DX vs FX lens on my nikon D3100?

I understand that DX means cropped sensors for nikon cameras which adds an extra 1.5X focal length to the FX lens. What about the DX ones? E. G the nikon 35mm f/1.8G DX, will it be a 35mm lens on my nikon d3100 or will the crop factor apply to it as well making it a 50mm lens? If it applies then what's the difference between DX and FX lens as long as the crop factor applies to all of them?

You can use ANY AF-S Nikkor lens on your D3100

Focal lengths do NOT change, only the angle of view changes when you place a FX lens on a DX body

Here is how different focal lengths see when mounted on DX an FX cameras

http://imaging.nikon.com/...simulator/

I know what you are asking. The answer is yes, the same effect will occur with your camera whether you use a FX or DX lens.

The focal length is a measurement of the distance from the sensor to the center of the lens focus point. In that regard, the measurement is exactly the same whether you are using a FX or DX body, so the focal length is the same.

What differs with a DX lens is that since only the center of the lens is used on a DX camera, it is less costly to make a smaller diameter lens - i.e. Leave the outer glass area off the lens that only a FX camera would use.

Although describing "effective" focal length by 1.5x is a popular activity, I don't like to use it as it just causes confusion. If anything, using the term 'digital zoom" would be more accurate for a cropped DSLR.

For example, if you used a 35mm lens on a FX vs DX camera, the crop factor changes but the lens's characteristic perspective distortion will be the same for either camera. That is because the lens is still a 35mm lens.

Also, when using a given lens on a DX body, the depth-of-field will be increased vs. On a FX body. But this takes some explanation. First, to compare FX and DX bodies, you have to "normalize" the photo. That means, you need to create the identical photo in both cameras. The way to do that is to back up with the DX camera until the photo is the same as with the FX body.

But when you back up, you increase the focus distance, and when increasing the focus distance, you increase the DoF.

However, by using the term "effective" focal length when comparing a DX camera, you are thinking in terms of a longer focal length - which decreases DoF.

So that is why I dislike to use the 1.5x effective focal length with DX bodies as (1) the dof behaves the opposite to what it should, and (2) the lens characteristics remain the same.

This causes confusion, and it would be far better in my view to just realize that a DX body results in a cropped photo - sort of, a digital zoom (although there's no quality reduction since you are not magnifying the sensor's image). Or just realize you need to backup when using a DX body with everything else the same (which is how I keep it straight). Call the 1.5x a backup factor if you wish - it is more accurate than using a 1.5x effective focal length.

DX lens is cheaper and lighter than FX lens.
The crop factor would still apply.
One way of dealing with this conundrum is to be comfortable with the DX system and forget about 35mm comparison, although people /camera manufacturers won't let you do it.