Lens Reviews / Sigma Lenses i Lab tested

18-250mm	$500 average price

SLRgear Review
June 22, 2009
by Andrew Alexander

Sigma announced this superzoom lens at the beginning of 2009, initially for Sigma, Canon and Nikon mounts but as of June it's also available in Sony and Pentax mounts. This category of lens (also known as the ''vacation'' lens) offers the benefit of having both wide-angle and telephoto focal lengths in the same package, and Sigma has added their optical image stabilization technology to offset camera shake problems.

The lens is designed on the DC specification, meaning it's intended for cameras with cropped-frame sensors (APS-C). On these cameras the lens will offer an equivalent field-of-view of 29-400mm (Canon) or 27-375mm (Nikon and others). The lens doesn't feature a constant aperture; as the lens is zoomed out towards the telephoto end, its widest aperture offering decreases. The following table shows the largest and smallest apertures at various focal lengths:

Focal Length (mm)	18	24	35	50	80	120	250
Max. aperture	ƒ/3.5	ƒ/4	ƒ/4.5	ƒ/5	ƒ/5.6	ƒ/5.6	ƒ/6.3
Min. aperture	ƒ/22	ƒ/25	ƒ/29	ƒ/32	ƒ/36	ƒ/40	ƒ/40

The lens takes 72mm filters, comes with a petal-shaped lens hood, and is available now in Sigma, Canon, Nikon, Pentax and Sony mounts for approximately $500.

Sharpness
The 18-250mm provided sharp test results, even when used wide open, to about 80mm. Wide open in the telephoto range, things start to get a bit soft, especially in the corners. Stopping down helps alleviate these issues.

At the wide end, the lens performs very well. At 18mm and ƒ/3.5 we do note some slight de-centering but the central region is still very sharp at around 1.5 blur units. The corners are slightly softer at around 3 blur units. Stopping down improves this equation, until optimal sharpness is achieved at ƒ/5.6; results at this aperture are between 1.5-2 blur units across the frame and essentially stay that way until ƒ/11. Diffraction limiting sets in at ƒ/16, and by ƒ/22 the image is generally soft at between 3 and 4 blur units across the frame.

In the mid-range - 24-50mm - the lens showed its best performance. Wide open at ƒ/4 or ƒ/5, the lens provides nicely sharp images, with around 1-1.5 blur units in the center but some softness in the corners (4-5 blur units). At 50mm the lens is balanced quite well, showing around 1.5-2 blur units across the frame. Stopping down to ƒ/8 or ƒ/11 improves any corner softness until the image is essentially smooth across the frame at around 1.5-2.5 blur units. 50mm is the ideal setting for the lens, providing the ''smoothest'' performance without corner softness; at ƒ/8, it's the sharpest the lens gets, at just over 1 blur unit across the frame. Fully stopped-down performance is still acceptable by ƒ/22, around 3 blur units, but at the extreme ends (ƒ/32-36) we note generalized softness in the range of 4-5 blur units.

Upper left corner, 250mm, ƒ/6.3. Note CA fringing and softness.

On the telephoto end, the lens shows its shortcomings. Wide open, it does a little better than previous Sigma 18-200mm offerings, but even by 80mm - just a third of the way through its entire focal range - it's showing significant corner softness wide open. Results at 120mm and 250mm are similar, with particularly soft corners at 120mm (9 blur units in the corners!). There also isn't much of a central area of sharpness to speak of; there's a bit at 80mm and 120mm, but by 250mm everything is slightly soft. Stopping down to a smaller aperture does help amend this softness, especially at 80mm; at ƒ/8, it becomes respectably sharp, at just under 2 blur units across the frame. Not so much for 120mm and 250mm. At these focal lengths you'll need to stop down to at least ƒ/11 to achieve optimal sharpness (2-3 blur units across the frame). Diffraction limiting sets in at ƒ/16 so stopping down further only makes this less sharp.

Fully stopped-down performance above 80mm (where you'll see apertures of ƒ/36 or ƒ/40) should be avoided, unless you like the soft-focus look: we note results in the 6-10 blur unit range, across the frame.

Chromatic Aberration
Chromatic aberration performance is fairly good for a lens in this category, showing some significant fringing in the wide-angle and telephoto ranges only. Here's another indication the lens is designed for mid-range performance: at the 50mm and 80mm settings, CA is almost non-existent.

I'd look at the interactive CA graph and sample images for further detail, but the settings where CA is particularly problematic are: 18-24mm at ƒ/3.5 or wider, and pretty much any aperture at 250mm.

Shading (''Vignetting'')
The 18-250mm OS performs well with respect to light falloff. In fact the only point where corner shading is much of a factor is with the lens set to 18mm and using the ƒ/3.5 or ƒ/4 aperture settings. In these cases, the corners are 1/2 and 1/3 EV darker than the center, respectively. There's also a slight bit of light falloff when using the lens at 250mm and ƒ/6.3; just over 1/3 EV. These aren't significant amounts. And at any other setting, the lens shows 1/4 difference or less.

Distortion
The complex array of lens elements that allows such a vast range of focal lengths in one lens leads to some dramatic results for distortion. When used in the wide angle configuration, the lens provides uniform barrel (''bloat'') distortion up to around 21mm, with noteable distortion in the corners at 18mm (+0.75%). After 21mm, distortion across the frame remains consistently barrel-distorted, but at a moderately low level (around 0.2%, on average) and the extreme corner distortion turns into the pincushion (''squeeze'') style. The worst results are seen between 24mm and 50mm, where the corners show -0.6% pincushion distortion. Post-processing would be required to correct for these effects.

Autofocus Operation
The 18-250mm OS employs Sigma's HSM technology, which allows for fast and quiet autofocusing. Indeed, the lens was able to focus between infinity, close-focus and back to infinity in around one second. However, most of Sigma's HSM lenses allow the user to override autofocus results by just turning the focus dial; the instruction manual with this lens advises not try to focus manually if the switch is in the AF mode.

Macro
The lens offers fair macro performance, with a maximum magnification rating of 0.29x (1:3.4 reproduction). Minimum close-focusing distance is 45cm (just over a foot and a half).

Build Quality and Handling
The 18-250mm makes a compact package, weighing in at a pound and a bit (630g). The lens features a black-matte finish, constructed in plastic; the lens mount is metal, and the filter ring is plastic.

The lens is adorned with a few noteworthy control and information surfaces. A distance scale is provided on the manual focus ring, but don't expect to use it for any critical application: there are only five distance markings, and then infinity. There's no depth-of-field scale. On the lens barrel, there is a helpful scale representation of the macro reproduction ratio at the given focal length; of course, this will only be seen when the lens is zoomed out past 18mm.

There are three switches on the lens: one enables or disables autofocus, one enables or disables optical image stabilization, and one enables the zoom lock feature.

The zoom ring is the larger of the two, at 1 1/4 inches wide with a texture of raised rubber ribs. The ring requires a 75 degree turn to run through the range of focal lengths, and is smooth, but not too tight. Below 50mm, zoom creep is not an issue, but at 50mm or greater, the lens has a tendency to creep towards 250mm. The zoom lock is useful for keeping the lens locked to the 18mm focal length.

The focus ring is mounted at the front of the lens, and isn't great for manual focus functionality. The ring only provides 45 degrees of range, and the fidelity isn't very good; you will need a very careful hand to achieve accurate manual focus results. The short focus throw probably explains the fast autofocus results. The focus ends with hard stops at the close-focus and infinity points. Happily, the front element does not turn during focus or zooming operations.

Optical image stabilization is built-in to the lens, and does work well to offer a few stops of camera-shake resistance. It's especially useful at the longer focal lengths, as things get pretty wobbly at 250mm, and the maximum aperture of ƒ/6.3 at that focal length will take a toll on your shutter speed unless you push the ISO. We're testing the lens in our image stabilization lab, and will hopefully be able to produce those results in the near future.

The petal-shaped lens hood comes with the lens, and reverses on the front for easy storage. It's a basic lens hood with internal ribs to reduce stray light, and adds 1 1/2 inches to the overall length of the lens.

Alternatives

Sigma 18-200mm ƒ/3.5-6.3 DC OS HSM ~$430
The obvious question is, has Sigma improved from its previous version of this lens? The answer is a resounding yes: the corner softness issues that plagued the 18-200mm have been dramatically improved in the 18-250mm version. CA, distortion and corner shading are essentially the same. It's also better than the previous non-OS version, however both the OS and non-OS 18-200mm were perhaps a little better in the telephoto range.

Tamron 18-270mm ƒ/3.5-6.3 Di II VC LD Aspherical IF Macro AF ~$620
A very comparable lens from Tamron, the 18-270mm offers a slight edge on telephoto performance, but not enough to sway most users. Optically, this lens tested very well, and to compare the two takes a keen eye to spot the differences. In the end, I'd give the edge to the Tamron; just slightly sharper, especially in the telephoto end, and slightly better with CA. However, distortion is much greater in the wide end, where it's better-controlled by the Sigma. Both lenses feature image stabilization; the Tamron is slightly more expensive, and at present, only available in Nikon and Canon mounts.

Major Manufacturer comparables
All the manufacturers Sigma is competing with in this lens category offer equivalent lenses, and all of the body / lens combinations offer image stabilization.

Canon EF-S 18-200mm f/3.5-5.6 IS ~$600
Sharper across the board, and better CA tolerance; worse distortion and more significant corner shading.

Nikon 18-200mm f/3.5-5.6G IF-ED AF-S DX VR Nikkor ~$700
Sharper across the board, and better CA tolerance; worse distortion and more significant corner shading.

Pentax 18-250mm f/3.5-6.3 ED AL IF SMC DA ~$500
Not yet tested. Comparable range of focal lengths.

Sony 18-250mm f/3.5-6.3 DT SAL-18250 ~$550
Slightly sharper, but not by much; CA is comparable. Corner shading and distortion are much worse.

Conclusion
Like most ''vacation'' zooms, the trick is to know the parameters where they function best. Sigma's latest iteration of this lens offers a bit more telephoto performance, for which the integrated optical image stabilization is crucially useful. Unfortunately the lens' maximum aperture decreases quickly as the lens is zoomed out, to the point where at 80mm, you're using a ƒ/5.6 lens. This design allows the lens to be kept as small as it is, but at that point you're needing either a sunny day, an external flash, or a steady hand (even with the image stabilization) to get reasonably sharp photos.

The testing on the lens shows it to be very capable when used in the wide-angle or mid-range settings, but telephoto performance leaves something to be desired. In all cases stopping down helps to improve the image, and using the lens at 250mm and ƒ/11 only serves to underline the previous point. This is a walkaround lens, but really only if you're walking around outside on a sunny day.

With those caveats, it's important to note that most lenses in this category have to struggle with the same limitations, and this lens perhaps comes out better than most. The whole point of the lens is to combine multiple lenses into one, and with the competitive price point, it serves the function admirably, with the side benefit of saving you a little money in the process.

Sample Photos

The VFA target should give you a good idea of sharpness in the center and corners, as well as some idea of the extent of barrel or pincushion distortion and chromatic aberration, while the Still Life subject may help in judging contrast and color. We shoot both images using the default JPEG settings and manual white balance of our test bodies, so the images should be quite consistent from lens to lens.

Still Life shot

VFA target

As appropriate, we shoot these with both full-frame and sub-frame bodies, at a range of focal lengths, and at both maximum aperture and ƒ/8. For the ''VFA'' target (the viewfinder accuracy target from Imaging Resource), we also provide sample crops from the center and upper-left corner of each shot, so you can quickly get a sense of relative sharpness, without having to download and inspect the full-res images. To avoid space limitations with the layout of our review pages, indexes to the test shots launch in separate windows.