I think i saw some people talkin about "grabbing a sharp copy" of a lens at a shop, how do you acctually test which lens is the sharpest? also heard something about backfocusing or something can anyone explain to me? Thanks
Back-focus: Do a search here or on google. Explained 10,000 times.
Sharp copy: Really hard to tell from camera alone. Many will recommend you bring a laptop, take the same picture with the same settings using several lenses, then get the one that loks sharpest edge-to-edge.
It is hard to tell sharpness as you need to zoom in and compare pics... which is really difficult with that tiny LCD screen when you are at a shop. However, you should first do your research before going to buy any lens. Read the reviews, and also what people say about it. See tests results, etc. Weigh all that you have researched, then go to the shop and do a test. By that time, you should already know based on reviews which lens is sharper, and now you would want to test the handling issues.
bring a piece of paper, a piece of aluminimum to the shop
then try to use the lens to cut those things, if it can cut both, it will be "sharp"
using lcd alone cannot tell, and frankly every lens has an indviidaul "sweet aperture" - usually around f/8 to f/11 will be fair enough if you ask me
for me i don't really care, lcd zoom in sharp enough can already
basically you should just use
a) lowest iso possible
b) put it on a solid base if tripod not available
c) use sweet aperture as mentioned above, if don't know, USUALLY f/8 should be ok, or 2 or 3 stops down from maximum aperture
d) zoom in to see lor