How many steps difference?
If you get rid of your 75mm and 85mm lenses you'll never replace them by walking forward with a 50mm lerns. Perspective (what's in the background) will be very different.
But here's an idea -- you own the two lenses. You own two feet, presumably.
Get up from your computer, put the 28-75 lens on the camera, focus it at 75mm and then a 50mm, and actually look through the viewfinder.
Try this for a subject inside your living room, and for a subject across the street.
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