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09-07-2007, 01:14 PM   #1
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why does Pentax lens get longer when going wider

My Pentax 16-45 f/4 lens gets long when I shorten the focal length, and gets shorter when I lengthen the focal length. This makes no sense to me. I don't have any other Pentax zooms right now, but I don't remember this being the case with the kit lens, or the 50-200 or the 70-300 when I had those lenses. Is this the way they all work? WHY do they work this way? I find it slightly disorienting. My Tamron and Sigma lenses all work the way I expect -- long = tele, short = wide.

Will

09-07-2007, 01:19 PM   #2
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The 18-55 kit lens is shortest at about 35mm, longer at wide and at tele - has to do with optics and which elements/groups they decide to move to get the various focal lengths. I'm sure there is some optical website you could look up to see whats going on...

This is actually quite common. Like the closer you focus, the longer the lens gets.... seems backwards as well..
09-07-2007, 01:23 PM   #3
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Will

I think the answer is that as you transition from wide angle to normal / tele is that the separation between lens groups changes, and as a result the front element (group) has to move out again.

My 28-105 and 28-70 do the same thing, minimum length seems to be about 50mm setting,

For a better answer you need a lens designer
09-07-2007, 01:25 PM   #4
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Most 28-80's act in a very similar manner as well...

09-07-2007, 01:30 PM   #5
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Funny - Just by the topic I knew exactly what lens you were talking about.

The 16-45 is shortest at 45 and steadily gets longer until it's longest at 16..

Like the others have mentioned, it's about lens placement within the tube in relation to the front element.
09-07-2007, 02:06 PM   #6
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My FA* 28-70mm 2.8 becomes twice as long at 28mm.

Tom
09-07-2007, 03:11 PM   #7
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QuoteOriginally posted by ennacac Quote
My FA* 28-70mm 2.8 becomes twice as long at 28mm.

Tom
Retrofocus lens desing. In oder to behave normally the film to lens element distance needs to be smaller than possible w/ the SLR design.
Retrofocus and Tele Lenses
09-07-2007, 03:49 PM   #8
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QuoteOriginally posted by WMBP Quote
My Pentax 16-45 f/4 lens gets long when I shorten the focal length, and gets shorter when I lengthen the focal length. This makes no sense to me. I don't have any other Pentax zooms right now, but I don't remember this being the case with the kit lens, or the 50-200 or the 70-300 when I had those lenses. Is this the way they all work? WHY do they work this way?
Modern zoom lenses have multiple groups of lenses: some "positive" (converging) and some "negative" (diverging). When you zoom, these groups shift relative to each other, changing the effective focal length of lens. If there was just one group of lenses moving, then it would work as you expect. But with multiple groups of lenses shifting inside the barrels relative to each other, the physical size doesn't determine effective focal length. For example, with the DA 16-45mm f4, you can also see the rear group moving closer to the sensor plane as the front group moves away (and I would expect other lens groups are moving in-between).

A short while ago some links to Pentax zoom lens patents were circulating on dpreview. They discuss the internal design of the DA* lenses and how zooming and focusing was accomplished. It will give you some idea of contemporary lens design practice.

09-07-2007, 08:40 PM   #9
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BTW, both the Canon 24-70mm F2.8 L and the 28-70mm F2.8 L are the same way. I'm not an optical engineer, but I currently have the 16-45mm Pentax and I've shot with both the Canon lenses and I can tell you that they all had very, very low distortion.

One feature the Canon's have is the lens hood doesn't zoom with the front lens element. So the lens doesn't seem to "trumpet" as much.
09-07-2007, 08:53 PM   #10
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QuoteOriginally posted by JMI Quote

One feature the Canon's have is the lens hood doesn't zoom with the front lens element. So the lens doesn't seem to "trumpet" as much.
Maybe more like a trombone than a trumpet.

With the lens hood design, the Canon design is quite clever as it allows the hood to be more effective over the zoom range. I have a Pentax D-FA 50mm f2.8 Macro, which has a similar design: The lens hood is fixed to the outer main body of the lens, but the lens inner section with the front element extends for close focusing. This allows more hood at longer focusing distances, and less hood with a close subject where the hood may interfere with the lighting.
09-07-2007, 09:13 PM   #11
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Now what is funey is the new 16-50 DA* is the oppisite. IMO I think it should be the same and than they can have a long hood (atached to the main lens body that soesn't move) that A-protects the extended barrel from the elments and B-Provides better flair control for the tell-setings

But need not worry, I thought the same when I got my 16-45. Its a great lens I must say. I have beat the S out of mine in the past 1/5-2yr and it loves it. Its slightly looser and wobler than a new one. Not bad IMO for not being a "pro" lens like how I use it as.
09-07-2007, 10:43 PM   #12
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My Sigma 28-70 f2.8 is the same as well.
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