Hi Freak,
I advise you to read some of the lens tests.
You've got a very fine high res camera, you will see a lot of sharpness difference between different lenses.
A good place to start with is:
Camera lens tests, user reviews, camera accessory reviews - SLRgear.com!
You'll see that most Sigma long zooms suffer from lack of sharpness at the long end.
Quote:
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Sharpness
The Bigma offers a mixed bag of performance; for the most part, between 50mm and 200mm, the lens offers respectably sharp images, with a few notable and problematic focal length / aperture combinations. Above 200mm, image sharpness degrades noticeably. I suspect our sample of the lens may have a slight issue with one or more de-centered elements, owing to the curious performance we occasionally found.
At 50mm, the lens performed admirably well on our D200 test body, showing an average of 2 blur units, skewed slightly towards the top of the frame. Stopping down the lens at this focal length improves image sharpness, until it reaches its optimal aperture of ƒ/8 where we note 1.5 blur units across the frame. Even fully-stopped down at ƒ/22 images didn't exceed 2 blur units across the frame.
Of course, you don't buy a lens like this to use solely at 50mm. The lens has a curious speed bump of performance when set to 100mm, where we note excessive corner softness wide open at ƒ/5 (specifically, the right-hand corner) - in the order of 8 blur units. This corner softness actually gets worse when stopped down to ƒ/5.6, then improves as the lens is stopped down further. This result may have something to do with the lens' configurable use with teleconverters, which can only be used with the lens locked to a minimum focal length of 100mm.
Wide open at 200mm we see some excellent results for sharpness - around 2 blur units across the frame - even though by this point we're reaching a comparatively slow minimum aperture of ƒ/6. Things here get a little better stopped down to ƒ/8 - a little better than 2 blur units - and a bit better still at ƒ/11.
Wide open at the 300mm mark (ƒ/6.3) we note significant corner softness, with one corner off the charts, and average overall image sharpness (around 3-4 blur units on average). It's marginally improved with wide open performance at 400mm and 500mm, but not much better. In all these cases stopping down the lens to ƒ/8 does little to improve sharpness, one needs to stop down to at least ƒ/11 if not ƒ/16 to get the sharpest results from the lens at these focal lengths. At these settings you'll see results of 2-3 blur units.
Performance fully stopped down is fairly good below 200mm, where the smallest aperture is ƒ/22-29. Smaller than that, and at longer focal lengths, the images produced become very soft - between 4-5 blur units across the frame.
Results for image sharpness are almost identical on the full-frame D3x, with perhaps slightly more exaggerated corner softness above 200mm.
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End Quote.
Also, you need to consider the weigth.
Other good options are: DA* 60-250mm + 1.7TC or the DA* 300mm.
The 300mm is very sharp! You will appreciate that.
Combined with a Tamron 90mm Macro you will be low weigth and versatile with very good IQ.
I did a lot of pixel peeping on the Internet and decided not to go for the Bigma.
I have the DA*60-250 + TC now, and it's very good stuff.
Here a quote on the DA* 60-250:
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Sharpness
Wide-open performance for the Pentax 60-250mm is very good generally, and between 80-200mm, excellent. Our copy of this lens was slightly de-centered towards the bottom of the frame, evident at ƒ/4. At 60mm and 250mm we note some corner softness, but the sweet spot of sharpness is fairly generous at 60mm. At 250mm at ƒ/4 the lens doesn't provide tack-sharp results, but it's still exceptional: just above 2 blur units.
Between 80 and 200mm, performance is excellent at ƒ/4 - around 1.5 blur units across the frame. Stopping down to ƒ/5.6 provides tack-sharp results at 1 blur unit, and there is a similar increase in sharpness at 60mm and 250mm as well. At ƒ/8 the lens is essentially as sharp as our tests can note, at around the 1 blur unit mark (there is some statistically relevant corner softness at 60mm and 250mm, but barely worth noting).
Diffraction limiting sets in at ƒ/11, but the effect is negligible until ƒ/22 where overall sharpness begins to surpass the 2 blur unit level across all focal lengths. I'd avoid using the lens fully-stopped-down at ƒ/32, where image sharpness becomes somewhat uneven and hovers between 3 and 4 blur units.
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The Sigma 70-200mm looks also Ok, but gives in a lot of speed and some IQ with the Sigma 2x TC.
A DSLR does a lot less sharpning with its in camera processing than your FZ30, so your pictures may look rather dull with standard out of the camera JPEG files.
You should either up the sharpness in the camera (see also firmware upgrade for the K-7 for additional settings), or shoot RAW and do this in Post Processing.
Whatever you decide, show us some picture when you've got your new lens!
- Bert