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12-02-2008, 07:34 AM   #16
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QuoteOriginally posted by Lowell Goudge Quote
there is an interesting article somewhere about this.
I'd be interested to read that if you can remember how to find it.

QuoteOriginally posted by Lowell Goudge Quote
it is true of virtually all zooms but more so of super xooms. due to the internal focusing (almost a requirement for AF) the lens designs are such that focusing changes the focal length also and that true focal length is only at infinity.
I checked the 55-300 against my D FA 100 mm and FA 50mm and it was off by a small amount. Nothing at all like what the internal focussing 18-250 did to the stated FL.

QuoteOriginally posted by Lowell Goudge Quote
I looked at several lens reports and no one reports on the effective focal length as a function of distance. There must be a test somewhere yor all the measurebaters out there to report on this
I agree, those figures should be reported. It's ridiculous that they aren't, because FL is as relevant as any of the characteristics that are measured. I'm interested for example in how the Bigma compares in FL to the new 150-500mm. I read a few reviews and didn't even manage to find out whether the Bigma is an internal focus design. From its spec I imagine it is? How is actual focal length not seen as important in lens tests? Here's the testing I did:
Picasa Web Albums - Dan - FL compare

12-02-2008, 04:35 PM   #17
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Does this only affect internal focussing? The 80-320 shoots in and out when focussing!
12-02-2008, 04:51 PM   #18
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QuoteOriginally posted by Arpe Quote
Does this only affect internal focussing? The 80-320 shoots in and out when focussing!
I don't know.

The best way is to do some tests, with a known subject height / size at a known distance, then repeat these tests by moving in.,

I think it really only impacts the super tele's, and I think the article I saw was in a receint photo magazine in response to a reader letter.
12-02-2008, 09:56 PM   #19
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QuoteOriginally posted by sharko Quote
You shouldn't use a TC of any sort with the 18-250 superzoom as it will not autofocus at all and the viewfinder becomes so dark its hard to see anything because of the loss of light.
actually, in daylight, i've not had a problem with the 18-250 Auto focusing with a Tamron 1.4TC on K10. It does require a little more light. The problem I had with trying to catch birds through brush conditions, is that MF is harder to do because the additional focus linkage of the Tamron 1.4TC adds more friction and the movement is not smooth.

12-04-2008, 04:49 AM   #20
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The January 2009 Shutterbug reported that Pentax showed a new 1.4TC at Photokina in Cologne, due in U.S. stores this coming spring. Fully compatible with SDM lenses/bodies, too.

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