How common are 'duff' lenses?

You sometimes come across lenses on Lens Test Sites that aren't up to scratch and they sometimes get a 2nd lens to test which performs much better.

My Mk 1 and Mk2 18-55mm Kit lenses were both good, but I have seen bad results from a few Mk 1's.

Quite likely - I'm still a bit green where this photo manipulation stuff is concerned and tend to just tweak brightness/contrast and more recently levels until I get something which looks similar to what I remember seeing on the day!

I have seen some really odd reasons for giving a lens a poor review, from "I won't give it full marks because I only give those to Ltds" to apparently basing their entire opinion on the fact that the lens was a two touch zoom design rather than a one touch like the rest of the M series zooms.

I've got the M 40-80mm (for example) which has absolutely stinking reviews on the other site, but it really isn't that bad. It does have barrel distortion which shows up if your subject has long straight lines, but it's sharp enough, relatively fast, and the macro function works well. I shot a couple of rolls of film with it on an ME Super last October and the distortion is only painful in one shot from the eighty-odd slides, which was of a spiral staircase on the side of a rather angular building with the lens set to 40mm.

The answers to the OP perhaps suggest that the question could equally have been 'how common are variations (affecting IQ) in any particular model of lens?'

My Sigma 30/1.4, Sigma 17-70 and Sigma 135-400 are some of my best lenses.

I bought the 17-70 as part of a package with my ( now sold ) K10D and at first I thought it was rubbish. turned out that the AF on my K10D simply needed calibrating for it. It's actually a stunning quality lens for it's price range.

I'm sure many people who claim their lens is 'soft' aren't calibrating the AF on them.

If a lens required AF recalibration would this need to be done with every body the lens was used with? I have a K10 and a K20 and get the identical slightly unsatisfactory results via the Sigma 105 from both...

Depends of the lens. Some of my lenses haev needed more or less the same calibration on my K10D, K20D and the 3 K-5s I have now owned. Others needed calibration on one or two of the cameras and not on others. Not an exact science.

I make sure I check AF calibration on all lenses I use on my cams. Even the manual focusses ones in order to use the AF confirmation

Just out of curiosity, as this is the first time I ever heard of AF calibration, How do you know if this needs doing and secondly how do you check then thirdly how do you do it.

Bernard
PS I'm male -"I don't read instruction manuals". Actually I did but couldn't find anything.
PPS I have a K10D with 18-55mm zoom

Thanks for all the responses folks - useful and interesting - but I second Frogherders question - What's the best way to tell if AF needs calibrating for a particular lens? Sorry to be so green...

if your finger is bleeding, it probably needs a bandaid. If it is not bleeding then it does not.
apply that logic to the problem.
if your lens is consistently front or back focussing at wide open apertures... then.....

Well it's pretty obvious that after checking the accuracy of the AF for your lens that it's AFing correctly then there's no need to apply any compensation.

After shooting a multitude of test shots I've found that you don't necessarily need any expensive or complex test charts ( I even made one myself at one point )

Simply AF on a pattern or print on something at an angle and take a shot at the widest aperture of your lens. look at the shot and see whether the intended target is in focus or areas in front or behind are in focus. A tape measure is a good item to snap. Take several shots to allow for variations.

If after taking several shots you are getting the sharpest results on what you are actually focussing on your lens needs no calibration

If it's either back or front focussing then go into the camera's AF adjustment settings and make a compensation adjustment and reshoot, repeat until you are satisfied.

Thanks Smeggy (and Don) - clear and simple advice. Much appreciated.