EF Zoom through focus? at DVinfo.net

Annen James · May 28th, 2013, 10:42 AM

Hello,
Does the C300 retain focus when zooming using a EF zoom lens (like 24-70)?

Matt Davis · May 28th, 2013, 11:23 AM

This is the function of the lens, not the camera.

You're looking for the behaviour of most ENG video lenses - they are 'Parfocal', in that they retain the same focus regardless of the zoom setting.

This is quite difficult to do, and as the size of the sensor increases, so does the complexity of the mechanics and the optics. A small camera with a small 'chip' doesn't need to sweat the details because a wider lens will cover its sensor and they have a shallow depth of field.

As you scale up the sensor size, so you need a longer lens to cover the sensor, and so the depth of field decreases due to the focal length, and it gets exponentially harder to maintain focus through the zoom range.

Modern cameras with fixed lenses often have an extra lens group that moves via an electronic guidance system, according to the zoom setting, which helps the focus. Photographic lenses have no such need, and so you can't zoom and hold focus at the same time.

Cine lenses that ARE parfocal tend to be prohibitively expensive because of their complex construction and design.

The Canon 24-105 is 'almost' parfocal, hence its popularity as an interview lens with large sensor cameras. Even between lenses of the same model and manufacturer can vary in their degree of parfocal accuracy.But it's a rare design. Can anyone else nominate parfocal or near-parfocal lenses?

Mark Dobson · May 29th, 2013, 02:50 AM

For those on a budget and wanting that little bit extra at both ends I'd recommend the
Canon EF-S 18-135mm f/3.5-5.6 STM IS. Whilst not having the elan of an 'L' series lens it is a very bright and sharp contender.

The first thing I checked when I bought it from Park Cameras was its parfocal performance. Coming from video camera I'd always learnt to zoom in, check the focus then zoom out to commence filming.

By nature of it's construction it is also vey easy to accomplish acceptable live zooms, in or out. And it's 'fly-by-wire' focus can be accurately controlled with a light touch. I look forward to some of it's features being incorporated in the optically superior L series, some of these have not seen a mechanical update since the last century.

Matt Davis · May 29th, 2013, 03:05 AM

Quote:

Originally Posted by Mark Dobson

I'd recommend the Canon EF-S 18-135mm f/3.5-5.6 STM IS.

And IIRC, this is at least one lens Canon have chosen to support in the C100 for 'Continual AF' - which has its uses in pro Videography! :)

The Sony SEL-18200 was not parfocal, but you could *just* get away with a sneaky zoom when AF was engaged, so hoping that - if not actually parfocal - the Canon 18-135 can do this with the Canon C100 update. Also hoping that the 18-200 would do this too.

Mark Dobson · May 29th, 2013, 03:57 AM

Ha - had to look up IIRC - it's the same sort of technical acronym that David Cameron used in his emails to Rebekah Brooks.

IMHO the biggest downside of these fabulous large chip Canon Video Cameras is the lack of a custom built E mount servo video zooms.

Sony have produced the Sony SCL-Z18X140 18-252mm / T3.8 (wide) servo zoom lens with FZ mount for the PMW-F3.

Quite a price at £ 7,772 (inc vat) but also quite a snip compared to the Canon CN-E30-300mm T2.95-3.7 LS EF at a mere £35,994.00 (inc vat)

But yes I would like one.

Matt Davis · May 29th, 2013, 04:52 AM

Quote:

Originally Posted by Mark Dobson

yes I would like one.

Looks great on an F3! A friend had one for a couple of weeks, and tried it hand-held. I'm sure I saw his right hand forearm muscles develop a certain asymmetry. It went back. He uses Nikkors now.

Or how about this:

http://www.bhphotovideo.com/c/product/839228-REG/Canon_6142B001_CN_E_30_300mm_T2_95_3_7_L.html
"Boggle!" (for our non-UK readers, boggle - Idioms - by the Free Dictionary, Thesaurus and Encyclopedia. - just in case)

I think it's all testiment to how difficult it is to do properly. Alister's solution is 'good enough' for those of us with a real need to make our large sensor cameras cross-dress as ENG cameras.

My one little worry about the MTF B4 adaptor is that I really think it needs rod support. The B4 mount is solid, and usually runs into the chassis in an ENG camera. Here, it mates to an adaptor, which then hangs on the eMount. I would want a baseplate with rods and a support arm under that lens. I've seen similar situations with the DoF adaptors of a few years back, and you could SEE the 'droop' without support.

I've also had the sudden and awful demise of a heavy lens that luckily snapped at the lens to retro-fitted-mount end - i.e. luckily not ripping the camera mount out. Having said that, I've seen photos of cameras with ripped lens mounts, and hence my pre-occupation with rods.

Chris Malley · May 29th, 2013, 05:34 AM

Theres a list I found courtesy of Doug Chinnery's own blog.

Doug Chinnery Photography: Canon Parfocal Lenses

Listing a fair few (and quite surprising!) lenses which even I didn't expect!

I can vouch for the 70-200 f/4L being parfocal however, fantastic interview lens too.

May 28th, 2013, 10:42 AM	#1
Annen James Regular Crew Join Date: Sep 2009 Location: Ogden Posts: 161	EF Zoom through focus? Hello, Does the C300 retain focus when zooming using a EF zoom lens (like 24-70)?

May 28th, 2013, 11:23 AM	#2
Matt Davis Trustee Join Date: Aug 2005 Location: Berkshire, UK Posts: 1,562	Re: EF Zoom through focus? This is the function of the lens, not the camera. You're looking for the behaviour of most ENG video lenses - they are 'Parfocal', in that they retain the same focus regardless of the zoom setting. This is quite difficult to do, and as the size of the sensor increases, so does the complexity of the mechanics and the optics. A small camera with a small 'chip' doesn't need to sweat the details because a wider lens will cover its sensor and they have a shallow depth of field. As you scale up the sensor size, so you need a longer lens to cover the sensor, and so the depth of field decreases due to the focal length, and it gets exponentially harder to maintain focus through the zoom range. Modern cameras with fixed lenses often have an extra lens group that moves via an electronic guidance system, according to the zoom setting, which helps the focus. Photographic lenses have no such need, and so you can't zoom and hold focus at the same time. Cine lenses that ARE parfocal tend to be prohibitively expensive because of their complex construction and design. The Canon 24-105 is 'almost' parfocal, hence its popularity as an interview lens with large sensor cameras. Even between lenses of the same model and manufacturer can vary in their degree of parfocal accuracy.But it's a rare design. Can anyone else nominate parfocal or near-parfocal lenses? __________________ Director/Editor - MDMA Ltd: Write, Shoot, Edit, Publish - mattdavis.pro EX1 x2, C100 --> FCPX & PPro6

May 29th, 2013, 02:50 AM	#3
Mark Dobson Major Player Join Date: Jan 2009 Location: Horsham / United Kingdom Posts: 328	Re: EF Zoom through focus? For those on a budget and wanting that little bit extra at both ends I'd recommend the Canon EF-S 18-135mm f/3.5-5.6 STM IS. Whilst not having the elan of an 'L' series lens it is a very bright and sharp contender. The first thing I checked when I bought it from Park Cameras was its parfocal performance. Coming from video camera I'd always learnt to zoom in, check the focus then zoom out to commence filming. By nature of it's construction it is also vey easy to accomplish acceptable live zooms, in or out. And it's 'fly-by-wire' focus can be accurately controlled with a light touch. I look forward to some of it's features being incorporated in the optically superior L series, some of these have not seen a mechanical update since the last century.

May 29th, 2013, 03:57 AM	#5
Mark Dobson Major Player Join Date: Jan 2009 Location: Horsham / United Kingdom Posts: 328	Re: EF Zoom through focus? Ha - had to look up IIRC - it's the same sort of technical acronym that David Cameron used in his emails to Rebekah Brooks. IMHO the biggest downside of these fabulous large chip Canon Video Cameras is the lack of a custom built E mount servo video zooms. Sony have produced the Sony SCL-Z18X140 18-252mm / T3.8 (wide) servo zoom lens with FZ mount for the PMW-F3. Quite a price at £ 7,772 (inc vat) but also quite a snip compared to the Canon CN-E30-300mm T2.95-3.7 LS EF at a mere £35,994.00 (inc vat) But yes I would like one.

May 29th, 2013, 05:34 AM	#7
Chris Malley New Boot Join Date: Mar 2013 Location: Manchester, UK Posts: 22	Re: EF Zoom through focus? Theres a list I found courtesy of Doug Chinnery's own blog. Doug Chinnery Photography: Canon Parfocal Lenses Listing a fair few (and quite surprising!) lenses which even I didn't expect! I can vouch for the 70-200 f/4L being parfocal however, fantastic interview lens too.