I used the Sigma 18-250mm when preparing the article ‘Crop or Cram‘, partly to see how it held up in comparison to the 70-300mm SSM G and the Sigma 70-200mm f/2.8 HSM. Here’s one the shots from the Alpha 700, at 210mm and f/8, showing that good geometry is important even when straight lines are only incidental to the subject:
And here is one at full 250mm from the Alpha 380:
These have been processed with CA correction, the one aberration which the 18-250mm shows. It is not a strong CA colour fringe, and it is also very sharp, meaning that once you hit the right correction values using Adobe Camera Raw, Lightroom or any other CA-correcting process (including Photoshop>Distortion>Lens Correction filter) the effect disappears entirely leaving a sharp image. Clicking either shot will open a pBase option including a full size version and EXIF data. This is a direct URL for the full size 14.2 megapixel shot above:
I suggest you take a look at this regardless of bandwidth. It’s complimentary to the abilities of the new Alpha 380, and even more so to the sharpness of the Sigma. It’s an action shot and even at 1/1000th you can see the effects of motion blur, but if you know how to read image sharpness, it’s easy to tell from focus error or lens aberration. The Sigma 18-250mm has one quality which our Sony 18-250mm lacks, and that is dead-accurate focusing, no FF or BF on any of the bodies we have tested it on. This is just as well, as the view through the Alpha 380 finder is so small and becomes visibly dark as you zoom to 250mm. Manual focus would never be an option on this camera.
How about 18mm? Well, the good news is that the lens shows excellent geometry again, and of more note is the absence of mechanical vignetting to the corners. As owners of the Carl Zeiss 16-80mm, or the Sony 16-105mm, or the Sony 18-200mm and 18-250mm lenses will already know it’s not unusual for a dark corner to appear. Sometimes this is consistent, other times it seems to be a result of the SSS tracking the image to a darker place. I certainly know that I end up cropping the height of most shots on our 16-105mm taken at 16mm because the top right corner vignettes.
The huge 72mm front glass of the Sigma, and its overall generous optical design, seem intended to remove vignetting – probably a necessity with the OS mechanism. The lens must have extra coverage to allow the moving element/group to do its work without losing resolution or brightness. Consequently, this lens (like the even bigger Canon 18-200mm f/3.5-5.6 IS design) has been made to a different standard than non-stabilised superzooms.
Here you can see 18mm on the Alpha 350. Some CA correction improved the shot, but this was just -18 Red/Cyan, compared to a typical value of -30 Red/Cyan needed by nearly all other lenses at 18mm. You can see the geometry – just a hint of barrel distortion, but not enough to make those flagpoles look wrong. Depth of field at f/11 is just right for the 14.2 megapixel resolution, without diffraction sharpness loss setting in as it does at f/16 and f/22.
Finally, a shot which is taken under all the wrong conditions. Wide open, 18mm, by natural light at 1/8th, hand held, with the photographer crouching on the top of a desk opposite the bookcase – a very unstable shooting position, ducking down to get the shot in below a chandelier and avoiding treading on my Mac wireless keyboard after hearing it crunch under my heel once… it took a few shots to get the focus right, the light level at ISO 320 on the A700 was fairly low, and the natural movement of the shooting stance meant firing very quickly after securing focus. Otherwise, as with many shooting postures, the camera could move two or three inches fore and back with just the stabilising movement of remaining balanced.
You can see that at close range, the 18mm barrel distortion is more visible – this is always the case, and a good reason why test charts should never be used to test distortion with wide-angles. Of course, the weight on those shelves is responsible for some of the curvature otherwise the distortion would be of a type never seen before in lenses. American Red Oak is a good choice for bookcases but after 20 years the shevles could do with reversing!
There is hardly any vignetting (the window light is more responsible for the brighter centre of the image, along with the coincidence of some lighter coloured book spines). I have not corrected CA in this image at all, and you can view the full size version by clicking the image for the pBase page, or directly here:
Considering the conditions of shooting, the OS has worked well and the lens shows good sharness corner to corner. Every book title is legible right down to the old copy of ‘The Family of Man’ lying flat on top of the centre bottom shelf.