|Sigma 50mm f/1.4 ART on Canon 5D Mark III|
Sigma's new 50mm ART started off life with some bold claims about its resolving capabilities: as good as the Zeiss Otus. It has (mostly) lived up to the hype surrounding its launch, and is legitimately a "premium" lens. You can mention it in the same breath as the pricey Canon 50mm f/1.2L, so how does it fair against the venerable Canon 50mmL in real world terms?
Incidentally, if you are wondering why there is a sudden interest in creating premium 50mm-ish lenses (Zeiss Otus, Nikon 58mm, Sigma ART) it's because that's the prime that most people will buy. Many photographers will look down upon 50mm as a neither-nor focal length, the fact of the matter is that more people will buy an premium 50mm lens than they will a premium wide-angle or portrait-length prime. That's the economics of it.
Because lens design is a complex topic, this comparison will be simplified to two variables: sharpness in the center of the lens (with central focal point) and the quality of the bokeh behind the point of focus. Essentially, this is an ad hoc way of simulating close-mid range usage with a 50mm lens on a full frame body. Here is the test scene. The 3" terracotta warrior figurine is approximately 5' away from the camera. The test images are with out of cameras JPEG's from a Canon 5D Mark III at identical settings.
|Sigma 50mm ART, f/4|
Focus was placed in the center of the frame on the figurine. Note that since this isn't a flat target, the following samples don't (clearly) illustrate optical aspects such as field curvature and spherical aberration. Click each image for 100% crop view. The first is a comparison of the Canon at its f/1.2 and f/1.4 settings.
|Canon f/1.2 vs f/1.4|
What follows is a side-by-side comparison from f/1.4 through to f/8 in whole-stop increments. The difference is stark: the Sigma has more pronounced contrast and detail rendition throughout thew whole aperture range. By f/5.6 the Canon pulls close, but even then, the Sigma is producing crisper edges throughout. (Again, click on the image for 100% crop view.)
These are taken from the extreme right of the image. For reference, the display rack is a further 6' behind the figurine, or roughly 10' away from the camera. Click for 100% crop view.
|Canon: f/1.2 vs f/1.4|
Though there is a difference in bokeh between f/1.2 and f/1.4 on the Canon, its visually not as much as would be expected of the 2/3 stop difference. The reason because of this is because there are diminishing returns with how shallow the depth of field can be on a digital image sensor. The physical surface of the sensor is not flat, and the wiring and interconnects between the light wells form "walls" that shade the light well from extreme-angled light rays. (An extended discussion of this phenomenon can be found at DxOMark with this infamous article. The point being made was valid, but the article was controversial at the time because of the inflammatory way in which it was being promoted.)
When comparing the Canon against the Sigma, the Canon's out-of-focus rendition is generally smoother at each aperture setting, but not by much. The difference is more pronounced at f/5.6 and above, where the Sigma's high resolving power seems to be "trying harder" at pulling detail out of a scene, even in the out of focus parts. However, the difference is not overt, and by most definitions, the 50mm ART has a pleasing bokeh that isn't overly nervous. The difference between it and the Canon is not as great as even the small difference between the Sigma 35mm ART and the Nikon 35mm f/1.4; same story, but the Sigma in that comparison does have a visually harsher bokeh, though not by much.
Chromatic Aberration (Not Depicted)
Though not possible to depict in the even lighting of the camera store, the Sigma also shows a marked superiority to the Canon with lateral and longitudinal chromatic aberration control. That is to say, the Sigma is extremely well-controlled for a fast prime, but that the Canon exhibits higher than normal levels. The 50mmL isn't alone in this regards, the Nikon 58mm displays heavy longitudinal chromatic aberration in bright back-lit conditions as well. In general purpose shooting, this puts the Canon at a disadvantage against the Sigma, as you have to be more mindful of avoiding the weakness of the lens.
Given the price difference and the difference in overall sharpness, most people will go for the Sigma over the Canon. However, the 50mmL has a subtle way of rendering a scene that some people will find more appealing than the Sigma. In the case of portraiture and people photography, the Canon will be more forgiving with skin textures, bu the Sigma will pull in more detail in the eyes and hair. The amount of detail that the Sigma generates is formidable, even with the lens held wide open.
On a canon full frame system, 50mm is not a priority for the majority of people. Most will opt for a normal zoom like the 24-70mm f/2.8 first, whereas the more specialized photographers will go for longer (portrait length) or wider (landscape). Both the Canon 50mm f/1.2L and the Sigma 50mm ART make for a compelling case, as the Canon produces gorgoeus-looking bokeh that a standard zoom can't, where as the Sigma simply trumps all other lenses in terms of resolving power. Both lenses are keepers,. but the lower price of the Sigma makes it easier to have.
With thanks to Broadway Camera.