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Old Jul 9, 2006, 2:15 PM   #11
NHL
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DMW wrote:
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Maybe "One shot" will help.
There's a fundamental difference between 1-shot AF and the AI modes:
o In One-shot AF, the shutter won't release unless at least one of the AF point has locked (i.e. focus priority)
o In the AI modes the shutter will release even before the AF has locked (i.e. shutter prioruty) and in the subsequent shots the camera will try to focus as best as it could to maintain the frame rate (i.e. focus priority but with a timeout)

-> Try it: lock yourself in a darkroom where the camera can't focus - In One-shot AF, the camera won't release the shutter. In the AI modes the camera fire away regardless...
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Old Jul 9, 2006, 2:24 PM   #12
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I don't see an MTF chart for at http://www.photodo.com

But, looking at user performance surveys at http://www.photozone.de , users of this lens rate it's peformance as Poor on the wide angle end at wide open apertures, and on Poor the long end at wide open apertures.

So, you're probably better off using it stopped down a bit from wide open if you need the best results. Most lenses are sharper two or three stops down from wide open anyway.

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Old Jul 9, 2006, 2:31 PM   #13
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NHL wrote:
Quote:
DMW wrote:
Quote:
Maybe "One shot" will help.
There's a fundamental difference between 1-shot AF and the AI modes:
o In One-shot AF, the shutter won't release unless at least one of the AF point has locked (i.e. focus priority)
o In the AI modes the shutter will release even before the AF has locked (i.e. shutter prioruty) and in the subsequent shots the camera will try to focus as best as it could to maintain the frame rate (i.e. focus priority but with a timeout)

-> Try it: lock yourself in a darkroom where the camera can't focus - In One-shot AF, the camera won't release the shutter. In the AI modes the camera fire away regardless...

I misunderstood the meaning of AI Focus, I thought that AI Focus meant the camera would use One Shot AF unless it detected the object moving closer or further away at which point the camera would switch to AI Servo.


Quote:
I don't see an MTF chart for at http://www.photodo.com

But, looking at user performance surveys at http://www.photozone.de , users of this lens rate it's peformance as Poor on the wide angle end at wide open apertures, and on Poor the long end at wide open apertures.

So, you're probably better off using it stopped down a bit from wide open if you need the best results. Most lenses are sharper two or three stops down from wide open anyway.
I am not surprised by poor results but for it not to focus while on a tripod and shooting a stationary object that made me think it was faulty. I am still confused as to why the focus points are not in focus on the Harley shot. :?
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Old Jul 9, 2006, 3:05 PM   #14
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I am not surprised by poor results but for it not to focus while on a tripod and shooting a stationary object that made me think it was faulty. I am still confused as to why the focus points are not in focus on the Harley shot. :?
It's probably just that soft at wide open apertures.

That's what users mean by "Performance" in the surveys. It's just a soft lens at wide open apertures.

Looking through some comments from users, I saw one user that said it wasn't usable at 24mm and f/2.8, and another saying it was soft at 24mm at wider apertures, and another saying it was even still a little soft at 135mm at wider apertures.

But, most comments seem to warn against trying to use it wide open on the wide end of the lens.

I'd suggest using it stopped down some from wide open. I'd probably try to stop down to f/5.6 to f/8 on the wide end, or around f/8 to f/11 on the long end. That should improve sharpness considerably.

Basically, you're trying to use the lens where it appears to be softest (on the 24mm end at wider apertures). Most lenses are not going to be tack sharp at all apertures, and this one rates at bit worse than most at wider apertures according to users, especially at wider focal lengths.



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Old Jul 12, 2006, 1:24 PM   #15
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Update:

You may want to try another lens if your vendor will swap it out (or send to Sigma and let them look at it),

I have seen a lot of comments on this lens being soft at wide open apertures. But, a KM DSLR owner on another forum just posted some examples from one wide open at f/2.8 on the wide end at 24mm, and they look just fine to my eyes (nowhere near as soft as your lens).


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Old Jul 18, 2006, 8:18 PM   #16
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I have made similar comments about a Sigma lens: the 24-70mm f/2.8 version. Long story short, it was so blurry at f/2.8 (especially at or around 24mm) that I could not even entertain the notion of using it at that aperture. Things improved by f/5.6, but jeez, I didn't buy a 2.8 lens to be able to use it only at 5.6! The fact that my little plastic Canon 18-55mm f/3.5-5.6 lense BLEW IT OUT OF THE WATER, clarity-wise, just threw salt in the wound. Maybe the Sigma was a defect, but I did want to waste another trip the the camera shop. I returned it and instead purchased a Canon 24-105mm f/4 IS USM, and haven't looked back. Sure, it's only an f/4, but I can actually USE it at that aperture.


JimC wrote:
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Quote:
I am not surprised by poor results but for it not to focus while on a tripod and shooting a stationary object that made me think it was faulty. I am still confused as to why the focus points are not in focus on the Harley shot. :?
It's probably just that soft at wide open apertures.

That's what users mean by "Performance" in the surveys. It's just a soft lens at wide open apertures.

Looking through some comments from users, I saw one user that said it wasn't usable at 24mm and f/2.8, and another saying it was soft at 24mm at wider apertures, and another saying it was even still a little soft at 135mm at wider apertures.

But, most comments seem to warn against trying to use it wide open on the wide end of the lens.

I'd suggest using it stopped down some from wide open. I'd probably try to stop down to f/5.6 to f/8 on the wide end, or around f/8 to f/11 on the long end. That should improve sharpness considerably.

Basically, you're trying to use the lens where it appears to be softest (on the 24mm end at wider apertures). Most lenses are not going to be tack sharp at all apertures, and this one rates at bit worse than most at wider apertures according to users, especially at wider focal lengths.


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Old Jul 19, 2006, 4:22 AM   #17
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Well, more tests have proved that I cannot use this lens at 2.8. Removing the Skylight filter helped but the entire shot is still out of focus when the lens is wide open.

Now I don't know what to do, the lens performs fine when stopped down slightly, it does have a fantastic range and it feels nice on the camera. I have called Sigma and they want me to send them the lens and the body but I can't afford to be without my camera while they sort it out.

After more hunting on the web I see many people that claim to have the same problem with this lens, some mention Sigma 're-chipping' the lens so I will give them a call and see if thats an option.

Thanks for your replies.
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