At 18mm
Very sharp at the center (10mm radius. think of the circle on your viewfinder). Past that, it is sharp, though some objects may not be as much because only the Sagital (S) lines are sharp, not the Meridional (M). Not good for landscape wide open. At f8, There is a minor improvement in the corners. It's basically similar wide open at the corners. It is excellent at f8 in the center, even up to 20mm radius from the center (or 8mm on the graph). Past that, it is soft though some shots will appear sharp if the lines align with the Sagital (S) line which is very, very high.
Contrast, the same as resolution at the 10mm radius or (5mm from the center) excellent wide open. Same call as resolution. Amazingly, it is still contrasty wide open even in the corners! In fact, at f8 just improved a bit in contrast. It is above board in contrast even in the corners! And this will help give the illusion of resolution making the corners/edges of the images appear sharp from a distance.
18mm conclusion - Excellent contrast wide open, and more so at f8! Very good and above usable at f3.5 in resolution, but the edges and corners aren't as good. It's not bad, and in terms of resolution, if we compare it with other lenses, it is on par or maybe even better as far as resolutions go.
Let's see if it vignettes or has severe barrel distortions though!
At 200mm
Wide open (f5.6) at 10mm radius (5mm on the chart), it is very sharp (excellent). Very sharp and very good maybe up to 16mm radius from center (or 8mm on the chart). Corners can be terribly soft. Not good if you want to have corner-to-corner sharpness. This is nice for portraits of head and you want the corners to be soft anyway. But not good for copy work where corner sharpness is required.
Contrast is very high though, whether at f5.6 or f8. They are about the same. This lens is very contrasty.
Overall conclusions based only on the charts:
Very high resolution in the center. Very contrasty all over the frame even wide open at all focal lengths, most espcially in the long end. This high contrast will give the impression of sharpness. And the high S values will add to the resolutions for some types of subjects. Very is very good to almost Excellent performance overall for this class of lens. It will be a surprise! Just remember, this is not the 55-250 IS or other lenses. don't expect it to be on par with the 17-55 IS, though I think, it will be very close for the same apertures.
I suspect some severe barel distortions at 18mm maybe up to 24-26mm. Some moderate pincussion at the 200mm, but MTF charts don't show this really. I think this lens is best at 40-90mm though I have no data to support this because there is no MTF chart for these FLs. This is just based on experience and based on how it performs at 18mm and 200mm. It didn't falter at the 200mm from 18mm. It was even better at the long end. The center rez also held from 18mm to 200mm. So, I can only guess that this lens will be at its best at around 35/40mm to maybe 90mm-100mm even wide open with a good balance of center sharpness and contrast and even at the corners. But only at 35-90mm. Again, I repeat, I am guessing here only based on experience. Photozone tests and other sites will verify if this "guess" is correct.
Also, the lens uses a 6 blade curved design, w/c the 55-250 IS uses. This means, that the bokeh will be greatly improved, and the specular highlights will be round, not 6 sided. In fact, for a 6-bladed design, it may be on par with regular blades of 8-blades. They should start using this curved blades in all their future lenses as it does render bokeh nicely. I would gamble again here, I think the bokeh will be creamy. No, not prime creamy, but for it's aperture and it's zoom range, it will be a surprise! Why do I say that? Because the 1-S & M lines are very close. 2-the use of the curved blades 3-experience with the 55-250 IS bokeh (I have this lens). For sure, the specular highlights of this lens will be round, and not a geometric figure!
My final words, and I am staking my reputation here, barring any other issues (as listed below), this lens will not be a mediocre lens. This will be in line with the new ef-s lines we are seeing. What it means is that it will not disappoint and its sharpness will be in the same breath as the new 18-55 IS and the surprising 55-250 IS. But the difference is that this lens will be more contrasty. And it will show. This probably is due to the 2 CA glass and the 2 UD glass (w/c is the reason for the high contrast and center resolution).
A shame it is not ring USM, but this will be one smokin' lens! I don't want to offend the
Nikon 18-200 vr, but darn, this one is 2 steps way above the VR version! It is closer to the tamron 18-250 di-2, though I suspect, that this lens will probably be crowned the king of the 18-200 range for crop bodies as it will likely unseat the tamron, and neuter the advantage of OS of the sigma 18-200 OS!
Well, the tamron 18-270mm VC could probably dethrone it, but then again, that lens will be out end of the year yet.
Also keep these in mind, the MTF charts don't measure or show the following:
- CA
- vignetting
- flare resistance
- barrel and/or pincussion distortions
- actual bokeh in real life
- AF speed
- AF hunting
- AF accuracy
- handling
- Efficacy of the IS
- No measurements of f-stop transitions (e.g what is the f-stop reading at 35mm, 50mm or 100mm) across the FLs.
Also, we only see the 18mm and 200mm FLs. We don't know the MTF somewhere in-between. So we do not know how it really does at 35mm, 50mm, 80mm, 100mm, 135mm, 150mm.
Finally, remember, to compare this to an EF lens, read the EF lens only up to the 15mm mark as this is a lens for a crop body.
So that's my mini-review based on the MTF charts only! Real world samples and experience will verify if these readings are correct!
Sigma 18-200mm f3.5-6.3 DC OS vs Canon EF-S 18-200mm f/3.5-5.6 IS
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