1. Introduction 2. Design and Operation 3. Test results 4. Test results 5. Conclusion & samples

Design

The 18-250mm is fairly typical in design and build for a modern Sigma lens, which means a lightweight but solid-feeling black plastic barrel, and relatively smoothly-operating zoom and focus rings. It uses a 'double trombone' design to extend to its longest setting, with impressively little play of the barrel when set to 250mm. As we'd expect of a lens at this price, the mount is metal.

Two large, positive switches on the side of the barrel control the autofocus and image stabilization mechanisms, while a smaller one on the zoom ring locks it at the 18mm position for transport. All three are well-placed for operation by your left thumb while holding the camera in the normal shooting position.

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. This tends to be most problematic if you habitually carry the camera with the lens pointing downwards, either in-hand or using a sling-type strap.

The 18-250mm does well here; the feel of the zoom ring is impressively even across the range, yet never excessively stiff. Our review sample didn't suffer too much from zoom creep either, at least when retracted to 18mm. Like most lenses of its type it also has a switch to lock it at the 18mm position, which is unusually well-placed for quick operation by your left thumb.

Compared to Tamron 18-270mm F/3.5-6.3 Di II VC PZD

Here's the Sigma 18-250mm F3.5-6.3 Macro alongside it's nearest rival, the Tamron 18-270mm F/3.5-6.3 Di II VC PZD, both in Canon mount. The two lenses are practically identical in size when retracted to the 18mm position. They also have very similar control layouts, with side-mounted switches to control autofocus and image stabilization, and a lock switch towards the top of the zoom ring. Both feature built-in optical stabilization too, but while the Tamron has a longer zoom range, the Sigma can focus closer. This is illustrated below:

Sigma 18-250mm f/3.5-6.3 Macro	Tamron 18-270mm F3.5-6.3 PZD
250mm, distant subject	270mm, distant subject
Macro: Closest focus = 29cm	Macro: Closest focus = 43cm

On the camera

Even on a small entry-level SLR such as the Canon EOS 650D shown here, the Sigma 18-250mm F3.6-6.3 Macro feels nicely balanced, which isn't something that can be said of all superzoom lenses. It's notably smaller than either the Canon or Nikon 18-200mm F3.5-5.6 superzooms, and much the same size as the Sony and Pentax 18-250mm F3.5-6.3 models (which both share their essential specifications with the old Tamron 18-250mm F/3.5-6.3 Di II LD Aspherical (IF) Macro).

Flash shadowing

One problem often seen with 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 couple of Canon bodies, including the entry-level EOS 650D, and never really saw any semblance of shadowing, even at 18mm and focus distances close to the minimum - a distinct advantage of the compact design. 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, particularly older models on which the flash unit doesn't lift quite so high above the lens axis. But again its compact size means the problem should be minimal.

Autofocus

The 18-250mm uses Sigma's Hypersonic Motor for autofocus, and when shooting with the optical viewfinder our Canon mount sample was near-silent in use, and impressively fast and decisive even at full telephoto. It's clearly quicker than the Tamron 18-270mm F/3.5-5.6 PZD used on the same (Canon) body. 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.

Switch from the optical viewfinder to live view, though, and the story changes. Focusing slows right down (although the exact speed is highly dependent on the camera used), and if you refocus during recording the clicking of the AF motor will be audible on your soundtrack as the camera fine-tunes focus.

Dependence of effective focal length on focus distance

This lens's angle of view widens dramatically on focusing from infinity to 0.35m, 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 the 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 Canon, Nikon, Pentax, Sigma and Sony versions; 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 62mm - unusually (and economically) small for a superzoom. 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. Sigma has even added a ribbed grip to make it easier to remove. 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 24mm wide 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). In typical Sigma fashion the action is smooth and even. The front element extends 71mm on zooming and feels impressively solid when fully extended, with just a little lateral play.
	A small switch on the side of the barrel locks the zoom at 18mm, preventing the lens from extending under its own length when you're carrying it around. This is especially welcome if you like to use a sling-type strap to carry your camera. The switch is notably well-placed for operation by your left thumb while shooting.
	The focus ring has a 8mm-wide grip, and rotates 50 degrees clockwise from infinity to 0.35m, matching Canon and Sony lenses but opposite to those from Pentax and Nikon. Its action is nice and smooth, but the limited travel can make precise manual focus tricky. A basic distance scale is marked in feet and meters. The focus ring travels slightly past the infinity position, and rotates during autofocus.
	Two chunky, positive switches on the side of the lens barrel set the focus and image stabilization modes. You can't adjust focus manually when the lens is set to AF (or at least, you shouldn't try).
	A slightly curious scale on the outer sleeve of the 'double trombone' zoom mechanism shows the image magnification when the lens is set to its minimum focus distance of 0.35m. The numbers correspond to each of the focal lengths marked on the zoom ring, except for 18mm.

Reported aperture vs focal length

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

The 18-250mm maintains essentially the same maximum aperture through the zoom range as its predecessor up to the 80mm setting. However at 135mm it reports itself to be 1/3 stop slower, showing F6.3 compared to F5.6. Sensibly, Sigma has limited the minimum aperture to F22 throughout (at least on our Canon mount sample) rather than allow smaller apertures at longer focal lengths, which would show excessive image quality degradation due to diffraction.

1. Introduction 2. Design and Operation 3. Test results 4. Test results 5. Conclusion & samples