Of course, now that I have an M50 f1.4 and a M28 f3.5, that zoom has been consigned to the spare-lens drawer :-)
S
Cotty wrote:
Okay folks, here's a gen-u-ine question with gen-u-ine Pentax content.
In Washington I was able to crack my pile (blecch) and get to grips with a new lens, the Canon 24-70 2.8 - and immediately I noticed it did one thing which surprised me.
At 70mm, the lens is compact (LOL) and at it's 'normal' physical size, but when I zoomed out to 24mm, the lens increased in length by some inches. First, I was not expecting this so it was a total surprise, and second, I didn't like the idea of this because I've always had zooms that remained physically the same size (with internal focus etc).
Then it dawned on me what was going on. The lens shade is very deep - so deep that at 70mm, it is perfect for the focal length (although not necessarily for a smaller digi sensor). Zoom out to 24mm and the hood becomes far too deep, and will obviously produce bad vignetting. So, to counter this effect, the front group of elements shoots forward during the zoom and sits much closer to the front of the lens hood, in turn providing a perfect hood length for the wider focal length.
In the past, I have only had Sigma and Tokina zooms, and neither did this. In fact, with the Tokina 28-70 2.6/2.8, the hood is very shallow indeed - and the front lens group does actually move in and out similar to the Canon, but within a lens barrel recess, so that the hood (which is attached to the outer barrel) does not rotate. The front element does rotate, but you get around this with polarisers etc by having them screw into the outer barrel filter ring, which does not rotate. Apologies if this is not clear.
So, eventually I ask the question. Zoom lenses - especially fast 2.8 zooms - do any others, particulary from Pentax, do this? Does the 28- 70 2.8 Pentax move the front elements backwards/forwards during zooming, to dynamically alter the effective lens hood depth ? Does this change the physical size of the lens, or does this happen with the constant outer confines of the lens?
Is this activity engineered to provide a dynamic lens hood depth? Or is it an engineering necessity that just so happens to provide good lens shading through the relevant movement of the optics? Or both?
Any relevant comments?
I ask because I have never had a Pentax zoom and am curious.
TIA
Cheers, Cotty
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