constant f2.8 - why is it important?

Sorry, but I have to ask a beginner question-

I see that everyone likes and reccomends the zooms that are a constant f2.8., and all the expensive zooms have this ability.

Why? How does is affect picutres taken outdoors or with a flash if you are going the use a smaller aperture anyway?

I want to but a good starter lens for my 20d, maybe the tamron 28-75 but I want a little more zoom, so I am considering the canon 28-135, but this does not have constant 2.8. - what capabilty will I loose with the canon?

It lets you shoot at lower ISO, Faster Shutter speeds, more possible handheld shots, more control of depth of field, build bigger biceps and makes you feel like more of a man (or woman).

It can be the difference between whether or not you have to bring a tripod with you (depending on the lens and situation).

It's all about options and convenience.
You May or may not need the options at this time.... but you may in the future.

Many people like shooting in available light (no flash), they may well need 2.8 for a certain condition (overcast day, low light in doors) and if they can't get that when they zoom out then they may well miss a shot because the aperture becomes too small. Flash is not going to light up a landscape, apertures going to do that

For that reason i'd never have a zoom lens that does not have a constant aperture.

well actually i have the kit lens (always forget about that) so i should say i'd never buy another lens that never had it

Zoom lenses that have variable aperature (non-constant) will change with change in focal length. Eg, 35-135 f/4.0-5.6, at 35mm the aperature will be f/4.0 and at 135mm the aperature will be f/5.6. Because of the change in the aperature, more light is needed or slower shutter speed for correct exposure.

That is not accurate. Some of us like our Bokeh and the larger aperature provides that. The other situation is when flash is not possible,eg church weddings.

One good reason for the constant aperture lens is so that when you zoom you don't have to change (or concern yourself with changes to) the exposure settings on the camera. For somebody who's under pressure to produce with the camera - such as at weddings, news events, or sports - this is definitely a plus.

Including what they said above, it also helps you blur the background and get rid of the distractions.

http://img99.echo.cx/i​mg99/3999/crw2549rj3fj​.jpg

Of course, the more you fill the frame with the subject, the more it'll blur out the bg.

grego - is this photo shot at 2.8?

Compare the image if you took it with 2 lenses, a 28-70 2.8 shot at 70mm, f5.6 and the same scene shot with a 28-135 f4-5.6 shot at 70mm f5.6 - will the picutre look the same?

You will also get a brighter viewfinder image, and usually better AF, especially in low light with lenses that have a smaller f-stop.
So a constant F2.8 will give these benefits, as opposed to a lens with a variable aperture, or a constant, slower aperture.

Fixed aperture designs are kind of traditional for professional lenses.The main reason they started making them like that is so studio photographers could set the aperture and not have to worry about resetting the (complicated) lighting when they zoomed in and out.

They could design professional lenses to have a variable aperture but it doesn't cost a whole lot more to make them with constant aperture instead.Or to put it another way--adding this design feature to an allready expensive lens doesn't change the apparent price much.If they did the same to cheap consumer lenses the difference in price WOULD be noticable

I might be nice to have a 70-200 F1.4-F2.8L IS.

(I guessed the wide open aperture).

Yes, and I was pretty far away. That was my first time covering baseball for the newspaper. As I got more comfy(during the season), i moved in closer. But that was from the entrace(where the fence is, which is considerably far, considering the wide foul lines).

At the same distance, you wouldn't have the same amount of blur with a smaller aperture.

I can only imagine the chromatic aberrations at the wide end.

The constant aperture lenses are professional glass. They are *(supprosed to be)* shart wide open as well as stopped down and that's usually the case with the L glass. To get sharp, consumer lenses have to be stopped down to f/8

f/2.8 will let in 8 times more light than the f/8... it will also blur the background MUCH better.

And you also feel like da man, having that big heavy lens hang off your neck all day

DocFrankenstein wrote:
And you also feel like da man, having that big heavy lens hang off your neck all day

You do get a lot of questions and comments from people, I must say. So if you like attention, the lens is defintely an attention getter.

If you want attention, try using a flash bracket and a LightSphere II. Everyone's a comedian when that thing comes out. Use it at the same time as the 70-200 F2.8 and you want have to say "look at me for the photo", everyone'll be looking already.