Originally posted by Tommot1965 Ive never heard of those terms before, thanks for bringing it to my attentions,
after a quick google with results at this very forum..it turns out that the lens is a varifocal design.... obviously the aussies aint got a clue about what they are selling!!, as the ets I was told to do.. wouldst work, and shouldn't work
great forum , many thanks for the very helpful advice
Hi Tommo,
I think a Google search will show that this lens is notorious for focusing issues across many brands with FF on the wide end being common.
In addition, even very good zoom lenses will not hold perfect focus at all focal lengths. It is also usually not going to give good results on most any zoom to focus at one setting and zoom to another without a re-focus at the new focal length, but you probably already know this.
I adjust my zooms one of two ways:
Best focus where I use it the most.
Best focus at the longest focal setting as the wide end will be more forgiving of a focus error.
BTW, I have a Sigma 18-50 f2.8 that isn't a bad little lens, but it REALLY doesn't want to focus properly on the K5. It was always ok on my K10 and K20. This makes me wonder about what data the camera is reading from the lens chip and if the Sigma lenses need an update for the K5?
Sigma is pretty good about re-calibrating and re-chipping products if needed, so it might be worthwhile to contact them about the lens.
Alternately, you could just buy a DA* 16-50....
Ray