Is a 50mm f/1.8 or 50mm f/1.4 a good lens for portraits in low light?

The 50mm f1.8 (or the 'nifty fifty' as both this example and Nikon's version are often known) would be a great step up for portraits from the kit lens, regardless of low-light or not. In other words, it's a good first portrait lens AND works well in low light.

I got one at Christmas last year, and have very much enjoyed learning how to shoot differently with it compared to the kit.

You will notice a huge diffence with the 1.8 compared to your kit lens.

As a former owner of the Canon 50mm/f1.8 lens, and a current owner of the Canon 50mm/f1.4 lens, I can say the most noticeable difference between the two is build quality. I was very pleased with the photos I took with my 50mm/f1.8. I didn't replace it until I dropped it (from about 4 feet onto soft ground), and it broke in two. I then decided to upgrade mainly as a safeguard against future breakage. The 50mm/f1.4 is clearly much more solidly built, but it costs about 2.5x the money. Either one works very well in low light (although the f1.4 obviously has a slight edge here).

If you're starting out, I wouldn't hesitate to buy the 50mm/f1.8 Canon lens.

Whether it's worth the extra price for the f1.4 lens probably has more to do with how gentle you are on your equipment than on the noticeable optical differences.

Update: The EF 50mm f/1.8 STM introduced in 2015 gives you most of the best of both the older EF 50mm f/1.8 II and the EF 50mm f/1.4. It's build quality is much better than the EF 50mm f/1.8 II, the optics are as good as the EF 50mm f/1.4 at f/1.8 and up, and it's as cheap as the EF 50mm f/1.8 II was.

I have both.

I regret buying the 50mm f/1.8 after seeing how well built the 50mm f/1.4 is.

Like Flimzy, I dropped my lens together with my camera, the difference is that the one I dropped is the 50mm f/1.4, it did not break. Nothing happened to it, I picked it up and it just work.

On a side note, the bokeh is far better on the 1.4.

The site featured image from November 21, 2011, Jelly Fish, is shot by me, with the 50mm f/1.4 lens.

I think its well worth its price, while the 50mm f/1.8 is cheap, having both I would say I could have saved $100 for something else if I have bought the 50mm f/1.4 the first time.

One thing I would love is IS on this lens.

I personally have the 1.8 and my friend the 1.4. Obviously the 1.4 is much better build quality and fairly better optically, but the 1.8 is a bargain and still a good lens as long as you don't plan on throwing it around. Also more easily replaced if it breaks. Both give pleasing pictures and both will be better in low light than your current lens... but..

..in your example you give these lenses would not improve the picture in the way you want. The christmas tree would still be the source of light and the people would still be underexposed in front of it. The lenses would both make the ability for faster shutter speeds or lower ISO's, but the lighting ratio in your picture would still be the same.

To get the picture you are after you would still need some illumination on your subjects to expose them better with the tree. As it is dark a reflector wouldn't be much use, so it's more likely you will need to use some flash, but don't put it on auto. Use your camera (I'd prefer in manual) to expose for the tree, and then use the flash as fill light, probably dialling in some flash exposure compensation of -1 or -2 stops so that the light is mostly only lighting their faces and not affecting the already lit background so much.

It'll likely take some tinkering to get the right ratio of lighting that you want, but using a faster lens is only going to mean more bokeh (which will be nice for the tree potentially) and the ability to shoot faster, it won't magically bring your subjects out of the low light whilst leaving the background as it was.