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11-28-2007, 07:52 PM   #1
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Is the Pentax or Tamron 18-250 a parfocal lens?

Can anyone who has either lens tell me?

Thanks, BobG

11-28-2007, 08:35 PM   #2
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You need to check, but I have not seen a varifocal lens in a long time. Most zooms are parfocal, i.e. true zooms that retain focus as they are zoomed.

Varifocal lenses were popular with one ring zooms, since you could more or less manually refocus at the same time as zoom, but a two ring zoom was too difficult to manage if you had to refocus as you zoomed.
11-29-2007, 11:30 AM   #3
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QuoteOriginally posted by Lowell Goudge Quote
You need to check, but I have not seen a varifocal lens in a long time. Most zooms are parfocal, i.e. true zooms that retain focus as they are zoomed.
Are you sure? I thought most zooms nowadays are varifocal.

QuoteQuote:
Varifocal lenses were popular with one ring zooms, since you could more or less manually refocus at the same time as zoom, but a two ring zoom was too difficult to manage if you had to refocus as you zoomed.
Actually, most zoom lenses are designed for auto focus - and that's why most people don't care if it is varifocal or not. The AF would take care of business. And I believe varifocal and variable open aperture zooms are cheaper to design.
11-29-2007, 11:38 AM   #4
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QuoteOriginally posted by BobG Quote
Can anyone who has either lens tell me?

Thanks, BobG
My Tamron 18-250mm always refocuses when I zoom in or out.
So I guess its not. My old Panagor zoom was.
Since it was manual focus, you best got the focus right zoomed in, than zoom out.

Nowadays the camera does the focusing most of the time.
So, would that be a problem for you?

- Bert

11-29-2007, 12:23 PM   #5
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With varifocal zooms (most of the current models) you cannot use the distance markings on the lens. However, most people wouldn't use those markings anyway. The annoying thing is if you look at them after focusing, you might think that either the lens is defective or the marking is wrong. This is a result of the (probably cheaper) varifocal design; the same object sitting at the same distance will be seen at different distances if you look at the scale on the barrel. For example; at 70mm focal length the scale would show 7m; and at 20mm 5m. The real distance of the object might very well be neither! You should just ignore those values if the shots turn out to be normally focused. Of course the same situation occurs even when you focus manually.
A parfocal zoom lens, on the other hand, allows you focus using its highest tele setting and recompose your shot changing the zoom setting to a wide angle one. With a variofocal one, you have to re-focus whenever you change the zoom setting.
11-29-2007, 01:26 PM   #6
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Just as a quico test I looked at my 18-35 pentax KAFJ, my 28-80 pentax KAF2, my sigma 70-200 and they all are parfocal, i.e. focus is constant when I zoom. ANy re-focusing is a function of slight errors , or more precise selection of the focus point when zoomed all the way in.

Althoug I agree varifocal are easier and cheaper to build,. none of my zoome seem to be varifocal. I still have to check my sigma 10-20 but with almost infinite depth of field that will be hard
01-11-2009, 03:51 PM   #7
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Just for others finding this thread through a search. I don't think the Tamron 18-250 is parfocal. See this new thread.
01-11-2009, 08:45 PM   #8
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QuoteOriginally posted by bc_the_path Quote
With varifocal zooms (most of the current models) you cannot use the distance markings on the lens. However, most people wouldn't use those markings anyway. The annoying thing is if you look at them after focusing, you might think that either the lens is defective or the marking is wrong. This is a result of the (probably cheaper) varifocal design; the same object sitting at the same distance will be seen at different distances if you look at the scale on the barrel. For example; at 70mm focal length the scale would show 7m; and at 20mm 5m. The real distance of the object might very well be neither! You should just ignore those values if the shots turn out to be normally focused. Of course the same situation occurs even when you focus manually.
A parfocal zoom lens, on the other hand, allows you focus using its highest tele setting and recompose your shot changing the zoom setting to a wide angle one. With a variofocal one, you have to re-focus whenever you change the zoom setting.
I've noticed this on my Sigma 10-20 and wondered what was up with it.

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