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Old Jul 30, 2010, 6:08 PM   #1
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Default IF on Pentax 60-250mm f/4

I thought that the term 'IF' ment internal focus therefore the lens wouldn't extend zooming out but i see that this is still the case with this Pentax 60-250mm f/4

Why is it so ?
Pentax 35mm f/2.4 AL SMC DA
Pentax 10-17mm f/3.5-4.5 ED IF SMC P-DA Fish-Eye
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Old Jul 30, 2010, 7:00 PM   #2
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Hi GV,

Internal Focus is not the same as Internal Zoom. The benefits of IF are

1. the lens doesn't change length or balance while focusing at a given FL, and
2. probably more importantly, the front element doesn't rotate when focusing.

Internal Zoom, while a nice feature, is probably impractical in a lens with almost a 5x zoom ratio (most of the IF/IZ zooms are only about 3x max), and the size of the lens would probably have been impractical as IF/IZ lenses are usually considerably larger (their fixed length has to be close to the FL at the long end (from the sensor plane), possibly with a bit of added length for the movement of the internal focusing elements). This is all assumption, as I'm no optical engineer, but specs of actual lenses seem to bear this out.

An example of this can be seen with a comparison of the DA* 50-135 (which is both IF and IZ) specs to the DA* 200 (which is IF only, of course) where the 50-135 is 136mm long and the 200 is 134mm long. The Tamron 70-200/2.8 (also IF/IZ IIRC) is 194mm long to compare apple to apples in FL. With

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