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06/17/2007 01:03:51 PM · #1 |
If I press the shutter halfway to get focus set, do I have to do it again if I zoom in or out? I've always done it, but now I'm reading more and it seems like, at least in some cases, I don't.
What terms or feature description should I look for to not have to refocus?
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06/17/2007 01:09:38 PM · #2 |
Depends on the lens. The term for a lens that does not change focus when zooming is "parfocal."
ETA: Even a parfocal lens usually loses this desirable property if an extension tube is added. Teleconverters, however, don't affect whether a lens is parfocal.
Message edited by author 2007-06-17 13:10:37. |
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06/17/2007 01:21:06 PM · #3 |
Even when using a parfocal lens, I don't trust the focus to be precise when zooming IN, though it seems to do OK zooming OUT. This is a problem partly of precision of focus (focus on a given spot more accurate when zoomed in on that spot) and partly of DOF (the longer you zoom to the less DOF you have and the more critical is precise focus).
Technically, a "true" zoom lens will by definition be parfocal, and a lens that is not is a "varifocal lens"; a lens that can be set to various focal lengths but not a "zoom" lens. I'm not sure how valid this distinction is anymore. I think that most modern zooms take at least a nominals tab at holding focus throughout the zoom range. Certainly, if you want to work that way you should zoom in tight, set focus, hold focus, and zoom back out to compose.
R.
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06/17/2007 01:29:59 PM · #4 |
Originally posted by Bear_Music:
...I think that most modern zooms take at least a nominals tab at holding focus throughout the zoom range. Certainly, if you want to work that way you should zoom in tight, set focus, hold focus, and zoom back out to compose.
R. |
Canon's consumer zooms (and at least one "L" zoom) give up being parfocal as a design compromise; Here is a listing of the ones that are, as well as some info on why the ones that arent, aren't. Originally from Chuck Westfall of Canon USA.
I would think that they still try to minimize the focus shift, to the extent that's possible in the design. |
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06/17/2007 02:03:43 PM · #5 |
A trick i found on my Sigma AF 70-210 (push pull zoom) when manual focusing is to zoom all the way in focus and then set the zoom where i want it. It helps me (bad vision) to get a hell of alot better focus.
That said my point and shoot's focus goes all whack when zooming. |
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06/17/2007 02:07:02 PM · #6 |
Originally posted by kirbic:
Canon's consumer zooms (and at least one "L" zoom) give up being parfocal as a design compromise; Here is a listing of the ones that are, as well as some info on why the ones that arent, aren't. Originally from Chuck Westfall of Canon USA.
I would think that they still try to minimize the focus shift, to the extent that's possible in the design. |
Kewl. I definitely thought my 70-200mm f/4L was a parfocal, and I see it is listed as one. My 10-22mm isn't, but if you focus at 22mm and zoom out to 10mm it seems to work fine...
R.
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06/17/2007 04:00:29 PM · #7 |
When Angineaux developed the first zoom lenses (late 1950's), his objective was to eliminate refocusing of variable power lens assemblies. However, the more extended the zoom ratio becomes, the tighter the tolerances are, leaving more room for position error. I have made variable power assemblies that did not use Angineaux's complex mechanics, but instead required repositioning two lens elements with respect to the objective lens. Works well for astronomical telescopes.
Shorter zoom ratios, ie. 3-5X, maintain accurate focus somewhat better than extreme ratio lenses such as the 10-15X lenses. Todays cad designed systems have overcome much of the limitations in lens systems but still require a bit of common sense in practice.
Message edited by author 2007-06-17 16:03:23. |
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06/17/2007 11:25:37 PM · #8 |
Thanks, everyone!
One more quickie: does "push-pull" mean you're literally shoving the zoom in and out instead of turning a ring? Any advantages to either method?
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06/17/2007 11:29:54 PM · #9 |
Originally posted by levyj413:
Thanks, everyone!
One more quickie: does "push-pull" mean you're literally shoving the zoom in and out instead of turning a ring? Any advantages to either method? |
Yes, push pull is what it is.
For sports it is a quicker on action, and your not tweaking your wrist and arm to get the focal length.
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06/17/2007 11:33:06 PM · #10 |
Originally posted by levyj413:
Thanks, everyone!
One more quickie: does "push-pull" mean you're literally shoving the zoom in and out instead of turning a ring? Any advantages to either method? |
I guess since i mentioned it thats why it came up. Yeah heres the lense:
I nearly broke the thing when it came in the mail trying to twist it lmao. |
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