front focussing lens, split image focussing screen?

[Dave Pickett]

As per title, despite some adjustment on the D300s it does seem that some of my images are front focused. The lens is a sigma 70-200 f2.8.

It may be my technique, but I have in any event ordered a split image screen, just like we used to have in the old days of manual focussing.

Has anyone fitted or used one of these, my thinking is that I can use this to check how accurate the autofocusing actually is.

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=190362891304&ssPageName=STRK:MEWNX:IT

 

[odd jim]

Can you not just stick it on a tripod and focus on something specific wide open? Putting on a split screen focusing screen seems like a bit of a convoluted way to check the focus on a lens...