No, the true focal length of your lens won’t change when using an adapter.

However, if you’re using a full frame sensor lens with an ASP-C or cropped sensor camera, this can change the equivalent or perceived focal length. The camera will apply a crop factor in the lens field of view, which will make the image appear more zoomed in than it did through your viewfinder.

Here is an analogy to help you think of this in a different way: Imagine you have a projector and it’s set up to create an 8×10 metre image – this is your full frame lens. But you only have a projector screen that is 6x8m – this is your APS-C sensor. The screen you have to capture the image is smaller than the image itself, so you can only capture a smaller part of it – your image will be cropped.