FOCUS issues with the 20X Lense

[Marty Hudzik]

Originally Posted by Steve House

Does it really keep a constant aperture? I ask because the /f number is a ratio of the aperture diameter to the focal length. At f/8 the aperture is 1/8 the focal length so at f=10mm it is 1.25mm in diameter while at f=20mm it is 2.5mm. So if the lens is set to, say, f/8 and you zoom from 10mm to 20mm focal length, the diameter of the aperture must also change in step with the focal length in order to keep the ratio constant. Some lenses are built so that when you set f/8 and zoom the lens the aperture automatically adjusts to keep it at the same /f number. On other lenses the aperture isn't linked to the zoom control and so it doesn't physically change in size with changes in the zoom setting. In that case setting to f/8 at f=10mm and zooming to f=20 results in the aperture becoming f/16. Which way are the Canons made?

I can't answer that question technically. But let me give you a real world answer on the Canon 16x manual lens. When I am in a dark room and at wide and the f-stop is wide open and I zoom all the way to the most telephoto end of the lens I see no loss in light. THe display stays the same also. This is to the naked eye and not scientific but it is easy to see. With the 20x lens or even the DVX 10x lens....if you are at 1.6 and zoom all the way you can see an obvious drop in light. Not the case with the 16x manual.

I have been told by people who really know lenses that this should not be happening with a lens of this caliber. Only $$$$ lenses are calibrated to hold aperture through the range. So all I can comment on is how mine behaves and I am glad I bought it. It may not be holding the exact aperture internally but the bottom line is the apparent light availablity does not seem to be diminsihed at all at full telephoto.

FWIW.

[Steve House]

Originally Posted by Marty Hudzik

I can't answer that question technically. But let me give you a real world answer on the Canon 16x manual lens. When I am in a dark room and at wide and the f-stop is wide open and I zoom all the way to the most telephoto end of the lens I see no loss in light. THe display stays the same also. This is to the naked eye and not scientific but it is easy to see. With the 20x lens or even the DVX 10x lens....if you are at 1.6 and zoom all the way you can see an obvious drop in light. Not the case with the 16x manual.

I have been told by people who really know lenses that this should not be happening with a lens of this caliber. Only $$$$ lenses are calibrated to hold aperture through the range. So all I can comment on is how mine behaves and I am glad I bought it. It may not be holding the exact aperture internally but the bottom line is the apparent light availablity does not seem to be diminsihed at all at full telephoto.

FWIW.

And the iris is on manual control?

[Marty Hudzik]

Originally Posted by Steve House

And the iris is on manual control?

Absolutely on manual. If it says 1.8 in the VF at wide.....it also says 1.8 when zoomed max. The more important thing is that the scene looks visually identical at both ends, in regards to light levels.

[Steve House]

Kewl - sounds like one of several good reasons to opt for that lens. I'm kind of old-fashioned and for stills I like the control one gets with a film camera with manual controls and a hand held light meter. <gasp!> For stills I even prefer fixed focal length lenses - after close to 40 years of serious still photography it was only last year the I acquired my very first zoom lens to fit my 25 year old Nikon F2.

[Ash Greyson]

When on a tripod, the 16X manual is incredible. The ONLY downside to the lens is the lack of OIS...



ash =o)

[Matthew Quinn]

I also have the same issue with setting my focus, zoom in to set the focus, pan out and it loses it, this is very annoying!

After reading that there is more than one person annoyed with this I will be sending it back to canon.

[David Lach]

Matthew, I can confirm that both my XL2s, when used with their stock lenses (20x) do not lose focus between both extremes of the zooming range, regardless of the aperture change. You'll see a change in DOF of course, but the focal point should remain sharp. If it doesn't there's a strong possibility there's a back-focus issue.

If you do send it back, it would probably be wise to ask Canon before if you need to send the camera too. Sometimes the problem can be with the lens, other times it can be the camera (or even both). At least that's what I heard, never had such a problem myself.

[Marty Hudzik]

Originally Posted by David Lach

Matthew, I can confirm that both my XL2s, when used with their stock lenses (20x) do not lose focus between both extremes of the zooming range, regardless of the aperture change. You'll see a change in DOF of course, but the focal point should remain sharp. If it doesn't there's a strong possibility there's a back-focus issue.

If you do send it back, it would probably be wise to ask Canon before if you need to send the camera too. Sometimes the problem can be with the lens, other times it can be the camera (or even both). At least that's what I heard, never had such a problem myself.

I was under the impression that the lens and body need to be calibrated together because the backfocus is an adjustment based on the area in which the lens mounts to the camera. If this mount is off by just a miniscule amount the backfocus could be off. Someway or another the Canon Tech gurus adjust for this in the camera/lens firmware so that you have a properly matched lens/camera. either one by itself would be hard to calibrate.

For what it is worth, I have the 16x manual lens and it can be backfocus adjusted by the end user via a small flange at the back of the lens. On my XL2 I have the backfocus tweaked. When I put it on a different XL2 I had to adjust for that one. In turn, when put back on mine I had to readjust again. This shows that there can be minor differences between 2 cameras and maybe between all cameras.

The auto lens is supposed to compensate automatically but there are times that it will need adjusted.

FWIW....

[Gary Barr]

I love the DOP I get from the 20x stock lens, but am considering the manual lens mainly because I can't stand it when I can't through or pull focus at the same time as zooming in or out. Is this a normal thing for an auto lens?

[Ash Greyson]

Yes...but the 20X you can set it up to do that for you... many people dont use this feature but it does work very well with practice. You can zoom manually and use a focus preset... or do a zoom preset... or do both...




ash =o)

[Marty Hudzik]

Originally Posted by Ash Greyson

Yes...but the 20X you can set it up to do that for you... many people dont use this feature but it does work very well with practice. You can zoom manually and use a focus preset... or do a zoom preset... or do both...
ash =o)

I am not 100% sure on the 20x lens but I am pretty sure that the older XL series servos were shared between the focus and the zoom. Meaning if you were in the process of zooming out slowly and tried to focus it would screw up. Either it ignored the attempted focus because the zoom was already in progress or it interupted the zoom to focus. I think.....That could potentially be a problem for pulling focus while zooming.

I will check it out this morning. I use my 16x manual lens almost exclusively so I am not as familiar with the 20x as Ash and many other on this board.

[Gary Barr]

yes Marty, that's exactly what happens, bit of a pain to work with when you want to focus somewhere else during a zoom.
thanks Ash, it's ok to use the preset feature when you've got the time but not a run and gun scenario which is my field. I find the presets a bit fiddly anyway actually, because if you are at the tele end and even touch the lens, never mind flick the switch to activate the preset, you're going to get unsatisfactory shake - maybe if it was included in a Lanc controller(is it?)

cheers.

[Chris Hurd]

Focus may be controlled remotely via LANC, but not the focus position preset function.

[Gary Barr]

yeah I've got a VZ Rock but seldom use the focus control, only the zoom and record really. still wouldn't help me though if I did use it to focus. I'd like to be able to pan/tilt and zoom with the VZ, and sometimes change focus on the lens barrel simultaneously...I'll have to get the manual lens won't I? Is it also controllable via Lanc?

[Chris Hurd]

The manual lens does not have a focus motor, therefore focus cannot be controlled remotely via LANC (although zoom is remotely controllable on that lens).