Though primarily considered a portrait lens(it is not a preferred portrait lens to quite a few photographers - it's more of an environmental portrait lens where you put more of the environment in the photo and leave the people at a distance for esthetics), I find that I use mine for anything but portraiture. There have been people complaining about the fact that the f/1.8 is rather soft wide-open. Frankly any lens is soft wide-open. You're letting in a lot of light and you're going to sacrifice image quality for speed, but nothing that can't be sharpened via post-processing.
Doing a portrait of my shooting buddy Toothy here. you'll find the shots (both full-size and crop in both shots) at both f/1.8 (wide open) and at the sweet-spot (the tipping point between sharpness and speed) of f/8 for this lens.
Full size shot at f/1.8
cropped in shot (100% at f/1.8)
Full size shot at f/8 (the 50mm f/1.8's sweetspot)
cropped in shot (100% at f/8)
So as you can see, there is a noticeable difference in image quality between wide-open and the lens' sweet spot. However it isn't much really to worry about.
Where you are going to be shooting wide-open is usually a portrait shot in dim light and then you are more than likely going to be going for an ethereal glow as opposed to a straight out studio shot portrait in terms of sharpness. If you need it any sharper, then go studio lights and stop down to f/8 or post-process in Adobe Lightroom or Portrait Professional.
The main reason why the 50mm is not preferred as a portrait lens is as follows: (once again using Toothy as the model)
Toothy at 50mm (notice how Toothy's face is rather flat, wide and unappealing... - OK a shark isn't all that appealing any way you look at it, but hey...if you really want to make a shark's face scary, use a 50mm)
Toothy at approximately 105mm (notice how Toothy has suddenly looked like he dropped a few pounds and is now looking like a svelte shark?) - shot with a 70-300mm f.3.5-4.5 lens.
That's the effect of a wide angle versus a telephoto on portraiture.
Don't ask me to do portraiture with models. I'd rather be slathered with honey and fed to a hungry grizzly.
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