I will give an example of comparing Canon Ef-s 10-18mm f / 4.5-5.6 is Stm with 17-40 in full frame. I mostly shoot landscapes at wide angle. So this is a huge difference. On a full frame, 17-40 gives an excellent focal range, from 10-18mm on a crop I constantly had to change lenses. it's like walking around with 40mm and 50mm primes and alternating between them throughout the shoot. With a 10-18 lens, the picture is sharp only in the foreground, everything that shrinks and blurs far away. On a full frame, even at 17mm, pixel-by-pixel sharpness throughout the plan and you can easily crop the frame.