Originally posted by Pål Jensen
Stricly speaking it is speed. You may want to shoot in lower light with your APS camera. In such circumstances it often a plus to have more DOF at the same exposure. As APS have more noise at higher ISO than comparable FF sensors, you can shoot at one stop lower ISO with APS than with FF for the same DOF. Hence, such a lens may reduce FF's high ISO advantage for low light shooting.
Unless the FF also uses a faster lens. And FF also has other advantages.
It's all relative.
The real issue is ISOless sensors and in-scene ISO manipulation. If sensors continue to improve on the ISO front most of the fast glass advantage will be nixed in favour of DOF shallowness. And that will have its limitations, mostly economic.
Imagine the tech where you would need no lens less than f/5.6 in low candlelight and the exact same DOF aesthetic can be achieved solely through instantaneous in-camera pixel manipulation and each section of the frame mapped differently for ISO read-out. And there will be no way to tell the difference pixel peeping. None.
And the FF version of that same camera can do it at f/8. On a CDAF system with almost perfect AF alignment on the pixels you want in sharpness. You'll get f/8 sharpness across the frame with software, not optics, controlling the bokeh, OOF, and aesthetics.
And once all data runs through the ADC and subsequent RAW processor, no one will be able to tell what was optically derived from software derived. You're f/8 lens will simply read your f/0.85 dial setting as a DOF attribute and blur/bokeh accordingly.
That's where we are headed, and why very fast glass like the Sigma is perhaps not a long-term investment. In the interim that's why primes at f/2.4 and 3.2 don't bother me as investments because as the sensor tech curve continues small,, not so fast lenses, stopped down only slightly, will have all the same attributes as a much costlier, more expensive, and much heavier lens.
In the meantime buy the Sigma and work on your "pro" technique until algorithms put the pro technical stuff out of biz, just as most pros in photography have seen their business evaporate. It's sad but a simple economic function of the technology.