

It took a long time after zoom lens was introduce before studio photographer accept them because of image quality not because of the variable aperature.

As far as cost I can't argue on the point of R&D and Manufacturing, but market cost is a different story.
1. A EF 75-300 f/4-5.6 IS USM goes for $400
2. A EF 70-200L f/4 USM goes for $600
3. A EF 70-200L f/2.8 USM goes for $1200
4. A EF 70-200L f/2.8 IS USM goes for $1600

If #1 is an inexpensive lens and #2 is a relatively expensive lens and #4 is very expensive lens. Can you explain how the cost increase by $1200??? and how is it not noticable???

I think what's meant is that you'd notice the increase in price of the consumer zooms...

The problem is that physics has it's laws and R&D guys have their specs and limitations.

Imagine building a 70-200 and you're constrained by:

Maximum diameter of the back element - EF mount constraint
Mirror box - can't be too close to the film
Weight - it has to be handholable
Front element can't move, can't rotate
Has to be internal focusing
For focusing, you can't have lots of glass moving inside, it'll slow down the AF speed
You have to fit in a motor somewhere
The lens has to be sharp
The cost should not be astronomical
The manufacturing process should be as simple as possible...

I'm sure there are tons more limitations. I'm surprised 70-200 work as good as they do.

And you can't really make the lenses go any faster than they can. IE if you have 200/2.8 at the long end... you theoretically have enough light entering the lens to make it 70/1.0

And that's just not happening... not if we want to cover full frame sensors.

Minolta or whatever, managed to make an f/2.0 zoom recently and there was some fuss about that. But what's the point?

When your system is to cut out "four thirds" and leave only a quarter of the chip, you can come up with something... but you're gonna have tradeoffs.

Hi snapper27

Not all expensive zooms have this ability, just have a look at the EF100-400 F4.5-5.6 IS L, most people who have used it will rave about it. yet it has a variable aperture depending on the zoom level and its not the cheapest of lenses

Smaller apertures will cause the depth of field to decrease making objects close to you subject to get blurrier quicker.

The 28-135 f3.5-5.6 IS L is a very good lens, it also has image stabilistaion which allows you to hand hold the lens at slower shutter speeds (you should have your shutter speed be at least 1/zoom level of the lens (eg a 200mm lens shouldn't use a shutter speed slower than 1/200 or you might get camera shake)) so instead of having 1/30 as your slowest shutter speed with this lens you get two extra stops with the IS and can hand hold it at (28/(2*2)=7 which is roughly 1/8 of a second).

All the best

BearSummer

I never spoke of Bokeh that is correct, but i did mention where a
constant aperture when zooming may well be needed, like indoors in low light (i spoke of available light) and over cast days. Reasons just as valid as Bokeh i have to say, so i'd not say incorrect just yet another reasons why constant aperture is a good idea

It does sounds like a GREAT lens doesn't it?!
Unfortunately i don't think it works quite like that but i can't explain why at the moment .I vaguely remember it's something to do with the constant aperture pro lenses allready using the front element to it's maximum potential... so that if the design was changed to be a variable zoom of (about) f1.4 at 70mm to f2.8 at 200mm then the front element size (at least) would have to be scaled up.
...Or i could be talking utter rubbish .I'm abit off tonight so i can't think very well

It would be facinating to ask someone that knows about these things

ron chappel wrote:
It does sounds like a GREAT lens doesn't it?!
Unfortunately i don't think it works quite like that but i can't explain why at the moment .I vaguely remember it's something to do with the constant aperture pro lenses allready using the front element to it's maximum potential... so that if the design was changed to be a variable zoom of (about) f1.4 at 70mm to f2.8 at 200mm then the front element size (at least) would have to be scaled up.
...Or i could be talking utter rubbish .I'm abit off tonight so i can't think very well

It would be facinating to ask someone that knows about these things

I asked this very same question to a pro-photog who has been shooting over 40 years. His response was very similar to yours. In order to obtain that type of setup in a lens, the front and middle elements would need to be SUBSTANTIALLLY larger, and of the highest quality. Therefore, one would end up with a lens that looks like a large telescope and would weigh in at around 8 pounds with glass that would rival a NASA Observatory Telescope tracking asteroids. That would put the price somewhere in the range of $8-10,000 for that lens. Ummm... personally, I think it would be easier to buy the primes with the large aperatures and deal with swapping lenses. But hey, that's just me. I could use the extra 10-grand to buy the rest of the L-glass in the lineup and have enough for a second 20D body as a backup. Of course, this is all in theory, and includes Canon's mail-in rebates!

Would be a neat lens though!
-GSL

Oh.... BTW..... a STOBOFRAME with a 580EX on it causes INSTANT attention.... no need for white lenses. At almost every ball-game I shoot, I get the same questions from players and fans alike.... "what paper are you from?" or "are you the league photographer?"

I've gotten more jobs this way, with the flash being in the OFF position! Just have it mounted.... people do get awestruck from silly frames and bulky flashes. I also notice a lot of fans looking at their P&S or DRebels and thinking... "hey... howecome my camera doesn't look like that?" If they only knew that $70 gets them that "pro-look".... but that's why they all use their built-in flashes and get "snapshots" and not professionally done photos. But that's our secret..... unless they find this forum that is...
-GSL

The 20D has a full cross-type AF (for apertures down to f5.6) at the centre which has a high precision mode available, but only when using a lens with a max aperture of 2.8 or better. So a lens like the 28-135 will give good focusing performance, but not as good as a f2.8 lens.

If you want to use the high precision mode of the centre sensor on the 20D then you need the f2.8 lenses, but the AF works pretty well even when not in high precision mode.