Constant F-Stop

[newgenphoto]

I can't believe I am asking this but I have too because I'm just not educated in L Glass or lenses. Is it true that the 24-70 L can be set to have F2.8 through the entire focal length? I only have kit lenses but I need to upgrade quick to keep up with the "jones" if you know what I mean.

[cactusclay]

Yes it's true.

[newgenphoto]

Hmmm... how does it work then? I mean, can you just slap on a 24-70L and set your f-stop to 2.8 and shoot away through the focal length or is it a setting on the lens? I know 2.8 isn't perfect for everything but it sure would've been nice a month ago shooting an indoor basketball game. I couldn't get below 3.5 and of course it was'nt constant so I couldn't zoom, etc... I definatley to upgrade now.

[robertwgross]

By Constant f/2.8, what is meant is that the Wide Open aperture is f/2.8 from short to long. The camera can still select f/2.8 or f/11 or anything else.

Other common lenses have a Wide Open aperture that varies somewhat from short to long, but still the camera can select f/2.8 or f/11 or anything else.

It does not mean that the aperture is permanently stuck at f/2.8.

---Bob Gross---

[newgenphoto]

Thats waht I thought. So if I mount the 24-70L on my camera, I just set my apeture at whatever I want on the camera and shoot regardless of my FL? If I want 2.8 @ 70mm then the camera won't automatically change like it does with a kit lens? I always shoot manual but it's a pain with kit lenses.

I'm paid as a photographer but I sound so ameture right now... I'm cracking up here! I just don't know about the high end lenses. I've been lucky so far with kit stuff I guess.

[robertwgross]

Yes, you can dial up f/2.8 for the short end or else f/2.8 for the long end.

A more common lens might be f/3.5 at the short end and then go to f/5.6 at the long end.

---Bob Gross---

[newgenphoto]

Exactly Bob,
Thats what happens all the time... can you say "frustration"???
Anyway, thanks again. I'm getting ready to spend some money here and just want to make sure I'm heading the right direction. I'm going to get a Mark II with the 24-70L. Great combo? I do mostly wedding/portrait stuff but I think this lens will cover me through a broad amount of work.

[RAitch]

Here's a question that may or may not be related.

Can you zoom in as tight as possible, manually focus the picture, than zoom out wider and expect the same focal point? Or will that change depending on the focal length?

Can you do that with a fixed FL lens... or can you do it with any lens?

[J Rabin]

If I understand RAitch question, I think he/she is asking if the 24-70 is a "parafocal" lens, meaning once you achieve focus at one focal lens, it stays in focus at other zoomed legths. Yes, I believe the 24-70 is parafocal. Many zoom lenses are today. Jack

[CyberDyneSystems]

Raitch,..

I have not put this to the test .. but my understanding in theory is that constant aperture zooms are by nature "parafocal"... and the reason that normal zooms are not, is because of the variable aperture.

I could have made that up though... :rolleyes:
But it's not a lie if you belive it's true.

[rent]

hi jack, i think the term you are referring to is parfocal, no parafocal.

as for which ef lenses are parfocal... i vaguely remember reading something chuck westfall of canon wrote. he stated that parfocal lens are limited to the L-series fixed aperture zooms and front-focusing zooms.

-alex

If I understand RAitch question, I think he/she is asking if the 24-70 is a "parafocal" lens, meaning once you achieve focus at one focal lens, it stays in focus at other zoomed legths. Yes, I believe the 24-70 is parafocal. Many zoom lenses are today. Jack

[Jesper]

I have the 17-40 f/4 L, which is also supposed to be a parfocal lens.

However, in my experience it's not perfectly parfocal - if I zoom in, focus manually and zoom out again, the image is not optimally sharp. So I wouldn't use this as a focussing technique (at least not with my 17-40).

[mbze430]

I can say that with my 24-70 it is parfocal.

Here is a list of Parfocal for Canon EF
http://www.rogercavanagh.com/helpinfo/30_parfocal.stm

[RAitch]

Sweet. I wasn't talking specifically about the 24-70 but just lenses in general.
If it had something to do with aperture, I couldn't figure out how it wouldn't work on a variable aperture lens.... assuming you had it closed enough to cover both ends.

I was wondering about the kit lens and the 70-200. thanks for the response.