Why is my picture soft?

[KarelBata]

I've got a pair of Canon 600Ds on loan from a college, I took a quick test shot using each before leaving them in a locker there overnight. They both have Canon EF 24mm lenses which were set at infinity. The metadata says the exposures were 1/2000 and 1/4000, so there's a stop difference.

Any ideas why is one is so much sharper than the other? I could take it back to stores and have a moan, but the place is about to close up for the summer and I'd rather fix the problem myself so I can hang on to them for the next six weeks. Any ideas?

[nanomad]

Assuming both pictures were taken using the same F/stop values, the second camera may have serious focusing issues.
It looks like it didn't even try focusing 

[KarelBata]

Both at 2.8.

I was in a hurry so I may have left one (or both) accidentally on auto-focus. But if it were a focus (or back focus, or lens mount) issue, wouldn't something in the foreground be in focus..?

[a1ex]

Focus can go beyond infinity

[ilguercio]

Check your focus with LV in 10x, don't just set both lenses to infinity as it may differ even slightly from lens to lens.