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03-26-2020, 10:06 AM   #1
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Is Pentax DA* 11-18/2,8 parfocal?

I think that the question is obvious. Does this lens change the focus distance when zooming?


Last edited by cport; 03-26-2020 at 01:03 PM.
03-26-2020, 10:57 AM   #2
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I've heard that it is parfocal.. but have not tested myself.
03-26-2020, 11:43 AM   #3
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It'd be pretty neat if it was. From what I see with my 50-135, that lens is, or is nearly, parfocal so it'd be pretty cool to have a 2nd nearly parfocal lens to add to it.
03-26-2020, 12:12 PM   #4
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I had to look it up. It is undoubtedly cool to have such a lens but it is only important to those making motion pictures. The word is parfocal, not parafocal, BTW.

What's a Parfocal Lens and Why Is It so Important? | Fstoppers

03-26-2020, 12:22 PM   #5
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QuoteOriginally posted by Wasp Quote
I had to look it up. It is undoubtedly cool to have such a lens but it is only important to those making motion pictures...
...and probably not that critical at the focal lengths in question for zoom pull where the time spent at other than the destination is scant.

FWIW, the "is this zoom parfocal?" question comes up every so often here at PF and usually with no clear usage in mind except that they read somewhere that such was preferred for video.


Steve

Last edited by stevebrot; 03-26-2020 at 12:28 PM.
03-26-2020, 01:05 PM   #6
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..

QuoteOriginally posted by FozzFoster Quote
I've heard that it is parfocal.. but have not tested myself.
It would be good this lens to have this property. With "focus clamp" it would make sense.

---------- Post added 03-26-20 at 01:06 PM ----------

QuoteOriginally posted by Wasp Quote
I had to look it up. It is undoubtedly cool to have such a lens but it is only important to those making motion pictures. The word is parfocal, not parafocal, BTW.

What's a Parfocal Lens and Why Is It so Important? | Fstoppers
Sure, my mistake. I have corrected it ...
03-26-2020, 01:56 PM - 2 Likes   #7
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QuoteOriginally posted by Wasp Quote
I had to look it up. It is undoubtedly cool to have such a lens but it is only important to those making motion pictures. The word is parfocal, not parafocal, BTW.

What's a Parfocal Lens and Why Is It so Important? | Fstoppers
It's also important to those who use manual focus zoom lenses: zoom in, focus precisely, zoom out. This second use isn't so important with an autofocus 11-18mm lens either.
03-26-2020, 02:40 PM   #8
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QuoteOriginally posted by stevebrot Quote
...and probably not that critical at the focal lengths in question for zoom pull where the time spent at other than the destination is scant.

FWIW, the "is this zoom parfocal?" question comes up every so often here at PF and usually with no clear usage in mind except that they read somewhere that such was preferred for video.


Steve
There are clear usages for being parfocal in video if the shot has a zoom during the shot and you don't want the focus to change. I think the Dolly Zoom is the most common usage, but there are other types of shots too.

Dolly zoom - Wikipedia

I don't think most people who want video in the DSLR are doing these sorts of things, though.

03-26-2020, 03:36 PM   #9
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QuoteOriginally posted by Mistral75 Quote
It's also important to those who use manual focus zoom lenses: zoom in, focus precisely, zoom out.
Have you actually done this? I do a ton of manual focus work and it is far more effective to magnify in live view than to attempt critical focus at 210mm with intent to shoot at a shorter focal length. (Hint: The focus throw stays the same, but with lower precision at longer focal lengths.)


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03-26-2020, 04:40 PM   #10
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^ I have in the past, with film cameras.
03-27-2020, 03:17 AM   #11
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QuoteOriginally posted by stevebrot Quote
Have you actually done this? I do a ton of manual focus work and it is far more effective to magnify in live view than to attempt critical focus at 210mm with intent to shoot at a shorter focal length. (Hint: The focus throw stays the same, but with lower precision at longer focal lengths.)


Steve

But a lot slower ?
Didn't try so dunno, just asking.
03-27-2020, 02:56 PM   #12
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QuoteOriginally posted by Mistral75 Quote
^ I have in the past, with film cameras.
The case I imagine that approach might work would be a super-wide to normal zoom where the magnification at the wide end is so low as to make manual focus difficult with the long end offering an advantage. That case is one reason why I use an aftermarket split-image focus screen where the split-image is still functional even with the diagonal FOV at 180.

Of course, the rub comes in regards to the practical definition of parfocal. While many designs may approximate such, it has been my understanding that one must go to a dedicated cinema zoom to find a lens that actually provides a robust delivery.


Steve
2 Days Ago - 1 Like   #13
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Ok, I bought this lens (but my wife seems to be strongly against me to answer all questions here ) . I did not do any tripod testing but based on hands-on observations this lens is parfocal - focusing distance is not changing for zooming and focus clamp ON.
2 Days Ago   #14
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It could be that DOF is enough to mask any focus shifts. I have a Sigma 10-20mm f/3.5. Focus is almost unnecessary on that bad boy. Almost.
1 Day Ago   #15
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QuoteOriginally posted by Wasp Quote
It could be that DOF is enough to mask any focus shifts. I have a Sigma 10-20mm f/3.5. Focus is almost unnecessary on that bad boy. Almost.
Do not worry, I will check it carefully
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