Here's the simplest method....
One word of warning... don;t get too hung up on 'pixel-peeping'. if your photos like fine, the lens is OK. If you do the test you'll find that, unless you have spent the equivalent of a small family car on a lens, it is not going to be perfect; the edge / corner detail at some apertures is going to look much poorer than the centre. Just remember that this is normal, and that enlarging your digital image to '100%' in photoshop to look at lens performance is equivalent to making a print about two metres square... what really matters is how the result looks at your normal maximum print size..
If you find that one side of the picture clearly looks much worse than the other, or one corner is obviously worse then the others, or not all of the sheet of paper is in focus (e.g. the centre is sharp but the edges are out of focus) then there may be a fault with the lens. This is pretty rare however; don't think that fuzzy corners when the lens is at it's zoom extreme and widest aperture means that it needs returning - it's typical behaviour..
You can get some idea of what to expect from reading lens reviews on a repultable testing site likehttp://www.photozone.de..
Thanks for that Mike and very helpful. So it really is a question of taking it home and testing? It is just the threads I read seemed to be saying you could do this in the shop in some way. I have to say it seemed unlikely to me unless you were blessed with 25/20 eyesight...
Trouble is, there are several schools of thought about what is a good lens..
Some like them sharp in the middle and don't worry bout the edges and some like them sharp all over and some like them vague (for portraits). You could/can even buy filters for soft focus for portraits..
Worse still it ends up with you wound up; I often look at old cameras and check the lenses (torch and a dark room works best). Then one day I made a silly mistake and tested all my lenses and was convinced that the whole lot of them were junk, full of dust and scratches. That lot included almost every major player and some minor ones, primes and zooms, new and elderly; I wouldn't have bought one of them secondhand..
Of course, a lot of the trouble is the design; zooms go in and out like pistons and will suck dust and dirt into their innards. And there's nothing you can do about it this side of a few hundred pounds (which I was once quoted for stripping down a pre-war lens and repolishing/grinding it and rebluing and edge blacking)..
Then I remembered the joke once played upon the sponsor of a seriously large telescope in an observatory years ago. The sponsor was happy with the 'scope once he'd tried it and the makers showed him that the objective had had a hundred (meaning a US dollar's worth of one cent) stamps stuck on it. Seems he'd criticised them in the past and said his standards were higher than their's....
Anyway, check them for the obvious, by all means, but pay more attention to the pictures you make..
Thank you David. It is, I suppose a short step from here to paranoia but what worries me is sample variation. Having just read a number of reviews on photozone.de there are several instances where the initial sample had to be changed because of problems and these were not cheap lenses..
Having read these forum extensively I now understand that bodies come and go but lenses go on forever and are a critical factor for image quality..
I remain undecided on which system to go for but I know I will first buy a fast (i.e expensive) walkaround zoom with the body and eventually a longer zoom and one or two primes. (all fast/good quality i.e expensive) I also want SSM/AFS/USM or equivalent lenses..
So how concerned do I need to be about sample variation? Is it much ado about nothing in real world conditions or is it a worry and therefore a lottery?..
No one will ever say that it doesn't happen, but it is quite rare, and my approach is to simply go out with a new lens as often as I can right after I get it and take a lot of pictures. I even deliberately shoot some things that will show up various distortions and things like soft corners. Then I look at them. If I can't see anything wrong, I assume there is nothing wrong..
Nothing is enough for the man to whom nothing is enough...
So how concerned do I need to be about sample variation? Is it muchado about nothing in real world conditions or is it a worry andtherefore a lottery?.
There's no more reason to be concerned about getting a faulty lens than there is about getting a faulty camera. Of course a small proportion will have faults... that is what warranties are for. An optical misalignment in a lens will not be as obvious in the first instance as an electronic fault with a camera, but a few simple 'newspaper pinned to the wall' checks will show them up and are certainly worth doing. As long as you have used a reputable dealer there should be no problem about exchange / replacement..
Big repeat spenders can get such treatment from some merchants in some cases. Don't expect that treatment if you walk into Joe's Camera and tell Joe that you want to cherry-pick from a half dozen copies of the $80 Canon 50mm f/1.8 lens..
[ e d @ h a l l e yc c ] http://www.halley.cc/pix/..
Thank you all for your responses. You have given me some comfort...
So don't worry about it but remember that you can always go back to them and ask if the thing is OK. Most makers are happy to check things and re-assure you because they know their reputation can be destroyed overnight by a genuine case of poor quality..
Above all ignore the trolls. And try this link for re-assurance:.
For the record, a good picture that's in focus and naturally coloured etc will beat anything and you don't need a brilliant lens to do it. I've family pictures in the album taken in 1910 and you can't fault them..
I blame the marketing people for all this paranoia: it's stirred up because they want to sell you a new camera every year even though you won't need one. But they like their bonus every year..
...maybe I'll go and get my old box Brownie out of the loft. Where can you buy 127 roll film?Mike..
Well, the box "Brownie" has HDR. and that's this week's fashion/lifestyle thingumygig..