I have an APS-C camera and want a lens that will achieve a normal 50mm look. Do I buy a DX 50 mm or get the FX 35mm? Will they have the same effect?
Do I buy a DX 50 mm or get the FX 35mm?
That's up to your preference.
Will they have the same effect?
A 35mm FX lens or a 35mm DX lens will have the same effect with your camera.
A 50mm FX lens or a 50mm DX lens would have the same effect with your camera. (Though I'm not aware of anyone who makes a 50mm DX prime lens for the Nikon F mount.)
50mm is 50mm and is a property of the lens. The angle of view changes due to the sensor size. It doesn't change the focal length of the lens.
Any 50mm lens on your camera will have the same narrower angle of view than any 35mm lens on your camera.
If you want the same angle of view as a 50mm gives with a FF camera, get any 35mm lens, FX or DX.
All lenses one uses on an APS-C camera, both FX and DX, will give a narrower angle of view than when an FX lens of the same focal length is used on a FF camera.
A 35mm lens on an APS-C body will have the same view angle as a 50mm on a full-frame body. Since you mention DX, I assume you're using Nikon. The Nikkor 35mm f/1.8 DX lens is sharp but has noticable barrel distortion which can be reduced in post processing. If you want a narrow depth of field, this will be an equivalent of about a f/2.8 lens on a FF body, one of the drawbacks of a crop sensor camera.
BTW, focal length doesn't change when going from DX to FX, i.e. a 50mm DX lens will give the same view angle as a 50mm FX lens. The difference between DX & FX lenses is the illumination area at the sensor - plus the DX lens will be lighter since smaller area glass is used.
It depends what you mean by 'normal 50mm look'.
If you want to fill the frame the same as an FX body with a 50mm lens, then you need the 35mm or close equivalent.
If, however, you want it to look like you took the photo on a 50mm & then cropped it, then you have to have the actual 50mm.
Personally, I'd stop thinking in terms of 'normal on an FX' & concentrate on what you get from an APS-C body. You get used to how it frames after a while & that becomes your 'normal'.
The 35 is always going to look more 'pointy' than the 50, big nose, little ears, because you have to stand closer to get the same framing. In a choice between the two I'd always go with 'use the 50 & take a step back'.
I have the DX 35mm 1.8 & the FX 50mm 1.4 & they are chalk & cheese [& a 4x price difference too]. The 35 has become just extra weight in the bag a lot of the time. It gets used if, & only if, I can't stand far enough back with the 50 & also am willing to sacrifice the shallow DoF potential. You can't get the 35 anywhere near as soft as the 50 if you're trying to isolate a subject from its background.
I'd had my 50mm 1.4 for quite a while & got the 35mm 1.8 for a specific shoot where I knew I wouldn't have the room. I assumed the 35 would be almost as shallow as the 50. I was wrong.
You can test the framing using the kit lens, but you can't see the shallow DoF potential on it. For that you need the real thing.
"Normal" occurs when the recorded image is ~ 55˚ (diagonal) FoV and is then displayed so as to occupy the same ~55˚ of your FoV... the typical/standard image size/viewing distance; which correlates to the human's primary/central 60˚ FoV. This means there is no stretching/compressing during the recording/display/viewing.
But "normal" also occurs when the subject distance is appropriate at the time of recording... and this is actually the more important aspect. It creates a normal/relaxed viewed image of monocular cues/details suitable for a 2D image and scenes more distant than ~ 10ft (or, arguably, scenes at any distance). I.e. the primary factor that makes the 35mm "normal" for an APS sensor is that it forces a normal distance/perspective in order to record the ~55˚ FoV (w/o cropping).
So, you can use **any rectilinear lens with a projected/recorded FoV is ≥ ~55˚ from a comfortable/normal distance... if the recorded FoV is wider you would just need to crop it for standard/normal display.
**disregarding any lens specific distortion such as barrel/pincushion
Depends on what you mean by "a normal 50mm look": field of view or magnification or both together.
On a full-frame (135 format) camera, a 50mm lens is actually "normal" in two different ways: both in field of view (scene coverage), and in magnification. I.e., 50mm is very close to the magnification of the unaided human eye. If you shoot with both eyes open with a 50mm-ish lens, the scene will match through in both eyes, regardless of whatever size sensor the camera is using and composition becomes simply framing, rather than framing and translating for lens magnification.
On an APS-C/crop sensor, you can only have one or the other. You can use a 50mm lens and get normal magnification, but you'll have a tighter than normal field of view. Or you can use a 35mm lens and get a normal field of view (i.e., focal length matching diagonal dimension of the sensor), but wider-than-normal magnification, as well as the slightly different rendered depth of field and distortion that comes with shooting with a shorter lens.
Whether the lens is DX or FX doesn't change either of these factors. It just changes the size of the image circle the lens projects so that it covers the entire sensor out to the corners. And if you're shooting crop, both DX's circle and the larger FX circle will cover the entire sensor. You really only need to worry about this if you're shooting an FX body.
But, if you really want the "normal 50mm look" of a 50mm on a full frame camera, both FoV and magnification, then you need a 50mm lens on a full frame camera.