Is a parfocal lens necessary? at DVinfo.net

Brian Luce · November 22nd, 2009, 04:31 PM

Seems to add a lot the price of a lens. For video, why not just use that zoom/FA button? The big problem with the 7d is holding focus, not getting it initially. Parfocal wouldn't seem to help pulling focus unless you're zoomer?

Daniel Browning · November 22nd, 2009, 05:25 PM

Quote:

Originally Posted by Brian Luce

Parfocal wouldn't seem to help pulling focus unless you're zoomer?

That's correct. Parfocality doesn't matter unless you're a zoomer.

Jerry Porter · November 22nd, 2009, 05:31 PM

But it does help you get it correct the first time and lets you set frame off of that.

Brian Luce · November 22nd, 2009, 06:58 PM

Quote:

Originally Posted by Jerry Porter

But it does help you get it correct the first time and lets you set frame off of that.

That's how I always focused zooms, zoom in to focus, zoom out to desired frame, push button. But with that two position zoom button on the 7d, that isn't necessary any more. One way to save some money.

Benjamin Eckstein · November 22nd, 2009, 08:20 PM

Interesting, I had to look up this term to see what it meant. Being used to video zooms, I sort of assumed that photo zooms all held focus throughout the zoom. Is that not the case? If so, how would the focus marks on the lens work, as they don't change as you zoom? So are most photo zooms not parafocal? I bought the 17-55 2.8 (Canon) with the 7D and am planning to get the 20-700.

Daniel Browning · November 22nd, 2009, 09:00 PM

Quote:

Originally Posted by Benjamin Eckstein

Being used to video zooms, I sort of assumed that photo zooms all held focus throughout the zoom. Is that not the case?

Correct, it's not the case. Many still zooms don't even try to be parfocal, and of the ones that do try, there is unit-to-unit variation.

Quote:

Originally Posted by Benjamin Eckstein

If so, how would the focus marks on the lens work, as they don't change as you zoom?

The focus marks are already useless on the non-parfocal lenses for other reasons, such as the very short travel of the focus ring. They would have to add a real manual focus before it would be good enough to have trouble because the focal lengths aren't parfocal. The parfocal lenses tend to have better manual focus features, but still not as good as lenses that are designed for it.

I think these lenses are intended primarily for autofocus.

Quote:

Originally Posted by Benjamin Eckstein

So are most photo zooms not parafocal?

Correct; most are not. None are guaranteed to be, but some are better than others.

Quote:

Originally Posted by Benjamin Eckstein

I bought the 17-55 2.8 (Canon) with the 7D and am planning to get the 20-700.

I don't know about the 17-55, but the 70-200's tend to be nearly parfocal, the focus shifts only a tolerable amount on most units.

Benjamin Eckstein · November 22nd, 2009, 09:24 PM

Good to know, Daniel. Thanks for that info. And thank god for the excellent focus mag feature on the 7D.

Jerry Porter · November 22nd, 2009, 09:42 PM

I use a Tamron 17-50 f2.8 and a Tamron 70-200 f2.8 and both are Parfocal. They really are great video lenses and are worth every cent spent on them. As far as the true parfocal nature of them they are very close through out the the full zoom range. If you are at a low f they wander a little but not much. YMMV.

Brian Luce · November 22nd, 2009, 10:24 PM

On the 24-75 f2.8 Tamron, I've found exactly what Daniel says, short travel of focus ring -- which makes focusing super critical and difficult on the LCD. The interesting thing is, that same lens, when viewed through the viewfinder, is an absolute piece of cake to focus, even at the widest zoom setting. Too bad Canon couldn't figure a way to use that viewfinder during video mode, it'd solve a lot of issues.

Michael Richard · November 25th, 2009, 11:09 AM

what is this two position zoom button you refer too? Where is that exactly?

Brian Luce · November 25th, 2009, 12:16 PM

Quote:

Originally Posted by Michael Richard

what is this two position zoom button you refer too? Where is that exactly?

Upper right corner of camera back.

Chris Hurd · November 25th, 2009, 01:07 PM

It's best described as an image magnification function, just to avoid confusing it with zooming as in changing focal length, which of course is possible only via the control ring on the lens barrel.

Brian Luce · November 25th, 2009, 01:26 PM

Quote:

Originally Posted by Chris Hurd

It's best described as an image magnification function, just to avoid confusing it with zooming as in changing focal length, which of course is possible only via the control ring on the lens barrel.

If it was up to me, I'd call it focus assist. That's what they call it on other cams, but even in the manual, there's no mention of it as a focus assist tool. The manual generically names it "Magnify Button".

November 22nd, 2009, 04:31 PM	#1
Brian Luce Inner Circle Join Date: Jan 2006 Posts: 2,290	Is a parfocal lens necessary? Seems to add a lot the price of a lens. For video, why not just use that zoom/FA button? The big problem with the 7d is holding focus, not getting it initially. Parfocal wouldn't seem to help pulling focus unless you're zoomer?

November 25th, 2009, 01:07 PM	#12
Chris Hurd Obstreperous Rex Join Date: Jan 2001 Location: San Marcos, TX Posts: 27,366 Images: 513	It's best described as an image magnification function, just to avoid confusing it with zooming as in changing focal length, which of course is possible only via the control ring on the lens barrel. __________________ CH Search DV Info Net \| 20 years of DVi \| ...Tuesday is Soylent Green Day!

November 22nd, 2009, 05:31 PM	#3
Jerry Porter Major Player Join Date: Aug 2005 Location: Satellite Beach, Fl Posts: 784	But it does help you get it correct the first time and lets you set frame off of that.

November 22nd, 2009, 08:20 PM	#5
Benjamin Eckstein Major Player Join Date: Jan 2007 Location: Boston, Massachusetts Posts: 380 Images: 1	Interesting, I had to look up this term to see what it meant. Being used to video zooms, I sort of assumed that photo zooms all held focus throughout the zoom. Is that not the case? If so, how would the focus marks on the lens work, as they don't change as you zoom? So are most photo zooms not parafocal? I bought the 17-55 2.8 (Canon) with the 7D and am planning to get the 20-700.

November 22nd, 2009, 09:24 PM	#7
Benjamin Eckstein Major Player Join Date: Jan 2007 Location: Boston, Massachusetts Posts: 380 Images: 1	Good to know, Daniel. Thanks for that info. And thank god for the excellent focus mag feature on the 7D.

November 22nd, 2009, 09:42 PM	#8
Jerry Porter Major Player Join Date: Aug 2005 Location: Satellite Beach, Fl Posts: 784	I use a Tamron 17-50 f2.8 and a Tamron 70-200 f2.8 and both are Parfocal. They really are great video lenses and are worth every cent spent on them. As far as the true parfocal nature of them they are very close through out the the full zoom range. If you are at a low f they wander a little but not much. YMMV.

November 22nd, 2009, 10:24 PM	#9
Brian Luce Inner Circle Join Date: Jan 2006 Posts: 2,290	On the 24-75 f2.8 Tamron, I've found exactly what Daniel says, short travel of focus ring -- which makes focusing super critical and difficult on the LCD. The interesting thing is, that same lens, when viewed through the viewfinder, is an absolute piece of cake to focus, even at the widest zoom setting. Too bad Canon couldn't figure a way to use that viewfinder during video mode, it'd solve a lot of issues.

November 25th, 2009, 11:09 AM	#10
Michael Richard Regular Crew Join Date: Dec 2003 Location: Midcoast, Maine Posts: 89	what is this two position zoom button you refer too? Where is that exactly?