Sigma and Canon 24mm - Any comparisons?
 

The Sigma is 2.8 and the Canon is 1.4. The price difference, in Japan, is about 1500USD with the Sigma at 600USD and the Canon at around 2000USD. In low light, a 1.4 of any lens is great, but for those that own one of these two lenses, can you see a real difference in quality when you stop down the Canon to 2.8?

The Canon is fairly soft wide open at 1.4, but still useable, especially for video. When the Canon is stopped down to 2.8 it is sharper than the Sigma wide open at 2.8, although The Sigma creeps back performance from F/4 and matches it by f/5.6.

I prefer the Sigma EX AF f/1.8 DG Macro lenses (I have used both lenses extensively). Both the 24mm and 28mm f/1.8 versions are superb. The build quality is not as good as the Canon, and slightly soft wide open, but pulls in performance quickly from f/2.8 and ultra-sharp between f/4-f/16.
The Sigma lenses hold a huge advantage in their close-focus facility which makes them great for tight shots where the subject is close to the lens, or for macro stills/video where you want to show a lot of the background to provide a different view of a normal scene. These images were taken on the 24mm F/1.8 Macro, but the 28mm version offers similar performance (please note that these are only low rez jpegs uploaded via my mobile phone. The original high rez Tiff files are very sharp and high quality.):

Lens comparisons find me a bit confused with video in mind.

How much shows up with the line skipping? When I look at a lens to buy for still work, there is so much more detail being recorded. Does it transfer?

I would like to see if video from these two lenses differs very much or from any combination of expensive & affordable glass.

Sure distortion, CA and color/contrast change, but I am thinking more in terms of detail and overall image.

It all depends on which aperture is set, but even with video the better quality the optics the better the moving image quality. Of course stills are studied singularly at close range, frame by frame, where as video footage is generally viewed further from the screen and many frames per second. 1080 moving video is still a long way in image rez from a full 21MP single image.
Most modern lenses from Canon, Nikon, Pentax, Tokina, Sigma, Tamron etc set at mid-apertures f/5.6-f/8 would be very close in the sharpness stakes and be very difficult to tell apart if used for same video footage. Then colour depth, fringing, contrast, build quality, IS, flare and other values need to be considered.

When the ultra-wide lenses are compared for stills corner sharpness is a big divider.

I wonder with the much lower output resolution if these comparisons are still noticeable?

I have to apologize...I posted in hast. The Sigma is f1.8 not 2.8.

As far as stills are concerned (As I'm coming from a mostly stills work platform) I see very little difference in the 2 to warrant such a large gap in price. When I owned my Nikon D700 I had the 24-70 Sigma at less than half the price of the Nikon and still the quality held up against the Nikon. Video is different I'm sure and that's why I ask. PP done on a single photo is easy compared to video done over the course of a day...

I have that Sigma 20mm 1.8 and like it a lot especially the macro, but I'm using it on the 7D crop camera and so I see no soft corners or vignetting that came from reviewer that tested it on full frame, and so you might want to try it out at the store first.

While it makes total sense to me that general and corner sharpness is not as crucial in the video realm (video crops part of the corner anyway), my impression is that bokeh is one thing that isn't going to get any better by downscaling from 21mp to 2. Given the fact that shallow DOF is the real strength of these cameras I think high quality bokeh is something worth paying a few extra bucks for.

Bokeh is subjective and I wouldn't pay a cent/penny extra for it. There are other far more important factors why I choose each lens.

I had a 15mm Sigma 2.8 prime that I sold to get a more expensive Canon 17-35 2.8 L lens -- but I must say (without doing an a/b test) the Sigma seemed to be sharper than the L lens.

-- peer

I have the 17-35mm f2.8 L lens too. The Sigma has sharper edges at 15mm than the Canon at 17mm, but the Canon is very good from 20mm to 28mm (where most of my work with this lens is done) and the Canon does have the nice option of going 17mm extreme wide when you need it and zooming to 35mm for slightly tighter shots/footage. But if you only need a fixed extreme 15mm wide, then the Sigma is a valid option; although the Canon 15mm is also a decent lens. The Nikkor 15mm and Pentax 15mm fixed primes (I've owned both in the past) are superb options if you don't mind using an adapter.