FOCUS issues with the 20X Lense
 

We're pushing in to the eyes of an interview subject to get focus, as we pull out(with the lense on Manual focus) the camera seems to go soft on the subject and work on bringing the area behind the focal point into focus. Any ideas? Has anyone dealt with this problem before.

To re-summarize- The 20x lense is not holding sharp focus to the point it is focused to. It is on Manual. There is no accidental 'hitting' of the focus ring.

Stefan

Stefan, haven't had this problem. Sorry I can't be of any help to you on this.

Jay

What is your aperture set to? At full zoom you should try and keep it WIDE OPEN. It sounds like what you are referring to may just be normal, the shorter you make the lens the BIGGER the depth of field, therefore, more things will come into focus. At full zoom you have a tiny DOF and everything in the background is out of focus, the more you pull back (making the lens shorter) the larger the DOF and things that were previously out of focus, are now in focus...



ash =o)

Rather than being a DOF issue, I think it really is a lense issues. In using two cameras, and exchanging the cameras between our two set angles, the issue was resolved with the other camera(at the same aperature and shutter settings).. and the problem moved to our cut-away angle(the problem travels with the camera/lense).

Sounds like a lens issue to me, too. I've had no problem like this at many different aperture/iris settings. Might want to call Canon.

Kevin

I find if I try to do a close up focus, if you zoom that far that the aperture starts to close about 75%ish of the total zoom, you can only get away with maybe 1 stop less i.e from 1.6 to 1.8 for critical focus, if you zoom all the way in to 3.4.??(i Think) and then focus, once you pull out back to 1.6 the focus will be out....so basically zoom in till the aperture starts to close (about 75%) and that will be your best chance to get a close focus.


Mike

Never had it happen to me but heard that it does... just send the lens to Canon and they will adjust the back-focus or whatever the problem is...



ash =o)

Mike,
 

so why exactly does the aperature close up as I zoom in? This might be my problem, Ill work on focussing 75% in. However, folks on our crew said that today, even without a shifting aperature, the focus was softening as we pulled back out from the full zoom. So, it may be a back-focus lense issue. I'll get on the phone with canon, regardless.

stephan

as far as im aware this is what all zoom lenses do when used, the canon 20x has a spec of F1.6 at full wide and F3.4 (off the top of my head)at full zoom.

But like I said earlier, the aperture change only starts to happen at roughly 75% of the zoom.....once it starts to close more than a couple of stops, the depth of field changes enough to take the focus out once returned to the full wide setting

hope this helps and hope im right..!!!!! anyone..??

Mike

Because the lens has different maximum aperture values depending on what focal length you're using. At full wide, the maximum aperture is f/1.6 and at full telephoto the maximum aperture value is f/3.5. If you lock the exposure at f/3.5 then you won't see a change in aperture when you're adjusting the focal length.

thanks for the conformation chris.

Mike

right on. hopefully this is the problem. we'l do some testing.

You're only part right (if I understood correctly what you were saying). The iris changing will indeed affect the DOF, but it should not affect the subject on which you focused. only the space in front or behind him. The DOF doesn't move while opening or closing the iris (front to back), it is a constant ratio determined from the focusing point, so it just gets shallower or deeper in front and behind that point.

When you close in to focus on the eyes for example, the fact the iris changes doesn't matter, when you pull back, even though the F-stop change will affect the DOF, the eyes will (should) still remain sharp and clear since it is your focusing point (the distance between the subject and the focal plane determines the focusing point, not the iris nor the focal length, that's true for all manual and auto lenses). If the subject doesn't remain in focus, there's a back-focus issue.

Also, some lenses will have one fixed minimum aperture for all focal lengths. My 14x manual lens does this. But the 20x stock lens cannot keep it's minimum value of F1.6 throughout the whole zooming range.

My 16x manual lens keeps a constant aperture throughout the zoom range also. Part of the reasons I bought it.

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.

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.

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.

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



ash =o)

I have same issue too!
 

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.

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....

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?

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.

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.

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

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?

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).

As long as you have the little servo motor engaged on the zoom it should at least handle zooming functions. Obviously no focus features would work as the manual lens doesn't support this. The other functions are not lens specific so they should all work. I can't confirm this but it seems logical.

(Just to clarify the manual 16x lens has a manual and servo zoom function that you can engage/disengage as you need it. This allows you to do quick zooms in and out via hand or set it to allow the XL2 rocker to control zoom. But it can't do both simultaneously)

thanks guys, just to clarify though..if I got the manual 16x I could control the zoom via lanc at the same time as changing focus via the lens barrel as and when I wished?

Yes, on the black 16x manual lens, you can zoom remotely via LANC controller while changing focus manually by hand. If you want to do it all from the tripod pan handles, then you could get fancy and also control focus remotely with a separate mechanical focus controller, about $500 from VariZoom.

that's cool, might as well work in a TV studio again! nice idea, but just slightly too pricey for me at the minute though. would be great to control it all from the pan handles - would need a bigger and better LCD screen than the XL2's little thing! what's the best ext monitor for this cam in your opinion as the included one is ok for composition but pretty poor for focus? I do find it easier to focus with the screen though rather than the eyepiece magnifier, what do you all prefer, or have you all gone for the b&w 'proper' VF? (should I start a new thread somewhere else?)

Yes that is a new topic... but please don't start a new one as there are scores of existing threads here about LCD monitors for the XL series camcorders. Please search through those discussions first. Thanks,