conurus

[em_dee_aitch]

So - even though I now gladly own a CZ 24-85, I can't tell if this material is a light metal alloy or a really strong plastic. When I tap it with my fingernail, the sound I get could be either. Does anybody here have the lowdown on what these things were made out of?

Also, any commentary on whether the zoom mechanism is very susceptible to dust? The one I have is very minty, but still has a few dust specs in there already.

[conurus]

Hi,

The N24-85 employ a two-part construction:

1, an inner optical assembly with all glass elements mounted in metal zoom/focusing helicoils, and

2, an outer shell with AF motor, AF gear, zoom/focusing/aperture rings, circuit boards.

They built the lenses in such a way that the electronics and AF gears are totally separate from the optics. That way, we can service the former without affecting the later.

All structural points in both the inner optical assembly and the outer shell are made entirely of metal. You will find plastic parts in the N24-85, but they are never structural or bear weight or used to mount glass elements. For instance, it is not all that critical to use metal for the aperture ring and the focusing ring, so they aren't metal. Also, you will find the front-most section of the 3-section zoom to be made of plastic, but that is only a cosmetic outer shell. The real mount of the front glass element is metal.

You will find these lenses to be the last and the best autofocus lenses in terms of metal construction. There are exceptions to the rule, however. The 28-80 and 70-200 are intended to be budget choices and as such they have plastic structural elements.

That said, the N24-85 is poorly sealed against dust. Some dust behind the front group is not unusual. As I convert the N24-85, I would blow off excessive dust behind the front element for you. As a preventive measure, keep the 3-section zoom barrel clean so that as you zoom to 24 the dust on the barrel does not get sucked back into the lens and land behind the front group.