Sigma's new 18-250mm f/3.5-6.3 DC OS HSM is designed as an all-in-one lens for digital photographers with APS-C-sized (DX) sensors who don't want to carry more than one lens. Covering focal lengths ranging from the 35mm equivalent of 27mm to 375mm with most camera brands (28.8mm to 400mm on Canon bodies) it offers a 13.8 times zoom ratio. A built-in Hybrid Optical Stabiliser is provides for shake reduction in both shots and the viewfinder image.
Build quality is above average for the price of this lens -which is . The mounting plate is stainless steel and the polycarbonate cladding has a solid feel. The zoom movement is smooth and positive and a lock is provided to prevent it from extending when the lens is carried pointing downwards (although it showed no evidence of doing this in the course of our tests).
The Sigma 18-250mm f/3.5-6.3 DC OS HSM lens. (Source: Sigma.)
Optical construction is quite complex, although similar to the company's current 18-200mm F3.5-6.3 OS lens. Both lenses have 18 elements in 14 groups and include four Special Low Dispersion (SLD) glass elements and three aspherical lens elements to counteract common aberrations. Both have the same f/3.5-6.3 maximum apertures.
The new lens has a minimum focusing distance of 45 cm at all focal lengths and a maximum reproduction ratio of 1:3.4 (roughly one third life size), allowing it to be used for close-up photography. Internal focusing allows users to fit angle-critical attachments like polarisers and graduated filters
Sigma's Hyper Sonic Motor (HSM), which is driven by ultrasonic waves, is built in to ensure quiet autofocusing at relatively high speeds. Super multi-layer coatings on the lens elements reduce the incidence of flare and ghosting.
Extending approximately 100 mm from the camera body (without the lens cap attached), this lens protrudes a further 78mm when zoomed out to the 250mm focal length. The design consists of three barrels, the innermost containing the optical elements, the middle one engraved with 'macro' reproduction ratios that range from 1:3.4 at full zoom extension to 1:12.8 at the 18mm focal length.
The focusing and zoom rings are mounted on the outer barrel. The former is approximately 22 mm wide with a 10mm wide ridged, rubber-coated band that provides a comfortable and secure grip. Engraved on the ring just in front of this band are distance markings in feet and metres that range from 0.45 metres (the closest focus) to infinity. In manual focusing mode, these distances can be lined up against a mark on the front of the lens (where the innermost barrel flares out to accommodate the front element).
Behind the focusing ring (i.e. closer to the camera) lies the zoom ring, which is approximately 48 mm wide and carries a 33 mm thick, ridged rubber coating that provides an excellent grip. An engraved scale on the trailing edge (nearest the camera) carries markings for the 18mm, 24mm, 28mm, 35mm, 50mm, 80mm, 135mm and 250mm focal length positions. These are lined up against a white mark on the non-moving part of the lens barrel.
On the left side of the main barrel are two sliders; one for switching between auto and manual focusing and the other for turning the OS stabilisation on and off. The zoom lock is located at the rear of the zoom ring, just above the AF/MF slider. Filters screw into the 72 mm threaded ring in front of the front element.
The OS image stabilisation system has only two modes: on and off. You can't set stabilisation to operate in only one direction to get the best results with panning shots but the system appeared to handle panning reasonably well. The viewfinder image is also stabilised, making it easier to compose both pans and normal shots with longer focal length settings.
This lens will be offered with mounts for Sigma, Nikon, Canon, Pentax and Sony entry- and mid-level DSLRs. It is supplied with a petal-shaped lens hood, front and rear caps and a soft case (although the lens hood and case were not provided to Photo Review with the review lens).
Handling
The metal mounting plate and generally higher build quality of this lens give it a much solider feel (and slightly heavier weight) than cheaper plastic lenses. It was a comfortable match with the Canon EOS 40D we used for part of our tests but felt slightly front-heavy (although still quite usable) on the lighter EOS 400D body.
We found a few instances where the review lens appeared to have difficulty communicating with the EOS 40D body. This led to an inability to access the automatic focusing and exposure adjustments. These problems were resolved by detaching the lens and re-fitting it again so we assume the problem is related to the way in which the mounting plate connects to the contacts on the camera body. No problems were found with our EOS 400D body.
The focusing ring can only be moved when the lens is set to manual focus mode and, even then, it will only travel through about 30 degrees, which is a little too short for precise manual focusing. The zoom ring moves through roughly a quarter of a turn and we found the markings on the lens barrel to be close to accurate for the indicated focal length settings.
The ability of this lens to focus down to 45 cm across its entire focal length range provides good potential for close-up photography, particularly at higher zoom ratios. This lens has recently received a TIPA Award nominating it as the best entry-level lens of 2009. The supplied lens cap clamps firmly into place with spring-loaded clips.
Performance
Aside from the connectivity issues we found with our EOS 40D body, the review lens turned in a good all-round performance for its type and price tag. Like all extended zoom lenses, its relatively small maximum apertures affected autofocusing performance across the zoom range and hunting was common in low light levels.
Some shots taken with the lens set to f/16 and smaller apertures at the 250mm focal length appeared slightly soft, although whether this was due to the optical system we were unable to clarify. At shorter focal lengths, images appeared sharp and detailed and no significant instances of hunting were observed.
Imatest showed the review lens to be a competent performer for its focal length range and price tag. Although it didn't produce stellar results in Photo Review's standard tests, the review lens delivered good resolution at aperture settings between f/5.6 and f/16 for all focal lengths we were able to test. (Lack of sufficient distance in our test set-up prevented us from evaluating performance at the 250mm focal length setting.) The graph below shows the results of our Imatest tests.
For a lens with such an extended zoom range, lateral chromatic aberration was relatively low. A small amount of coloured fringing could be seen in some test shots taken in contrasty outdoor lighting. The graph below shows the results of our Imatest tests. Note: the red line marks the boundary between negligible and low CA values, while the green line separates low from moderate CA.
We found the OS system could provide up to four f-stops of shutter speed advantage at the full tele zoom - as long as we were prepared to discard roughly half of the test shots we took. Under these conditions we were able to use the 250mm focal length setting at shutter speeds as slow as 1/30 second.
Flare and ghosting were generally very low, thanks to the internal surface coatings in the lens. We found no fault with overall colour or tonal reproduction, even with relatively strong backlighting.
A small amount of vignetting was observed with the widest apertures at all focal length settings. However, stopping down by as little as one f-stop virtually eliminated this problem. Examples are shown below.
Vignetting at 18mm; 1/1024 second at f/3.5.
Vignetting at 250mm; 1/250 second at f/6.3.
Barrel distortion was visible at the18mm focal length setting but not by 35mm. Slight pincushion distortion began to appear at around the 50mm focal length mark but it remained relatively minor up to the 250mm focal length setting.
Close-up performance was good for the lens specifications and the focusing limit of almost 45 cm was usable for photographing flower clusters. However, best results could only be obtained by selecting the best AF point manually as the autofocusing system tended to lock on to the centre of the frame - and often took some time to find it. Bokeh was attractive with longer focal length settings but background details became quite busy at focal lengths wider than about 80mm.
Although relatively expensive for a 'consumer' lens, Sigma's 18-250mm f/3.5-6.3 DC OS HSM represents a worthwhile all-in-one solution for everyday picture-taking. It could also make a useful travelling companion for photographers with entry- and mid-level DSLRs.
Buy this lens if:
- You want a capable all-round lens for travelling, social events or general family photography.
- You want a zoom lens that can be used with polarisers and graduated filters.
- You require good flare resistance in backlit situations.
- You want a long zoom lens with effective image stabilisation for both the viewfinder image and the capture system.
Don't buy this lens if:
- You have a camera with a 'full frame' (36 x 24mm) image sensor.
- You need a fast lens across the focal length range for shooting action in dim lighting.
- You're looking for a light weight lens and/or 'bargain basement' pricing.
IMATEST GRAPHS
SAMPLE IMAGES
18mm focal length, ISO 100, 1/30 second at f/22.
Coloured fringing shown in a crop from the above image, enlarged to 100%.
250mm focal length, ISO 100, 1/30 second at f/16.
18mm focal length, ISO 100, 1/30 second at f/22.
250mm focal length, ISO 100, 1/30 second at f/16.
Without image stabilisation; 250mm focal length, ISO 100, 1/80 second at f/6.3.
With image stabilisation; 250mm focal length, ISO 100, 1/80 second at f/6.3.
21mm focal length, ISO 100, 1/15 second at f/18; with image stabilisation.
18mm focal length, ISO 200, 1/125 second at f/16.
250mm focal length, ISO 200, 1/91 second at f/16.
80mm focal length, ISO 200, 1/22 second at f/16.
Corner softening as shown in a crop from a 100% enlargement of the above image.
Close-up; 95mm focal length, ISO 200, 1/362 second at f/5.6.
250mm focal length, ISO 200, 1/181 second at f/8.
250mm focal length, ISO 200, 1/250 second at f/5.6.
Backlighting; 18mm focal length, ISO 100, 1/125 second at f/9.5. One flare spot can be seen near the centre of the frame.
Flare; 18mm focal length, ISO 200, 1/160 second at f/9.5.
Picture angle: 69.3 - 5.7 degrees
Minimum aperture: f/22
Lens construction: 18 elements in 14 groups
Lens mount: available in Sigma, Canon, Nikon, Pentax and Sony mounts
Diaphragm Blades: 7
Focus drive: Hyper Sonic Motor
Minimum focus: 45 cm at all focal lengths
Maximum magnification: 1:3.4
Filter size: 72 mm
Dimensions (Diameter x L): 79 x 101 mm
Weight: 630 grams