One of the headline features of a full frame camera is the extra amount of bokeh that you can achieve at a given aperture. That is to say, the depth of field will narrow by the equivalent of one stop compared to a DX system. There's more to good foreground/background isolation and composition than outright shallow depth of field, but let's be honest.... a creamy obliterated background is a satisfying thing. To that end, here is what the bokeh looks like as you go through the aperture range on a Nikon AF-S 50mm f/1.4G mounted on a D610:
At f/1.4, the background is almost completely obliterated; this is one stop past what any DX Nikon can achieve. Compared to the older 50mm f/1.4 D, the G version has less spherical aberration and produces a less nervous-looking bokeh. Note the coma aberration in the periphery of the frame, with the light sources stretched out into football-esque shapes. If you want better, then the AF-S NIKKOR 58mm f/1.4G is the ultimate in this focal length. However, as the the aperture narrows down, point light sources becomes more spherical and almost perfectly circular by f/4.
The f/2 example is not that far off from the f/1.4 shot. For not much worse in background bokeh, the rendition of the foreground is a bit sharper. The lettering and detail on the lens-mug is sharper, and in real-world use, the usable plane of focus is a tad bit more forgiving. Also note that the vignetting in the wide-open shot is much less pronounced at f/2. It's tempting to think that a DX camera would replicate this at the equivalent factor of f/1.4 and 35mm, but you would have to use either the Sigma or Nikon full frame lenses to do so. Nikon's DX 35mm does not open up to f/1.4, and Sigma's 30mm f/1.4 lens, while having decent bokeh, isn't as consistently sharp across the frame.
The f/2.8 sample would be roughly the equivalent of the Nikon 35mm f/1.8G AF-S DX used wide open on a crop frame DX D7100... except that it isn't. The 35mm DX has harsh fringe-y bokeh whereas the 50mm gives a consistent background rendition. This is the other reason why full frame gives better bokeh: it's not the amount of background blur that makes for a pleasing image, it's the quality of the blur. To that end, even though there are good DX lenses out there, FX lenses are usually built to a higher specification.
Going further into the aperture range, even though there's a bit of shake in the f/5.6 image, it's evident that the detail potential is there. Almost all of the Nikon 50mm lenses reach peak resolving power at f/5.6 to f/8.
50mm primes are often looked over by working pros because their priorities are often elsewhere. The AF-S 24-70mm f/2.8 is found in many kits, and for good reason. As you can see from the samples above, f/2.8 already provides workable foreground/background separation, and on the 24-70mm, the versatility of the zoom range outweighs the extra two stops of bokeh that the 50mm primes gives. However, as a general purpose lens, the 50mm f/1.4 G gives excellent results for the price; this isn't a "gold-band" lens, but given the consistency of its optics, it's almost like one. Remember: the price jumps by $1,200 USD to get to the next comparable offering in the Nikkor lineup.
With thanks to Broadway Camera.
With thanks to Broadway Camera.