I recently got a nikon d3400 with 35mm 1.8. I want to get an upgrade lens thats good with low light. Should i get 50mm or sigma 17-50

I recently got a nikon d3400 with 35mm 1.8. I want to get an upgrade lens thats good with low light. Should i get 50mm or sigma 17-50

The 35mm f/1.8 is excellent for low light because the maximum aperture is f/1.8. This lens is optically very good except for distortion. It would be hard to "upgrade" this lens since it is pretty close to the top in the DX lineup.

The Sigma 17-50mm lens has a max aperture of f/2.8 which means it is 1.3 stops slower (worse for low-light situations), but is a zoom lens with a very useful zoom range. This lens is better than the 18-55 kit lens because it is faster (better for low light than the kit lens) and optically better.

The 50mm f/1.8 lets in the same amount of light as the 35mm lens, so, won't be any better in low light situations. It does have a larger focal length by 1.4 times over the 35mm lens which means things will appear closer.

Lens choice is really up to you since you may have a different shooting style than other people.
Personally, I would go with the 17-55mm because zoom lenses are convenient to use. As you have probably discovered, prime lenses are optically better than a zoom, but they aren't as convenient since the zoom function is with your feet.

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You realize boke is the quality of the out of focus region, not selective focus. The 35mm and 50mm boke is nothing to do back flips over, kind of average if you're nit picking. However, for most people, the 35 & 50mm boke is good enough. Good boke would be had in something like the 85mm f/1.8 lens.

I think you are after selective focus. Longer focal length lenses are better at creating selective focus (narrow depth of field) which is why the 35mm lens has a hard time with selective focus.

Another thing is sensor size in your camera which is APS-C. Larger sensors, like a full-frame, are better for selective focus. Aperture, as you probably heard, is also important. Shooting at a wider aperture, like f/1.8, will give a narrower DOF than f/2.8.

Setting up your shots is important when trying to do selective focus. Your subject should be close to the camera (not too close where perspective distortion come in to play) and the background should be as far away as possible.

If you are trying to do portrait shots, then the 50mm f/1.8 or the 60mm f/2.8 macro on an APS-C camera will work well for portraits. Using the 85mm lens often gets you too far away from your subject.

Since you are going to China, a general purpose lens like the 17-50mm or 18-140mm would probably be better. If you're only shooting portraits, I would go with the 60mm macro so I could take close up shots of flowers and bugs.

Tell you what.

If you can't get decent shots consistently in low light with that f/1.8 lens, you never will even with a $23M Carl Zeiss Planar 50mm f/0.7!

Your 1.8 lens should be great with low light, as the only wider apertures available are 1.4 and rarely 1.2.

What types of scenes are you shooting in low light? Makes no sense to recommend a 35mm f/1.4 if you're shooting things far away such sports in a school gym.

Your 35mm f/1.8 is already very bright. A 35mm f/1.4 will only give you an extra 1/2 stop of light. But a lens that has an aperture of at least f/2.8 AND with VR will be a very good improvement on your already fast 35mm f/1.8. Vibration reduction (VR) will allow you to shoot hand held for an additional 3-5 stops. This means that you can avoid blurry shots caused by your movement (ie camera shake), but it will not help with blur caused the movement of your subject.

Having a fast lens like your 35mm f/1.8 is a great option for low-light photos, especially when you need fast shutter speeds to stop the motion of your subject. But having a fast lens AND vibration reduction is the best solution.

You are using a quite good prime lens that does not need an upgrade. If you want to add another quite good prime lens of a different length, the Nikkor 50mm f/1.8 is a good choice. It will give you a little more length. Adding fast primes as you want them is a good strategy, such as adding an 85mm, then a 105mm, etc. Some folks, myself included, prefer a single, walking around lens, so we choose a zoom lens such as the Nikkor 18-140mm. We give up the faster lens, and shoot higher ISO, for the convenience of a single lens. If you don't mind changing lenses, the 50mm is the way to go.