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Old August 24th, 2005, 03:15 PM   #1
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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
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Old August 24th, 2005, 03:22 PM   #2
 
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Stefan, haven't had this problem. Sorry I can't be of any help to you on this.

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Old August 24th, 2005, 04:15 PM   #3
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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...



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Old August 25th, 2005, 11:44 AM   #4
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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).
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Old August 25th, 2005, 11:58 AM   #5
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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.

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Old August 25th, 2005, 12:22 PM   #6
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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.


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Old August 25th, 2005, 01:54 PM   #7
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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...



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Old August 27th, 2005, 03:05 PM   #8
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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.
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Old August 27th, 2005, 03:17 PM   #9
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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
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Old August 27th, 2005, 03:17 PM   #10
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so why exactly does the aperature close up as I zoom in?
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.
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Old August 27th, 2005, 03:19 PM   #11
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Originally Posted by Chris Hurd
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
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Old August 27th, 2005, 10:32 PM   #12
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right on. hopefully this is the problem. we'l do some testing.
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Old August 28th, 2005, 12:43 AM   #13
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Originally Posted by Mike Hardcastle
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
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.
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Old August 28th, 2005, 11:24 AM   #14
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Originally Posted by David Lach

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.
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Old August 28th, 2005, 12:15 PM   #15
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Originally Posted by Marty Hudzik
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?
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