4) Lens Features
When it comes to lens features, the Sigma 14mm f/1.8 Art is perhaps one of the most advanced lenses Sigma has made to date. First, the lens features the largest 80mm glass mold in the industry, something the company is very proud of. Thanks to its complex optical formula comprising of a total of 16 elements in 11 groups (most of which are corrective elements), the Sigma 14mm f/1.8 Art produces images with very little distortion – something most other lenses in its class struggle with. To reduce various aberrations and produce exceptionally sharp images, Sigma engineers used a total of three “F” Low Dispersion (FLD), four Special Low Dispersion (SLD) and four Aspherical glass elements in the design. The new “F” Low Dispersion glass is supposed to be as good as calcium fluorite glass in terms of its refractive index, which is pretty impressive. Next, the lens is equipped with the latest generation Hyper Sonic AF Motor, which works quickly and quietly when relying on autofocus operation. Similar to other modern lenses, the Sigma 14mm f/1.8 Art features a rounded 9-blade diaphragm. To reduce ghosting and flare issues, the lens is coated with Sigma’s proprietary “Super Multi-Layer Coating” (see below on ghosting and flare performance of the lens). Lastly, the Sigma 14mm f/1.8 is compatible with the Sigma USB Dock, so if you have any issues with its AF accuracy, it is something you can calibrate yourself without having to send the lens to the manufacturer.
5) Lens Sharpness and Contrast
Considering all of the above and the fact that Sigma has been making some stunning Art-series lenses, it is obviously expected for the lens to deliver exceptional sharpness…and it certainly does! I was very impressed by my field experience (especially once I figured out how to focus with this lens) and my lab tests with Imatest confirmed my findings as well – the lens produced very nice sharpness results, even when focusing at close distances:
As you can see, the lens is capable of superb sharpness wide open and it produces very impressive results when stopped down to f/4, where it shows best overall performance. However, keep in mind that the above numbers are what I was able to achieve after getting my focusing figured out. Due to the field curvature of the lens, you must be careful about how you focus with it. Using live view contrast detection and placing the focus point in the center frame might yield even better center sharpness than the above, but at the cost of the corners. Take a look at the below graph, which shows what happens when you do that:
The center sharpness in this case peaks to very impressive numbers at f/4, but the corners become blurry as a result. In this particular case, I focused at the middle section of the test chart at f/1.8 and then took shots at different apertures. Wondering what kind of a difference one would see in real life? Take a look at the two crops below, from the left bottom side of the frame (as seen from RAW image, no post-processing applied):
This is what a corner looks like at f/4 when you focus well, as I did for the first graph of this review. And now take a look at a crop using same aperture, except when I focused in the center of the frame, as shown in the second graph:
That’s a pretty big difference, wouldn’t you agree? If you use this lens and see blurry corners when stopped down, you might need to tweak your focus a bit via live view, so that both the center and the corners look good!
Keep in mind that this behavior is not limited to shooting test charts at close distances. I observed the same behavior at longer distances – even at infinity. For example, take a look at this image, which I focused on correctly:
And here is the extreme corner from the bottom left:
Looks nice and sharp right? Well, take a look at the second image, which I focused on incorrectly this time – only used the center focus point and didn’t pay attention to the edges in live view. First, the full image:
And here is the same size crop from the bottom left corner:
Yikes! Both images were shot on a stable tripod, with a shutter speed of 10 seconds, so there is no chance for other variables that could have impacted the sharpness here. The two images look very sharp in the center, but the corners look drastically different, even at f/8!
When it comes to contrast and color, the Sigma 14mm f/1.8 Art does not disappoint – it is certainly excellent, as expected from a premium Art-series Sigma lens.
I am not sure what the appeal of out of focus areas would be on such a wide-angle lens – you would have to stand extremely close to a subject to get anything to look blurry in the background, at which point your subject is going to appear very distorted. Here is an image that I captured wide open at f/1.8, just a couple of feet away from the subject (lens correction applied in Lightroom):
That’s a pretty expected result…background highlights are going to be tiny and barely visible in your shots.
Any fast lens is going to vignette at larger apertures and the Sigma 14mm f/1.8 Art is not an exception. As you can see below, there is definitely some visible vignetting at f/1.8 and f/2:
Stopping down the lens to f/2.8 reduces vignetting, but not significantly – you will need to stop down to f/4 and smaller to get the best results. Here is the worst case scenario, shot at f/1.8, infinity focus:
Personally, I find vignetting to be a bit too extreme for my taste at f/1.8 and f/2, especially considering that this is not a portrait lens. If vignetting bothers you, it is very easy to remove it in post – Lightroom already has a built-in profile for the Sigma 14mm f/1.8 Art, so you can remove it with a single click via the Lens Corrections sub-module.
8) Ghosting and Flare
When shooting this wide, ghosting and flare are rarely an issue, because the points of light are so tiny in the frame. Unless you have a huge light source right next to your camera, you should not worry about it in your images. To make sure that the impact of ghosting and flare is minimal, Sigma coated the lens elements with its Super Multi-Layer Coating, which certainly does make a difference. Take a look at the below image, with the sun in the frame:
As you can see, the lens handled it very well. The worst case scenario I could come up with, was when shooting with the sun just right out of the frame, as can be seen in the below image sample:
Even then, traces of ghosting and flare are very minimal, which is great!
Sigma claims that the 14mm f/1.8 Art exhibits “virtually no distortion”. While I certainly agree that the Sigma 14mm f/1.8 Art produces the least amount of distortion I have seen on an ultra-wide angle lens, it certainly has some, especially at closer distances. Imatest measured 2.34% of barrel distortion, but that’s nothing compared to the amount of distortion we see on many other lenses, such as the Nikon 14-24mm f/2.8G (4%), Nikon 16-35mm f/4G VR (5.9%) or other third party ultra-wide angle lenses such as the Samyang 14mm f/2.8 (4.6%). Hence, if one compares distortion to other ultra-wide angle lenses, the Sigma 14mm f/1.8 Art is without a doubt the best of the bunch.
Take a look at the below image, with and without distortion correction applied in Lightroom:
As you can see, it is very minor – mostly towards the edges of the frame.
10) Chromatic Aberration
When it comes to lateral chromatic aberration performance, the Sigma 14mm f/1.8 Art exhibits a minimal amount of it at all apertures, which is pretty impressive:
I guess all that FLD, SLD and Aspherical glass in the lens help a great deal with keeping chromatic aberration under control. You might see some signs of it at the extreme edges, but it is very easy to fix via Lightroom’s lens corrections sub-module…
While the Sigma 14mm f/1.8 Art is perhaps one of the best lenses for astrophotography out there, it is definitely not free from coma. Take a look at the below crop from the top left side of the frame, shot at 5 second shutter speed to reduce star trails:
There is definitely some visible coma at the widest aperture at the edges of the frame, something we normally see on most lenses out there. Unfortunately, coma stays there at f/2, but gets drastically reduced as you stop down. By f/2.5 it is practically gone, but if you have very bright stars, you might still see some traces of coma at the edges of the frame.
Let’s now move on lens comparisons.