Sigma 120-300, Zoom and keep focus?

There have been many threads out there making the claim that the Sigma 120-300 f2.8 zoom can not focus while it is being zoomed. I am posting this to clear this point up. The shots on the link below were shot at a baseball game Wednesday using the Sigma lens + 1.4 tc on a Canon 1D. There are 18 frames that start at 310mm and come down to 168mm. No post processing was done except reduction for web and watermarking. No sharpening, and obviously no exposure or color correction. I wanted to show the files in their native state. I wanted to clear the air that this lens does not just produce the occasional in focus sharp shot, but does so reliably, even while be zoomed, even with a TC attached. I normally don't shoot long sequences like this, this one just happened to fall in my lap and thought I would share. Exif information should be there. Click on images to see a larger version. Don't try this with a 20/30D... you will not get the same results.

Here is the Link

http://www.buzzershot.​com/zoom_test

I've never heard this claim, but thanks for your demonstration.
It would seem to me that with AI servo, CF4-1 and your thumb mashed on * that this should not be an issue.

I like you and all, but you're an arse! Not only are you going to be the cause of my 120-300 purchase, you're also going to make me buy a 1D! WTH! Those are great shots!

I've not heard this before either. It may not be a well known fact, but the Sigma 120-300 is not parfocal, meaning that if you focus on a subject, zoom, you will have to refocus as the focus is lost. A lens that is parfocal, will allow you to focus once, and if you zoom the focus will still be obtained.

Ok, so I can only really think of one situation where that would be all that important - that being if you are manually focusing on a subject, zoom in to get a really good view to focus with, then zoom back out to take the image. Other then that, when would this be a big deal, particularly with Autofocus and fast focusing zooms? The sigma can obviously refocus as fast as a 1d can shoot. Is there other occasions when not being parfocal will make an impact?

I agree, its not a big deal with Auto Focus cameras of today. It's not well know, but it could bite you, especially if you are one that was used to focusing your lens and then re-zooming before taking the shot.

What do you mean don't try this with a 30D you won't get the same results? Why?

chrisvl wrote in post #2718120
What do you mean don't try this with a 30D you won't get the same results? Why?

im sure he just means the 1d series give's the best out of this combo so other camera's will not be as good although they will work ok.
Rob.

This was 18 out of 18 frames in focus all shot tracking a subject. 1D class bodies do this day in and day out. Unfortunately the 20/30D have a bit harder time tracking objects and not having a couple atleast front or back focused in a sequence. Maybe on an L it would work, but not with this Sigma. AF performace does take a hit between the two bodies. I bounce between the two all the time and am very familure with their differences.