The Chaos of Fullframe vs APS-C: A Test

Last time I tested fullframe lens on crop body (Sony 28-70mm f3.5-5.6 on A6300) and it did a pretty good job, the image quality of fullframe lens on crop body and crop lens on crop body was almost indistinguishable.
But this time it's getting more challenging and interesting, putting a crop lens on fullframe camera to test out side by side against fullframe lens.

Here is the video of the test, I took a handful of pictures on street with both lenses on same camera, identical shots. 

To sum up the video: 

1.You can't really tell the difference between the images produced by crop and fullframe lens IF you don't zoom in on the picture.

So if you are using the picture for casual shoot, you can definitely use crop lens on your fullframe body, honestly, not much is lost. 

2. In crop mode, your camera doesn't use all the pixels it has on sensor, the result is a smaller picture, less pixels, and of course lower resolution. When you zoom in on the identical pictures, it is very obvious which one is taken by fullframe and which one is by crop lens. The amount of detail and the range you can zoom are much better with fullframe of course. 

So if you are producing serious pictures, or really, any occasion that is not casual, it's always better to use full frame (Duh)

Here I have another video concerning the fullframe vs crop lens battle. I tested fullframe and crop lens on each other (fullframe lens on crop body + crop lens on fullframe body) and ran a little comparison, might be interesting for you to have a look. 

Conclusion:
Fullframe lens on Crop Body: totally works, very good image quality, but be careful with focal range: wide angle end will be limited while you get more tele range.
Crop lens on Fullframe Body: focal range remains the same in crop mode but resolution and image quality take a hit, usable but not ideal, avoid it if you don't have to, but it's not the end of the world.