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09-25-2008, 08:09 AM   #16
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That certainly sucks...

QuoteOriginally posted by audiobomber Quote
Here's what I found:

At 9 feet, the 18-250 has the same FOV as the 55-300 at 160 mm
At 30 feet, the 18-250 has the same FOV as the 55-300 at 200 mm
At about a mile, the 18-250 has the same FOV as the 55-300 at 230 mm

Picasa Web Albums - Dan - FL compare
does the Pentax equivalent do this as well?

Chers,
Cameron

09-25-2008, 08:23 AM   #17
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QuoteOriginally posted by Cambo Quote
does the Pentax equivalent do this as well?

Chers,
Cameron
Of course, it's the same lens. ALL internal focusing zoom lenses (that I've ever used) do this. Take a point of shoot with a super zoom and compare the field of view to an actual prime lens at it supposed focal equivalence -- it will do the same thing.

Lightweight, Macro capability, and faster focus come at costs.
09-25-2008, 09:50 AM   #18
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What confuses me is this entry in Wikipedia.

"An internal focus lens (sometimes known as IF) is a photographic lens design in which focus is shifted by moving the inner lens group or groups only, without any rotation or shifting of the front lens element. This makes it easy to use, for example, a screwed-in polarizing filter or a petal shaped lens hood. During macro photography, using an internal focus lens reduces the risk of the front of the lens accidentally hitting the subject during focusing as the front element does not move." Internal focusing - Wikipedia, the free encyclopedia

If you use the simple definition that the front element doesn't rotate, then the DA 55-300mm is an internal focus lens, and therefore not all IF lenses lose focal length at distances below infinity. I suspect the 55-300mm may be described as IF, and the reason it doesn't alter fov is because it is not a close-focus lens. The Tamron 70-300mm is a close focus lens, and it doesn't lose focal length, but it's not an IF lens because the front element rotates. I don't know anything about lens design, I'm just speculating based on a bit of research on the web.

Cameron, my 18-250mm is a Pentax. I'm not convinced there's any difference whatsoever in these lenses, including the coatings.
09-25-2008, 02:19 PM   #19
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QuoteOriginally posted by audiobomber Quote
If you use the simple definition that the front element doesn't rotate, then the DA 55-300mm is an internal focus lens
That's not the definition, though - it has to not move at all. As in, the lens cannot extend during focusing. *Lots* of lenses don't rotate.

QuoteQuote:
therefore not all IF lenses lose focal length at distances below infinity
Do you know for sure that it doesn't? Most zooms do; the bigger the zoom range, the more they lose.

09-25-2008, 02:51 PM   #20
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QuoteOriginally posted by Marc Sabatella Quote
That's not the definition, though - it has to not move at all. As in, the lens cannot extend during focusing. *Lots* of lenses don't rotate.
OK, now I see. I missed the "shifting" part in the definition.

QuoteOriginally posted by Marc Sabatella Quote
Do you know for sure that it doesn't? Most zooms do; the bigger the zoom range, the more they lose.
I don't have any exact way to check that, but I guess I could compare the 55-300 vs the 50mm 1.4. If the 55-300mm is losing mm's it should show up. I really don't think it does. If it does, it's certainly nothing like the 18-250mm.
09-25-2008, 04:30 PM   #21
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Some of you guys seem really lost. It happens with every IF zoom lens, regardless of the brand. It is far more severe with high ratio lenses, so a 15x lens is going to be really misleading.

This is why the whole 250mm thing is so stupid, it's just a gimmick. You could save size, weight, cost and lost sweet bugger all from the long end if ya went with a 16-100 instead.

Not to mention 16-100 is FAR more useful, a 250mm long end that is really 170mm more often than not but is slow to boot is just annoying.
09-26-2008, 05:45 AM   #22
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QuoteOriginally posted by Alfisti Quote
Some of you guys seem really lost.
Yeah, I guess you knew everything about photography as soon as you touched a camera.

QuoteOriginally posted by Alfisti Quote
This is why the whole 250mm thing is so stupid, it's just a gimmick. You could save size, weight, cost and lost sweet bugger all from the long end if ya went with a 16-100 instead.
Would that be the SMC DA 16-100mm ED F/4.0 VAPORWARE lens?

The 18-250mm goes well beyond 100mm. Superzooms like the 18-250mm and the Nikon 18-200 VR are a type of lens that a lot of people want, especially coming from p&s. I don't use mine any more, but it's a handy thing to leave on the K100DS my wife uses, because she has no interest in changing lenses. For more sophisticated users a "walk-around zoom" provides just another choice in the bag.
09-26-2008, 09:03 AM   #23
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Back when I had the FA28/200 (tamron clone) I noticed about the same thing. When compared to my FA*200/2.8 at close range, the zoom had a much wider angle of view (@200 mm).

After reading and searching a lot, I discovered that zoom lenses (focal length) is given at infinity focus point. At any shorter distances, the focal length also shortens. In my case, I discovered that the 28/200 zoom at its shortest focus distance, was only 135 mm (@200mm setting).

I remember reading this facts some years ago, in a Popular Photography Magazine on an article about the Tamron 28-200 zoom.

Robert B.

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