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01/03/2008 12:18:33 PM · #1 |
My Canon 70-200 2.8 IS is giving me a headache. When I shoot, I zoom all the way in to focus, zoom out and re-compose... So you would think the pictures would be pretty sharp. However! Come to find out that when I zoom out, even though the shutter button remains pressed halfway down, the lens falls out of focus. I've tested and re-tested this... It's not an issue with slow shutter speeds; it's definitely an issue with the focusing of the lens. Has anyone else had this? I wondered if it's a common problem or if it's just an issue with this particular lens.
Thanks!
-Kim |
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01/03/2008 12:22:20 PM · #2 |
When you change the zoom you're going to need to refocus unless you're in one of the tracking modes.
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01/03/2008 12:23:55 PM · #3 |
| there's really no need to zoom in for focussing. compose how you want the final image to look, then focus and shoot. not only will this save you time, it will ensure that you are indeed in focus, since zooming will change the focus anyway. |
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01/03/2008 12:48:46 PM · #4 |
It's of course an ideal situation of you can mount to a tripod, zoom in to get critical focus, then zoom out to compose. A lens that can do this, maintain its focus throughout the zoom range, is called a "parfocal" lens. Unfortunately, most parfocal lenses don't seem to be that precise. According to Canon (as of 2003) the following lenses in their lineup are parfocal zooms:
Current Lenses (as of 2003)
* EF 16-35mm f/2.8L USM
* EF 17-40mm f/4L USM
* EF 24-70mm f/2.8L USM
* EF 70-200mm f/2.8L USM
* EF 70-200mm f/2.8L IS USM
* EF 70-200mm f/4L USM
* EF 75-300mm f/4-5.6 IS USM
* EF 75-300mm f/4-5.6 III USM
* EF 75-300mm f/4-5.6 III
* EF 90-300mm f/4.5-5.6 USM
* EF 90-300mm f/4.5-5.6
Discontinued Lenses
* EF 17-35mm f/2.8L USM
* EF 20-35mm f/2.8L
* EF 28-70mm f/2.8L USM
* EF 28-80mm f/2.8-4L USM
* EF 50-200mm f/3.5-4.5 USM
* EF 50-200mm f/3.5-4.5
* EF 70-210mm f/4
* EF 75-300mm f/4-5.6 II USM
* EF 75-300mm f/4-5.6 II
* EF 75-300mm f/4-5.6 USM
* EF 75-300mm f/4-5.6
* EF 80-200mm f/2.8L
* EF 100-300mm f/5.6L
* EF 100-300mm f/5.6
That's the most recent list a quick search dug up.
That said, I have the 70-200 f/4L and it does NOT work especially well as a parfocal, so... Maybe  Kirbic has more info?
R.
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01/03/2008 02:14:55 PM · #5 |
Well, it's a relief to know that this isn't something I've done to the lens by dropping it or banging it against something. I am happy it's a known equipment issue although I really did think that I could expect the lens to maintain the focus point when I zoomed out. Thanks, Robert, for the research and the list of lenses; and thanks all of you for the information.
It was just driving me nuts, that I *should* have a tack-sharp image of a bride and groom kissing, and what I'd end up with was a tack sharp image of the altar behind the kissing bride and groom :)
ETA: Now I can work around the problem!
Message edited by author 2008-01-03 14:15:55. |
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01/03/2008 02:42:57 PM · #6 |
As Robert stated, "parfocal" lenses are made specifically to retain focus throughout the zoom range. This is one of the "hidden" features (good or bad) that aren't really advertised greatly.
Also, another "hidden" feature is when the front of the lens does not rotate during focusing. This is desirable (non-rotation) when using filters such as a Graduated Neutral Density filter. Otherwise, after focusing, you'd have to manually rotate your GND filter, which many GND filters do not allow. I get around this with one of my lenses by using the Cokin Filter System.
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