Sorry this is so long... I wanted to get my thoughts down to clarify the issues for me.

I've also posted this on 'Nikonites' and linked in the thread there to Photozone (positive feedback for PZ, I hope!). I hope that's not against some sort of 'etiquette' that I don't know about! <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='🙂' />

I have a Nikkor 16-85mm f3.5-5.6G DX

I'm considering adding a wider-angle lens (for landscapes, buildings) - I've read a number of reviews, particularly here on Photozone.

I've also read here about an increasing number of lenses showing 'decentering' effect... particularly third-party lenses. Klaus posted an "If I had to..." personal ranking of lens quality (control) in that thread, happily (for me) giving Nikon 2nd place. Klaus ranked Sigma and Tokina 9th and 11th respectively.

Some of the otherwise very good reviews of third-party lenses reinforce the hesitance I feel about 'investing' a substantial sum in one of these lenses as I'm not at all confident in being able to test for decentering. My alternative is a (more expensive) Nikon, bought second-hand.

On the other hand, it seems buying Nikon may not be a guarantee of faultlessness: BG_Home says in that thread tried two 12-24mm Nikon lenses and found both were decentered (bad luck or what..?!). So, buying second-hand might get me a decentered Nikon..!

The lenses I'm now considering are:

Sigma 8-16mm f4.5-5.6 DC HSM
Sigma 12-24mm f4.5-5.6 EX DG HSM
Nikkor 12-24mm f4G IF-ED DX (second-hand?)

I considered the Nikkor 10-24mm too but the image quality is reviewed as being (slightly) lower than the 12-24 in the overlapping range, and lower still at 10mm.

I also considered the Tokina 12-24mm f4 AT-X Pro DX - but I've now ruled this out, for reasons below.

All of these lenses are given 3.5 or 4 stars for optical quality in the reviews here. But there are caveats, particularly regarding sample variation for the two Sigma lenses.

Tokina 12-24: "Optical quality: ***.5" PZ review highlights the very high build quality and very good to excellent resolution, but also notes distortion and CA issues, and contra light problem. Klaus also placed Tokina lowest (11th!) in his "if I had to" ranking of lenses with decentering issues (despite generally higher build quality than other manufacturers). Hence, ruled out.
Sigma 8-16: "Optical quality: **** Therefore ... highly recommended (if you can find a good sample)!" This lens also can't take filters. But that IS wide
Sigma 12-24mm: "Optical quality: ***.5 Highly recommended ... if you can get a good sample (the tested sample in Canon mount wasn'tquite as good)."
Nikon 12-24mm. "Optical quality: ****" Generally a good review, but highlights field curvature and vignetting (though average for APS-C) at 12mm.

From the reviews the Nikon stands out, but the price (lowest I've found new is ~ Â£740) is hard for me to justify. That means I'm considering buying one second-hand... but wondering about the decentering issue and what I'd do if I got a bad 2nd-hand copy.

The other aspect of this is that I don't really know how much I would use a wider lens: i.e. how much difference does 12mm make, compared to 16mm? Does it make more difference on DX format, where it's 18mm compared to 24mm? From memory, on full frame (film), primes would have been 21mm and 24mm... I don't recall being aware of 18mm lenses back then (other than fish-eyes).

The attraction in getting a 12-24mm is that I think the extra width would be useful - I just don't know how much of the time. It would also cover the range up to 36mm equivalent, so I wouldn't be changing lenses very much when using for landscapes, trees, architecture, etc. whereas if I bought the 8-16mm Sigma, while it would extend my range more than a 12-24mm, alongside the 16-85mm, I can foresee that I would nearly always be carrying the 16-85mm as well and be changing lenses more often.

Are there people here who have (experience of) both lenses: starting from 16mm (16-85 / 16-35 etc) and 12mm - how much extra does the 12mm- range give..?
Would you be happy / recommend going for a Nikon second-hand (there are a few around for about Â£500) or is it better to go with the Sigma 12-24mm and return it if I get a bad copy? That's assuming I can develop the competence to check it adequately, and confident enough in my competence to return it!

Many thanks,

Ian