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August 24th, 2005, 02:15 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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August 24th, 2005, 02:22 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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August 24th, 2005, 03:15 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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August 25th, 2005, 10:44 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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August 25th, 2005, 10:58 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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August 25th, 2005, 11:22 AM | #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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August 25th, 2005, 12:54 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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August 27th, 2005, 02:05 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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August 27th, 2005, 02:17 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 |
August 27th, 2005, 02:17 PM | #10 | |
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August 27th, 2005, 02:19 PM | #11 | |
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thanks for the conformation chris. Mike |
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August 27th, 2005, 09:32 PM | #12 |
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right on. hopefully this is the problem. we'l do some testing.
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August 27th, 2005, 11:43 PM | #13 | |
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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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August 28th, 2005, 10:24 AM | #14 | |
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August 28th, 2005, 11:15 AM | #15 | |
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