DS + 16-45 autofocus

I have been playing with my 16-45 on my DS.
What I noticed is that at the 16mm the autofocus has some trouble at the infinity range. Sometimes it seems that the DS wants to adjust it beyond the infinity range.
I noticed this when shooting in nature, when you however select a fixed focus point and point the camera at a tree then it works better.

My DA16-45mm does exactly the same thing on my K100D. In fact it focusses slighty beyond infinity at 16mm. At 45mm it focuses at infinity. Remember it is a retrofocus lens.

Ok, nothing to be worried about then.

I have learned something again
http://en.wikipedia.org/wiki/Retrofocus

The term Retrofocus is new to me, but any lens with focal length less than about 40mm has the centre of focus behind the rear element of the lens, as this is the distance from the mount to the film plane. Similarly, long focal length lenses are generally physically shorter than their focal length, so the focal point must be in front of their front element. Do these have some other descriptive term applied to them? :

Is there a special design characteristic of these types of lenses that make them difficult to focus?

I just find it incredible that with a modern ultra- or hyper-zoom lens, the focal point moves from somewhere near or behind the rear element, to somewhere in front of the front element. Maybe when I have eternity to study it, I'll understand how it's done.

I understood that many lenses are designed to focus beyond infinity for visible light, as this enables them to be focused for infra-red. Although, if they don't have both visible & IR focal distances marked on them, setting them to the correct focal point would seem to be impossible.

Lenses that are physically shorter than a long focus lens would be expected to be are telephoto designs.

Standard wide angle lens designs would need the back of the lens to be very close to the film plane/CCD position and this would leave no room for the mirror assembly in an SLR. So lenses are designed with a longer back focus and these are retrofocus wide angles.

Lenses are sometimes made to focus beyond infinity. In the case of long telephotos this can be to allow for expansion and contraction due to temperature changes - the infinity point changes. I confess I have not noticed it in the 16-45mm, but then I didn't look for it either. In this case maybe it is not a 100% true zoom and the infinity position changes slightly at different focal lengths?

In any event, it's a cracker of a lens!

Just to expand on the term retrofocus in regards to wide angles. They are in fact inverted telephotos and work in the opposite sense. It is why, for a long time, you could get wide zooms, mid zooms or long zooms but the rnge was limited in each area. Until the computer age, it was difficult to make a lens which behaves with 2 of the characteristics.

Kim