DX vs FX

[Alexandra Hall]

So I am looking to get a prime lens but I am a bit confused. I have a Nikon D3000 which has a DX sensor. I am looking to get either the 35mm 1.8 AF-S DX or the 50mm 1.8 AF-S FX

Since the 35mm is a DX lens does that mean it will be a true 35mm on my DX camera or will I still have to multiply the crop factor which would give me closer to a 50mm? Because I want a 50mm focal length but I know the 50mm FX will be closer to 75mm.

Also, of these two which would you recommend for my D3000. I am getting into weddings and family portraits. But mostly family portraits at the moment.

[OsmosisStudios]

Focal length is focal length: it doesnt change.

What happens is that if you put a 50mm lens on a DX body, what you see is closer to 75mm. if you put it on an FX body, what you see is 50mm.

What matters is not that your LENS is a DX but that the body is. The thing is, DX lenses are always used on DX bodies (with very rare exceptions), so you'll always have a x1.5 multiplier. Just know that any lens you put onto your DX body will have to be multiplied by the crop factor, regardless of whether the lens is DX or not.

[Alexandra Hall]

So regardless of if the lens is DX or FX any 50mm will be about 75mm on a DX body. And any 50mm will be 50mm on an FX body? So is the following correct?

50mm DX on DX= 75mm
50mm FX on DX=75mm
50mm DX on FX=50mm
50mm FX on FX=50mm

If all this is true, what is the point of DX lenses? Why not just have them all be FX or all DX? What is the difference if it is only the body that affects the focal length that is displayed?

*Edit* I realize that I just completely reiterated what you said after re-reading. So I guess I just need to know the last few questions

[sk66]

Basically right except that DX lenses don't work on fx bodies. (you get vignetting, or worse)
The point of dx lens is the design takes advantage of the dx crop factor.
For example w/ the 50mm. As a 50mm DX only lens they can design the lens to optimize only the central 75mm fov that the dx sensor will see. This can actually result in an overall lens design that is notably different than what is required for a 50mm FX lens (as seen with Canon's EF-S lenses).

[inkista]

Designing for a smaller format lets you create sharper, smaller, and cheaper lens designs. A lot of complexity in lenses is for correcting issues of vignetting, chromatic aberration, etc.--all of which get worse as you head towards the corners. When those corners are closer in, you don't have to correct as much.

[Alexandra Hall]

Awesome! Thank you all so much! This helped a lot!