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Z-Cage

Sigma 20mm F1.8
EX DG ASPHERICAL RF Lens Review

Reviewed by Ron Risman -- November / December 2009




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Introduction

If you have been following my reviews here on Cameratown you already know that I love using the video capabilities of the EOS 5D Mark II and EOS 7D. One of the big advantages that video-capable DSLR's offer over traditional camcorders is the ability to shoot in very low light. This is why I wanted to review this Sigma 20mm lens. Its wide 20mm field-of-view (FOV), combined with it's fast f/1.8 maximum aperture, seems to make it a perfect video lens. Even on a cropped-sensor camera like the 7D, it still offers a decent 32mm wide-angle along with its fast aperture.

The Sigma 20mm also seemed like it would be an ideal lens for use on a Steadicam, as the wide field-of-view helps to hide vibrations and gives a great perspective when "flying" a camera. But it's the combination of the wide and the fast that make this lens so intriguing. There are plenty of wide lenses on the market and plenty of fast lenses on the market, but few that offer this combination. For example, Canon makes a very inexpensive 50mm f/1.8 lens that offers very good low-light capabilities, but the 50mm focal length just isn't wide enough for most indoor photography - and on the EOS 7D it becomes an 80mm telephoto due to the 1.6x crop factor of the APS-C size sensor. Canon also sells a 20mm wide lens that is actually priced below the Sigma, however its maximum aperture is f/2.8. The Sigma is over 2x more light sensitive, making it that much easier to capture good quality images in low-light.

In this review I'll share my thoughts on the Sigma 20mm lens and how it performed in a variety of situations with video as well as with still photography.

Sigma Highlighted Feature List

  • Super Multi Coating to reduce flare and ghosting
  • 9 diaphragm blades provides for a soft bokeh
  • Focus as close as 7.9 inches
  • Petal-type hood
  • Aspherical lens elements in both of the front and rear lens groups compensates for distortion, spherical aberration and astigmatism.
  • 13 Elements in 11 Groups
  • Minimum Aperture: F/22
  • Maximum Aperture: F/1.8
  • Filter thread size: 82mm
  • Available in the following mounts: Sigma, Canon, Nikon, Sony / Minolta, Pentax
  • Retail Price: $670 Street: $550-$599
Testing

Unlike other review sites I tend to shy away from test charts. While test charts are a great way to compare results of multiple products side-by-side (assuming that conditions are controlled and identical for each test), I prefer to test a lens on the same subjects that you and I shoot everyday - landscapes, close-ups, pets, family, living rooms - real-life subjects with uncontrolled lighting, lens flare, etc. - not static charts.

I have been using the Sigma 20mm f/1.8 lens over the past month with both the full-frame EOS 5D Mark and the EOS 7D with it's 1.6x cropped sensor. I have also used and tested the camera & lens on a Steadicam Merlin and an indiSLIDERmini portable slider / dolly.

First Impressions
One of the things I noticed first about this lens was its solid feel and heft. This is not a lightweight lens. It is 3.5" (L) x 3.5" (D) in size and weighs just over a pound (1.2lbs). The lens features a large, easy-to-grip, manual focus ring that is very smooth to operate, as well as a dual-focus mechanism that allows the manual focus ring to be disengaged when the lens is set to AF. This is important since the manual focus ring takes up most of the lens barrel and could be easily rotated in error just by handling the lens. The downside to this dual-focus mechanism is that it's overly confusing. In order to switch it from manual to auto you have to remember to make two changes: Switch the AF/M switch located near the lens connector to "AF," and slide the actual focus ring forward until it clicks into its (AF) position. This second step disengages the manual focus ring so that it can turn freely WITHOUT effecting focus or worse, going against the AF motor. If you want to switch back to manual focus you would set the main switch to "M" and then pull back on the focus ring, switching it to the "M" position as well. This engages the manual focus ring, allowing you to smoothly manual focus.



Video / Cinematography

With video, I want lenses that are fast and easy to manually focus. Prime lenses often have smoother focusing and faster maximum apertures, while staying somewhat affordable. The Sigma 20mm f/1.8 lens seems to be the ideal lens as it combines a very wide field of view with a fast wide-open aperture. As mentioned above, Canon makes a similarly priced 20mm lens, but its maximum aperture is f/2.8, over a full stop under that of the Sigma. Each stop represents doubling of light sensitivity.

Another beneficial feature both for stills and video is the rear internal focusing system. Internal focusing means the front of the lens doesn't rotate as the lens focuses, making it compatible with circular polarizer and ND filters (such as the VariND or FaderND). This also allowed Sigma to include a pedal-style hood which typically does a better job over standard hoods at reducing glare and flare from external light sources.

When using the Sigma 20mm on a full-frame camera, it exhibits quite a bit of wide-angle light fall-off when the aperture is wide-open at f/1.8 or f/2.8. While this would be a bigger problem if using wide apertures in daylight, it basically goes unnoticed when shooting in lower-lighting. For still photography this represents a bigger problem as wide-open apertures are often used both outdoors as well as in.

For use with a dolly / slider
A slider is essentially a very short portable track system that allows you to smoothly glide the camera across or toward a subject while video recording. This type of shot is very cinematic in feel and can add tremendously to a final production. Using a wide 20mm lens on a slider or Glidetrack allows the videographer to capture beautiful low-angle landscape clips while keeping important elements in the field of view. The wide 20mm view on a full-frame camera will pick up some of the track, even on a short 24" track if you use it to move slide toward or away from your subject.

For use on a Steadicam / Glidetrack
The 20mm wide field-of-view really shines when used on a camera stabilizer like the Steadicam™ or Glidetrack™. These counter-balance stabilization systems allow you to record while walking, running, climbing stairs, or even while moving in a car without picking up the jarring footsteps or bumps normally associated with this type of footage. Using a wide-angle lens while "flying" on a stabilizer helps to hide any subtle vibrations while also providing much needed wide-angle coverage of the scene. When shooting on a Steadicam or other camera stabilizer you normally want the d deepest depth of field possible. This works in favor of the Sigma 20mm lens as there is very little light fall-off when shooting you stop down the lens to f/8, f/11, or f/16 even with a full-frame sensor.

Play the short video below to see footage from the Sigma 20mm f/1.8 - mounted to the Canon 5D Mark II and Steadicam Merlin. The 20mm wide focal-length provides for a great perspective when shooting video and especially came in handy inside the Minicooper. You can also download the full version here.



Comparison Video shot with EOS 7D
In this next video clip I recorded a 15 second scene comparing the Sigma 20mm f/1.8 (32mm equivalent) to three other Canon lenses. Since the EOS 7D uses a smaller "cropped" sensor the lenses don't go as wide due to the 1.6 multiplier -- Canon EF 24-105mm f/4L (38mm equivalent), Canon 18-55mm "kit" lens (28mm equivalent), and the Canon EF 50mm f/1.8 II lens (80mm equivalent).



Photography

The Sigma 20mm f/1.8 lens offers the photographer some of the same advantages as it offers for video (wide FOV, bright f/1.8 maximum aperture), though for still photography the lens performs best when used with an APS-C sensor camera.

Vignetting / Light fall off
Vignetting is seen as light fall-off along the outer perimeter of an image. On a full-frame camera, the Sigma 20mm f/1.8 exhibits strong vignetting at larger apertures (f/1.8, 2.8, 3.5, 5.6), but since a 20mm wide lens is best suited for landscapes, you're more likely to use this lens stopped down (f/8, f/11, f/16) in order to obtain greater depth-of-field - at least outdoors. When you do want to use the lens wide-open you'll most likely be shooting in lower-lighting situations where corner vignetting will often go unnoticed or at the least be very unobtrusive. Either way, most of today's photo software can correct for all but the most serious vignetting issues.

On a cropped sensor camera, like the EOS 7D, vignetting, even wide open, was very minimal and virtually nonexistent at f/2.8 and above. The reason that vignetting all but disappears with a cropped sensor is that the sensor only 'sees' the inner 63% of the lens, which is beyond the area where light fall-off is most visible. This is also the reason why this lens becomes a 32mm equivalent when used on a cropped sensor camera.

To more easily demonstrate light fall-off I have taken a series of photos of a plain white wall using both the full-frame EOS 5D Mark II and the cropped-sensor of the EOS 7D (Example #1). This 'white wall" demonstration helps to demonstrate the different levels of light fall-off at each aperture. When shooting an actual image, vignetting really isn't as noticeable (Example #2).

Example #1
Vignette Comparison - Full Frame and Cropped Sensor


Example of how light fall-off becomes less distracting in a real scene vs. a plain white wall.

Example #2
Vignette Comparison - Full Frame and Cropped Sensor


Lens Flare
Another issue that is common on very wide lenses is flare. Despite providing a pedal-style lens hood designed to reduce lens flare, the Sigma is very prone to flare anytime the lens was facing the direction of the sun. Flare is expected on wider lenses and I personally love lens flare in some ultra-wide shots, however that tends to be an attributes of a really wide lens.

During my test period with this lens I shot quite a bit of video and didn't notice any lens flare. Then it dawned on me that most of the video I shot was captured on city streets where the sun is often blocked by buildings or at night where flare isn't much of a problem. When shooting into the sun care should be taken to block sunlight from hitting the lens unless you are purposely going for that look as I did in the test image below.



Auto Focus
The focusing system in the Sigma 20mm f/1.8 lens works very reliability and fast. It's a bit noisier than what I'm used too with Canon's USM lenses, but not to a point where it attracts attention. The focus was also dead on with no rear or front focusing issues. At 20mm you wouldn't normally expect perfect focusing to be as critical since the depth-of-field is much larger on a wide-angle lens, but you'd be surprised at how soft small details can look when the lens isn't dead on.

Chromatic Aberration
Chromatic Aberration (CA) is often seen as purple, red, or blue fringing around high-contrast edges. The Sigma 20mm does a good job at keeping Chromatic Aberration to a minimum in the center of an image and is only mild around the edges & corners of a full-frame image. With the cropped-frame EOS 7D, CA was very minimal even in the corners. Chromatic Aberration is also easy to minimize or delaminate in post production using programs like Adobe Lightoom 2.x, Adobe Photoshop or Apple's Aperture.



Perspective Distortion
The Sigma 20mm lens distorts images, that's the nature of wide-angle lenses. It is this distortion that makes distances seem greater and subjects look further apart than they really are. This optical 'bending' is often an artistic look that many photographers purposely use a wide-angle lens to achieve. Because of perspective distortion wide-angle lenses don't make for great portraits as they are designed to exaggerate reality. When shooting landscapes it's best to shoot open vistas with no large trees close by, otherwise they trees will look as if they're leaning in on the frame. When using the lens on a cropped frame camera distortion will be reduced, but not eliminated.

Conclusion

In testing, I have found that the Sigma 20mm f/1.8 lens is a near perfect video lens for hybrid DSLR users. Despite the confusing double focus mechanism, the smooth and large manual focus ring make it a breeze to focus while shooting, the fast f/1.8 aperture provides great low-light sensitivity and very shallow depth-of-field, and the 20mm wide angle of view is great for capturing in tight spaces, to add perspective and dimension to an outdoor scene, and to better help tell a story. Even on the 7D, the lens still provides a respectable 32mm field-of-view, offering much greater versatility than the more typical 50mm f/1.8 lens (which converts to 80mm).

Any of the lens weaknesses really don't affect video capture. First is lens sharpness. Since HD video is only the equivalent of 2-megapixel resolution, the lens has no problem exceeding the sharpness levels needed. Second is vignetting. Vignetting is most visible when shooting wide-open under bright conditions, but when shooting video outdoors, videographer's usually prefer smaller apertures in order to keep a slower and more video-friendly shutter speed of 1/30th - 1/60th second.

For full-frame still shooters the Sigma 20mm lens represents some trade-offs, but no more than any other wide lens. When shooting at larger apertures (f/1.8 - 4.5) there is quite a bit of light fall-off in the corners of the frame. Yes, light fall-off is very noticeable when taking pictures of a blank wall as our example illustrated, but it is much less noticeable when shooting a real scene with people, trees, ground, sky, etc.

If shooting with a cropped-sensor camera you'll lose the wide 20mm field-of-view, but you'll have virtually no light fall-off at any aperture and corner sharpness will automatically improve at f/1.8 since the sensor only uses the inner portion (and best part) of the lens.

Center lens sharpness was excellent at virtually all apertures except f/1.8, where it was still good. Corner sharpness was very soft at f/1.8 but quickly sharpened up at f/2.8 - f/16 and was just a bit soft at f/22. The Auto focus system worked very well, even in dim lighting, though low light focus performance my be more dependent on whether or not your camera or flash uses an AF assist lamp.

Lens flare was present anytime the sun was in the frame, which with a 20mm wide view is, well, often. When I shoot with a ultra wide lens, I typically look to add some lens flare to my images, but not everyone does. Your choices are either to shoot with your back to the sun or use a reflector, hand, or matte box (video) to help eliminate or minimize the chance of flare.

Image contrast was very high with the Sigma 20mm f/1.8 lens, an attribute that is often missing with wide lenses. Personally, there were some outdoor shots where I felt the contrast was actually too strong, so I would recommend using a more averaged metering pattern and setting up a custom style to reduce the contrast and saturation levels of the camera.

One thing that surprised me about this Sigma lens was its overall weight and size. This is not a small, lightweight lens. The filter diameter is a large 82mm, the weight is just over a pound, and the length is just slightly shorter than the Canon 24-105mm f/4L lens. I would have much preferred a filter size of 77mm to better match other lenses like the 24-105mm and 70-200mm f/2.8. 82mm filters are also much more expensive. When video recording, I love the size and weight of the lens, as it adds stability and a large area to grip and focus while shooting. For photography I would have preferred a smaller and lighter design.

See Differently
When you use a wide-angle lens don't make the mistake of shooting normal angles with it. A wide lens is designed to show things differently, so make sure you shoot things differently. Get up close to objects, shoot from low angles and high angles. With such a wide field of view you don't even have to look through the lens for many of your shots. Hold it above a crowd to get your shot, shoot low for pets and children, and just have fun with it - and you'll end up with a ton of great, unique photographs.

In conclusion I can honestly say that if you love wide-angle perspectives or if you shoot video with your DSLR you'll love this lens. They really have this segment of the market all to themselves, at least for now. The fact that this is a full-frame lens means you can buy it now for your APS-C size camera and it can stay in your bag when and if you upgrade to a full-frame camera in the future.

Buy / Compare Prices
This lens is available to purchase through Amazon.com. You can also use our comparison shopping page, powered by Pricegrabber, to get prices from multiple retailers.


Photo Gallery (Sigma 20mm on Full-Frame EOS 5D Mark II)

This first image was taken using a wide aperture at fairly fast shutter speed at ISO 800. I didn't take it from an exaggerated angle, but the wide FOV of the lens and shallow focus really makes Rizzy's (dog) face stand out. Settings: 1/160th sec., f/2.8 aperture, ISO 800



This next picture was taken as I ran up the street with Rizzy. I kept the overall shutter speed low in order to show action with motion blur.
1/30th sec., f/18 aperture, ISO 100



This next image of the Nubble Lighthouse was shot at 11:00 p.m. as the quarter moon started to rise. I exposed the scene for 35 seconds at f/1.8 with an ISO of 400. The red light from the lighthouse as well as the light from the moon created some lens flare in the photo. I personally like lens flare at times and I think it adds an interesting element to this scene.



Once again Rizzy is the star of the show. This image was shot from a very low angle, without looking through the lens. That's one of the benefits of shooting at 20mm -- the FOV is so large that you really don't have to worry about getting your subjects in the scene as much as you do framing. In this image I would have liked it better if I hadn't chopped of his feet. This image was taken at f/3.2, 1/160th shutter speed at ISO 800.



This next image demonstrates one of the great looks only achievable with a very wide lens. If you have a pet, getting up close with a wide lens exaggerates the nose and makes for a fun and interesting perspective. This photograph was taken at f/3.5, 1/60th shutter, and ISO 125.



This last photo demonstrates some of the unwanted distortion you'll get with an ultra wide lens if you're not careful keeping the lens horizontal. In this image I tilted the camera/lens upward to get more of the crisp blue sky into the image. This resulted in the house and trees looking as if they're leaning backwards into the frame. This slanted perspective can be corrected inside Photoshop or other software, though you will end up being forced to crop the sides a little bit, thus loosing some of the ultra wide view.



Buy / Compare Prices
This lens is available to purchase through Amazon.com. You can also use our comparison shopping page, powered by Pricegrabber, to get prices from multiple retailers.
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