It's the widest by FOV, which is probably the more relevant metric--assuming, that is, you care more about the final product than you do about arbitrary numbers. (Never a safe assumption about DPreview members, of course...)

Certainly going to be of interest to photographers of the night sky. Especially aurora shots where you don't necessarily want the shutter open for a long time (so the benefit of a wider angle for longer shutter times won't help).

Its not f/1,4 for bokeh, but for faster shutter speed. An aperture of 20/1,4 = 14 mm is not much in terms of bokeh anyway.

I would love that lens for shooting aurora and to get nice star reflections at night even when the water is not completely still.

Well, no need to look further, someone else actually did the job, and it looks is just as bad as the Canon. Greg VdB posted this one a few hours ago: http://www.lenstip.com/457.7-Lens_review-Sigma_A_20_mm_f_1.4_DG_HSM_Coma__astigmatism_and_bokeh.html

Thanks. Yeah, I use a Rokinon 14/2.8 on my Sony a7r2. What is the benefit of a 1.4 with astrophotography? I'm very curious.

More light?

Handheld night photography, and live music. At the venues I tend to shoot, my normal setting is ISO 8-10,000 at f/2 to get the shutter speeds I need. Being able to stop a lens down as opposed to shooting it wide open is very handy for image quality. There are certainly those of use that *need* fast lenses. f/2.8 is too slow for my needs. f/2 tends to be fast enough but, again, it is nice to not have to shoot wide open.

Great for sports too where you can get close. I have great shots of basketball for instance. Night life, event photography.... There are so many excellent applications, just not obvious at first. I really needed get one and start using it to really get how creativity got to a new level for me. Quite a killer combo with IBIS if your camera has that.

@Robert Holloway
4 times as much light? The ability to also see in the EVF so much better in a low light scene? From 2.8 to 1.4 the difference is huge! I managed to be able to manual focus at EV -6 and take a shot free hand 1 second exposure at only ISO 5,000. Moon lit shots hand held with ISO less than ISO 2,000?

+ weddings and other events in dark interiors.

You seriously can't figure out why F1.4 would be useful for trying to take pictures of the freekin stars?

Astrophotography - but the coma is pretty pronounced, so that'll be potentially annoying. But, then again, F1.4 for stars is really, really useful, especially given how tack sharp and contrasty the lens is in the center. Uncannily so.

Live concerts/music, where the extra distortion won't be so offensive.

But weddings/events... I don't know, I personally wouldn't go wider than 24mm b/c of the linear distortion near the edges. It's just too much, and you have to keep trying to reframe to not get people near the top, bottom, right, or left (or asking people to get out of your way, if you don't want them to look stretched out). I think that'll limit the utility of the lens in those use-cases, as exemplified by my people shots at the end of this gallery. Some unflattering distortion of people at the edges.

I'm definitely curious to see what people produce with this unique lens.

I doubt the lens is great for stars wide open. Stars really like to show off all the optical defects in a lens. But for night time lapses where you don't scrutinize the details the speed could be very useful.

In a few weeks they're having the Chinese New Year Dragon Parade in San Franciso--at night. World largest illuminated dragon, with a hundred people holding it. You're up close to a huge dragon, in the middle, with action and no light. The perfect lens.

Remember. This is a sharper than the best stopped down, so you have the best of both worlds.

I also wonder about its use in environmental portraiture where one does want subject isolation. Framed with a single subject in the center, with a wide FOV for a real dramatic view. Though a lot of the time, light is used for subject separation and the lens is stopped down so the environment is actually in focus/sharp... but sometimes blurred out backgrounds in environmental portraiture is very nice, in which case the F1.4 will be indispensable. Especially given how hard it is to isolate a subject a bit further away at such a wide focal length.

Also, band portraits?

Astro etc, but nice to get wide shot portraits with genuine narrow DOF.

Fast wide angle lenses are useful for filmmaking. In tight spaces where you would use lenses as wide as this, it's difficult to fit much lighting into the available space.

Everythingis1

No I can not figure out why f1.4 on a 20mm is useful. I have f0.95 on another lens, but the focal length is not wide. I have f2.8 at 14mm. I honestly can't see a reason to buy a lens like this.

I thought the sample gallery was technically fine, but saw nothing from a creative standpoint that said, i need a 20/1.4

To be clear I'm not saying it's a bad lens. I honestly don't see a use case.

I am with Robert. The lens is also huge as are the other F1.4 Art lenses.

Back in the days of film Olympus used to to make "two of everything" at wide angle with their Zuiko lenses.

They had a 21mm F2 and a 21m F3.5.
24mm F2 and F2.8
28mm F2 and F3.5 (later replaced with an F2.8)
and 35mm F2 and F2.8.

I owned the 21mm F3.5 and it was tiny with a 49mm filter thread. As did the 35mm F2.8 I also owned.

These days with ever improving noise performance from sensors I'd like to see an equally small set of lenses such as those F2.8 and F3.5 Olys. They would go really well with the Sony A7 size of FF body for these who don't need the speed.

Instead despite better sensors that require less light for good performance everyone seems obsessed with F1.4 lenses.

The Lenstip review has some good test shots showing coma on FF:
http://www.lenstip.com/457.7-Lens_review-Sigma_A_20_mm_f_1.4_DG_HSM_Coma__astigmatism_and_bokeh.html
Bad at f/1.4, but the situation improves quickly on stopping down.

We always try to shoot lens galleries at low ISO since the point is to show what the lens is capable of and not the camera behind the lens. For camera sample galleries, however, high ISO is fair game.

That's a weird comment. Canon sensors have a relative weakness (that's irrelevant for most people) in dynamic range at base ISO, but from about ISO 800 up they are as good or better than any.

Irrelevant for you perhaps. Canon has fantastic ergonomics,build quality and some killer lenses, but their sensors flat out not as good as Sony and Nikons. Perhaps good enough for you and others, but they are just not as good.

weak troll is weak

Troll are trolls:
show me how the 6D is any worse in high iso than the D750. Considering that it is a 3 years old sensors. Chan you read chart?
http://www.dxomark.com/Cameras/Compare/Side-by-side/Nikon-D750-versus-Nikon-Df-versus-Canon-EOS-6D___975_925_836

Df pulls away at pixel level, but at normalized resolution there is virtually no difference among D750, Df, and 6D.

Actually, Canon is worse at low ISO. But this should not stop you from bashing Canon.

There is no DR difference at high ISO. Only low. Canon and Sony sensors will look like equal hot garbage if you are pushing shadows on ISO 10k files.

@FastGlassLover: you are parroting things that you do not understand. High ISO Canon RAWs look no worse than any other brand with lifting shadows (meaning, they all look like garbage).

@FastGlassLover
actually the shadow recover issue is there only for low iso, it is just as good at high ISO. I am not sure where you are getting that.

"Until ISO 10,000 proved to be too dark still and you find yourself needing to lift the shadows or exposure a couple stops. Then Canon files look like garbage."

Please, do change your story again.

@ FastGlassLover
are you one of the many out here insisting on without hard evidence? Get down to the specifics: cameras conditions, optics etc to say the least if not give us examples. I am having enough of pompous trolling like this.

What does this tell you about actual ISO 800 shots? That shot is pushed in LR by 2 stops, so it is actually exposed as an ISO 800 one (for this brightness)! Also, shadows are pushed by +20 in LR.

Can we have some sanity here? Read noise at low ISO is no indication of performance at high ISO.

@misspiggy01
boy you are all over the places then.
1) Argument was not clearly stated "canons" is definitely not just the 5Ds
2) This is the highest resolution FF camera, compare normalized noise and level of details. Besides, this is not marketed as a high iso camera.
3) Your argument was about high iso embarrassment, now you are talking of low iso samples?

As JACS well put you cannot really judge fro these pictures either anyway. I.e. the D750 grain looks horrible in the pictures in the following article because of the post processing applied http://www.dpreview.com/articles/3609990856/dpreview-gear-of-the-year-2015-nikon-d750

True. A lot of people don't realize the complexities of lens performance. They also don't consider that humans (the thing holding the camera) tend to move after focus is locked, or field curvature, etc etc.

This is a specialty lens. If you do a lot of tight crowd/event shooting indoors, then this is the lens.

I think my 16-35 F4 IS will be fine for my needs though.

We specifically avoid applying corrections to these images in Photoshop because the point is to show what the lens is capable of producing on its own. It's true that you can often improve on this in Adobe Camera Raw, so we usually provide Raw files for download if you want to apply corrections on your own.