Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens Review

Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens Review

The Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens review is a reasonably-priced lens that comes in a small package and packs a really nice focal length range.

The wide focal length range covered by this lens is definitely its strongest asset. Before I go any farther, I need to specify that this is a Sigma "DC" or Digital Camera-only lens meaning it is only compatible with Canon 1.6x FOVCF DSLR bodies. From 18mm on the wide end to 125mm on the long end, few lens changes are needed to get the desired framing for a wide range of scenes. Experience this focal length range in the mouseover example below.

Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens focal length examples

18mm | 24mm | 35mm | 50mm | 70mm | 100mm | 125mm

 
This set of images was not shot specifically for this purpose - I sometimes like to explore a scene through all focal lengths of the lens for creativity purposes. The image framing jumps between the 70mm and 100mm frames - watch the bottom left wheat shock in the 70mm frame before going longer. This is the one visible in the 100mm frame. This wheat shock is the second from the left in the 18mm frame. These images were shot at f/11.

There are usually optical downsides to long focal length lenses - especially ones in this price range - and the Sigma 18-135 OS shows some of these. Barrel distortion is apparent at 18mm (this is no surprise), but pincushion distortion quickly sets in over the rest of the focal length range. Pincushion distortion is especially strong between 35 and 60mm and again in the low 70s. The pincushion distortion makes the ISO 12233 resolution chart results appear even worse due to the reduced detail size. This is not a real sharp lens even stopped down. Compare the Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens results to the results from the Canon EF 200mm f/2.8 L II USM Lens even wide open on the Canon EOS 40D to see the difference from a higher end reference lens. You will also see that contrast is not as high. In the mid focal length range of this lens (35-100mm), the right side of my lens is slightly less sharp than the left. Retesting delivered the same results.

CA (Chromatic Aberration), while not harsh, is apparent in the middle and worse at the corners of the frame at both ends of the focal length range. On the wide end, CA mostly clears up by 35mm and is once again apparent at and above 100mm. Color appears neutral.

Flare ranges from well controlled on the wide end to quite noticeable on the long end if there is a bright light in the corner of the frame. Some vignetting is visible in the corners at 18mm but there is not much to worry about in this regard at longer focal lengths. A standard thickness circular polarizer filter causes some additional vignetting at 18mm - you might want a slim version of this filter for this lens.

Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens product images

Retracted | Extended      with Hood: Retracted | Extended | Rotated

 
As you can see in the above examples, the Sigma 18-135 OS extends (1.9" / 48mm) during focal length increase, but does not extend (nor does the front element rotate), during focusing. While this is an HSM (Hypersonic Motor) driven autofocus lens, FTM (Full Time Manual) focusing is not enabled. To turn the manual focus ring, the AF/MF switch must be in the "M" position - otherwise damage could be done. Damage could also occur if the ring is held during AF (it turns). While the AF motor is quiet, the turning focus ring makes a relatively quiet "shhhhhh" sound with a little chatter at times as it acquires focus lock. AF speed is reasonable - but not real fast.

My experience with this lens' autofocus accuracy has not been the best. I am getting the best results from the widest angles with hit rates deteriorating as the focal length increases. Even the relatively wide DOF (Depth of Field) delivered at this lens' max aperture are not enough to hide the inconsistencies. Shooting at f/8 or f/11 gives me better results - better hiding any AF errors with deeper DOF.

With an MFD (Minimum Focus Distance) of 1.15' (.35m), the Sigma 18-135 OS delivers a nice MM (Maximum Magnification) value of about .26x.

ModelWeightDimensions w/o HoodFilter
Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens17.8 oz(505g)2.9 x 3.5"(74 x 88.5mm)67mm

 
The Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens is a really nice-sized lens. It is comfortable to hold and comfortable to carry for long periods of time. It seems solid and well-built with nearly no wobble in the components - only the focus rings shows some slight movement in the not-desired direction. Both the focus ring and the zoom ring are nicely sized for a lens of this size - they take up most of the lens barrel. Both are also smooth though the zoom ring occassionally (not frequently) catches at 125mm - becoming very hard to zoom back out - I can't make this happen on demand. The MF ring unfortunately has too much play in it - focus jumps when changing directions - making MF hard to use.

A focal length lock switch is provided - this lets you intentionally lock the lens at 18mm to prevent gravity-induced zoom creep. It is not needed on my lens at this point as the zoom ring is somewhat stiff (not bad), though I suppose that long term use could loosen things up to the point where the lock is needed.

Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens mounted on a Canon EOS Rebel XSi

 
The Sigma 18-135 OS is shown above mounted on a Canon EOS Rebel XSi with a Canon BG-E5 Battery Grip in the above pictures. The ball head is an Acratech GV2 ball Head.

Modelf/4.0f/4.5f/5.0f/5.6
Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens18mm28mm41mm73mm

 
This is a variable max aperture lens. The above chart shows the focal lengths the max aperture narrows at. Your are probably saying that this is an f/3.8 lens - why is the max aperture f/4 at the wide end? Good question - The Canon 40D and Rebel XSi / 450D DSLRs both report f/4 at 18mm. Sigma claims that you really are getting an f/3.8 aperture at 18mm but that the camera (and DPP) rounds up to the nearest 1/3 stop setting for display. I didn't conduct a test to determine if the extra 1/6 stop is really there or not.

This lens has a relatively narrow max aperture (which helps keep the weight, size and price down), but Sigma's OS (Optical Stabilization) comes to the rescue in low light situations. This OS implementation is rated at 3-4 stops. In a not-terribly-stable shooting position, I can achieve sharp images (at a decent hit rate) handholding this lens at shutter speeds of 1/5 second at 18mm and 1/20 second at 125mm. This is about 3- and 3+ stops of improvement respectively for me personally.

I was able to make sharp images at slower shutter speeds, but the keeper rate was very low. Respectively, I can make occasional sharp images below my rule-of-thumb non-IS shutter speeds as well, so the improvement amount remains similar. Interestingly, there were a good number of OS-on images in my test that were very blurred at shutter speeds faster than should have been needed for sharp images. It seems that this OS implementation is less forgiving or less-consistent at times compared to what I'm used to.

The OS settings for the Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens are simply On and Off. Even when the OS switch is off, the light whirring sound indicating OS is functioning can be heard while the camera meter is active. Sigma stated to me that OS remains active to hold the lens in an optically centered position - and you see the viewfinder framing jump to a new position when the meter is activated or deactivated. Keep this in mind when framing your shot - especially when setting up a tripod-based shot (keep the meter active).

The Sigma 18-135 OS comes with a center-and-side-pinch front lens cap that is easy to get off even inside the also-included relatively small lens hood. A rear cap is always included of course.

The Sigma 18-135 OS is available in Canon (reviewed), Nikon, Sigma, Pentax (no OS) and Sony (no OS, no AF unless the body supports HSM motor) mounts. My obligatory standard disclaimer: You should know that there are potential issues with third party lenses. Since Sigma reverse engineers (vs. licenses) manufacturer AF routines, the possibility exists that a new body might not support an older third party lens. Though not common, this has happened. Sometimes a lens can be rechipped to be made compatible, sometimes not. Second, there is the risk of a problem that results in the lens and body manufacturers pointing blame at each other. However, Sigma USA's 4-year warranty is far superior to Canon's standard 1 year warranty (though many credit cards will double the Canon warranty for you).

The combination of a narrow max aperture and somewhat slower autofocusing make this lens less attractive for sports. The Sigma 18-125mm f/3.8-5.6 DC OS HSM Lens could be a good choice for travel where it is not convenient to carry multiple lenses - or you need all these focal lengths mounted simultaneously. The price, especially for an optically stabilized lens, is good. The optical qualities leave room for improvement.

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