[phatio]

I am starting to know better my 17-70 bought along with my new K3. I just have noticed that the front element (I mean the moving part of the lens that moves when you change focal length) shows a pretty high lateral mobility if touched. Is that normal? I would say that the range is even 4mm (2 mm per direction). Any thoughts on that? Thanks a lot.

[kiwi_jono]

Not sure how much pressure you have to apply to get this movement but it does sound like quite a lot.

Edit: I misread your email and thought you were talking about Sigma version. I'm sure DA 17-70 owners will comment but still sounds like quite a bit of movement to me.

[blackcloudbrew]

My experience with my DA 17-70 was that the front part moves a bit. Never had a problem with it or from it - that I ever noticed. It's normal as far as I know.

[starbase218]

Yup, it moves quite a bit. I don't think it's 4mm but it's definitely noticeable. Still it's a very good lens imho. My copy is quite sharp in the center even at 70mm and f/4, with good corners as well.

I am starting to know better my 17-70 bought along with my new K3. I just have noticed that the front element (I mean the moving part of the lens that moves when you change focal length) shows a pretty high lateral mobility if touched. Is that normal? I would say that the range is even 4mm (2 mm per direction). Any thoughts on that? Thanks a lot.

It's not an IF lens so there has to be some room for the barrel to move. Its normal and its a good lens.

[starbase218]

Err... it IS an IF lens.

Err... it IS an IF lens.

Oops. You're right of course, I said "internal focus" and meant "internal zoom". And I know better because once I adjust a CPL, I'm done because of IF.

[starbase218]

Oops. You're right of course, I said "internal focus" and meant "internal zoom". And I know better because once I adjust a CPL, I'm done because of IF.

Is that because of the lens not extending, or because of the front end not rotating while focusing? There are non-IF lenses with a front element that does not rotate while focusing (e.g. the kitlens or 16-45).

Is that because of the lens not extending, or because of the front end not rotating while focusing? There are non-IF lenses with a front element that does not rotate while focusing (e.g. the kitlens or 16-45).

I don't know, I've never had a kit lens or a 16-45.

The barrel of the Pentax 17-70 extends to about 2.75 inches so the inner and outer barrels needs some clearance room. Its not like a machine tool way on bearings, just a polycarbonate tube. Its also an IF lens, which according to all the definitions I can find means "focus is shifted by moving the inner lens group or groups only, without any rotation or shifting of the front lens element".

My poorly executed intent was to reassure the OP that his lens was okay, having picked up mine and wiggled it about while extended and non-extended. Having had mine a few years and taken thousand of images, I have a lot of confidence in it.

I don't think too much about lenses, the ones I have are simply tools. All I care about is IQ, constant aperture and minimum aperture. The ones that zoom externally wiggle a bit. The ones that don't, don't. None of the front elements rotate while focusing but I never thought about why.