What am I doing wrong...?

[poloman]

I have found that the nifty fifty is not as reliable as it could be...
I always shoot 3 shots of any set up with that lens. With a depth of field that shallow, it is very easy for you to wobble a little.
You can establish if your lens is mating well with your camera body.
Set up three batteries or whatever in a diagonal array. Use a tripod. Carefully focus on the middle one with the center focus point. Your results will tell you what is going on.

[nice-n-cool-trig]

Thanks poloman, I'll do this tonight and post the results

[nice-n-cool-trig]

OK I've done the battery focus test thing (see attached image).

Nifty Fifty @ f1.8 1/1000s AI Focus on 10s Timer + 430EX + Tripod... (oh and using the centre focus point)

In the top two images the camera is trying to focus on the middle battery.
In the bottom left image the camera is trying to focus on the battery on the right.
In the bottom right image I manual focused on the battery on the right.

From what I can see this confirms that the lens is back-focusing, it would appear I purchased a Not-so-nifty-fifty (from eBay back in September).

Thanks for the help.

[poloman]

It is front focusing.
Try again with one-shot focus.
Do not use AI-focus.