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Design

The 18-250mm OS is a typical upper-mid-range Sigma design, which means a solid-feeling lens with an attractive matte-black finish and smooth operation of controls. In common with other lenses it in its class, the mount is metal and the barrel is constructed from lightweight but good-quality plastics. Like all superzooms the Sigma features a 'double-trombone' design for extension of the front lens group on zooming, but unusually it also uses a similar mechanism for moving the rear group internally. The lens is very similar in size to Tamron's 18-270mm F3.5-6.3 VC (although it doesn't extend quite so far at full tele), but noticeably heavier, perhaps reflecting a greater use of metal in the barrel construction.

Three switches are placed down the side of the barrel within easy reach of the right thumb. At the top is a zoom lock switch, which is slightly better-placed than that on the Tamron 18-270mm; like the other lenses in this class, this only works at 18mm. Below it are the controls for the focus and image stabilization mechanisms; these two switches are large and easy to operate, especially in comparison to those found on the Canon and Nikon 18-200mm lenses.

Zoom action / zoom creep

Superzoom lenses, with their long extensions and heavy front elements, tend to suffer from two related ergonomic issues - uneven zoom ring actions, and 'zoom creep', i.e. a tendency to extend under their own weight when carried around. The 18-250mm scores notably well on both accounts (and certainly much better than the Tamron 18-270mm); the feel of the zoom ring is impressively even across the range, yet never excessively stiff, and our review sample didn't really suffer from zoom creep at all. Credit to Sigma here - it's a small thing, but this makes using the lens just that little bit more pleasant.

On the camera

The lens feels ideally balanced on larger DSLRs such as the 7D or 50D, but can feel a little front-heavy on smaller bodies such as the 500D range (although it's still perfectly usable). Controls are well-placed, with both the zoom ring and the manual focus ring falling readily to hand. The zoom lock, AF and OS switches are also well-placed within easy reach on the side of the lens barrel; overall the Sigma scores highly in terms of usability.

Flash shadowing

One problem often seen with relatively bulky superzooms, especially on smaller SLR bodies, is that the lens will block the built-in flash at wideangle, resulting in a shadow in the lower center of the image. We tested the Sigma on a range of Canon bodies, from the entry-level EOS 1000D to the high-end EOS 7D, and only saw any semblance of shadowing at 18mm and focus distances close to the minimum. Do note, though, that if you shoot at wideangle with the hood attached you'll see considerable shadowing.

Based on our previous experience with such lenses, however, we would expect the 18-250mm to show a degree of shadowing with some of the smallest entry-level DSLRs, on which the flash unit doesn't lift quite so high above the lens axis.

Autofocus

The 18-250mm uses Sigma's Hypersonic Motor for autofocus, and our Canon mount sample was near-silent in use, and impressively fast and decisive even at full telephoto. It's certainly very much quicker than the Tamron 18-270mm VC, and probably on a par with Canon and Nikon's own lenses (although we were unable to compare them side-by-side). This makes the lens much more capable at getting quick grab shots of moving subjects - for example children. As always, though, it must be noted that focus speed and accuracy is dependent upon a number of variables, including the camera body used, subject contrast, and light levels.

Dependence of effective focal length on focus distance

This lens's angle of view widens dramatically on focusing from infinity to 0.45m, especially at the telephoto end. This is a common trait with superzooms (the Canon and Nikon 18-200s and the Tamron 18-270mm behave in just the same way), but at a focus distance of 2m that 250mm telephoto end has an effective focal length that's closer to 180mm. In context, it is worth bearing in mind that long telephotos generally tend to used more for distant subjects, in which case the lens naturally behaves as a 'true' 250mm; and at short distances you merely have to move a little bit closer to compensate.

Lens body elements

The lens comes in versions for Canon Nikon, Pentax, Sigma and Sony DSLRs; our review sample was in the Canon EF mount.

This view shows the somewhat unusual 'double trombone' mechanism which is used for moving the rear element on zooming.
The filter thread is 72mm. It does not rotate on autofocusing, which should please filter users.
The bayonet-mount hood is provided as standard, and clicks positively into place on the front of the lens. It's made from thick plastic, and features ribbed moldings on the inside to minimize reflections of stray light into the lens.

A white dot on the outside of the hood aids alignment for mounting, and the hood reverses neatly for storage.
The zoom ring has a 32mm wide ribbed rubber grip, and rotates 80 degrees anti-clockwise from wide to telephoto (the same way as Canon lenses, but opposite to Nikon, Pentax and Sony's). The action is smooth and even, in marked contrast to Tamron's 18-270mm VC. The front element extends fully 77mm on zooming and feels impressively solid when fully extended, with just a little lateral play.
The focus ring has a 10mm-wide grip, and rotates just 45 degrees clockwise from infinity to 0.45m, matching Canon and Sony lenses but opposite to those from Pentax and Nikon. Its action is nice and smooth, but the limited travel makes precise manual focus rather tricky - especially at the long end.

A basic distance scale is marked in feet and meters. The focus ring travels slightly past the infinity position, and rotates during autofocus.
An array of switches are placed on the side of the lens barrel within easy reach of the left thumb. At the top of the barrel (towards the right of this picture) is the zoom lock, which as usual works at 18mm only. Below that sites the auto/manual focus mode selector, with the image stabilization on-off switch at the bottom.
A slightly curious scale on the outer sleeve of the 'double trombone' zoom mechanism shows the image magnification when the lens is set to 0.45m (i.e. the minimum focus distance). The numbers correspond to each of the focal lengths marked on the zoom ring except for 18mm and 24mm (at which positions this sleeve is not visible).

Reported aperture vs focal length

Here we show the maximum and minimum apertures reported by the camera at the marked focal lengths.

Focal length 18mm 24mm 28mm 35mm 50mm 80mm 135mm 250mm
Max aperture
F3.5
F4.0
F4.0
F4.5
F5.0
F5.6
F5.6
F6.3
Min aperture
F22
F25
F25
F29
F32
F36
F40
F40

The 18-250mm maintains essentially the same maximum aperture through the zoom range as the older 18-200mm, and therefore is just marginally slower than the Canon or Nikon equivalents at the telephoto end (at 200mm the maximum aperture is reported as F6.3).

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