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03-19-2008, 09:12 AM   #1
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DA*50-135 Questions

Hello,

I just purchased a DA*50-135 last week and have been playing around with it a little. I have noticed that if I focus on something and then zoom the lens, the focus changes a lot. Is this normal for this lens? I tries my DA50-200 and it doesn't seem to do this. I love the lens, but I am not sure if it has focus issues and need to determine if I am gong to exchange it.

03-19-2008, 09:33 AM   #2
McD
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Are you zooming in and focusing and then zooming back out? If this problem is occuring when you do that, then yes, there is probably an issue with the lens. Focus shouldnt change at all when doing this. If your focusing then zooming in and it's doing it, it could just be that you think your subject is in focus, but youve just missed it.
03-19-2008, 09:36 AM   #3
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Also, AF on most lenses is slightly less accurate at the wide end than the long end, esp. at large apertures.
03-19-2008, 11:15 AM   #4
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It exhibits this behavior zooming both ways. If I zoom in and focus, and then zoom back out, I can see the focus changing in the viewfinder and when I press the AF button, it snaps back into focus. If I focus while at 50mm, I can see in the viewfinder that the subject is in focus (or close enough), but when I zoom in I can see the focus changing. Again, if I press the AF button, it snaps back into focus.

03-19-2008, 11:22 AM   #5
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QuoteOriginally posted by ticktock Quote
It exhibits this behavior zooming both ways. If I zoom in and focus, and then zoom back out, I can see the focus changing in the viewfinder and when I press the AF button, it snaps back into focus. If I focus while at 50mm, I can see in the viewfinder that the subject is in focus (or close enough), but when I zoom in I can see the focus changing. Again, if I press the AF button, it snaps back into focus.
That's completely normal, because if you change the focal length, you change the focal point! If you intend to keep a subject in focus while zooming, set your autofocus to AF.C (continuous) instead of AF.S.

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03-19-2008, 01:54 PM   #6
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It's so normal among zoom lenses that a lot of them have different focusing index marks with the focal length against them (I've got my old 80-320mm and 28-105mm lenses to hand, and they both show this).
03-19-2008, 03:10 PM   #7
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It may be normal behaviour for a cheap consumer zoom, but most disappointing behaviour for a professional grade zoom.

This is what happens when a once great marque cuts corners and dilutes the image of its top of the range line.
03-19-2008, 04:03 PM   #8
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Actually I see no problems with this. After all, AF is based on contrast detection. When one changes the focal length, the area the focus point is aiming at does change ever so slightly. So what is wrong if the focus is reset by the camera? The comment that it is normal behaviour for a cheap consumer zoom, but most disappointing behaviour for a professional grade zoom is without any basis.

03-19-2008, 04:32 PM   #9
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QuoteOriginally posted by creampuff Quote
Actually I see no problems with this. After all, AF is based on contrast detection. When one changes the focal length, the area the focus point is aiming at does change ever so slightly. So what is wrong if the focus is reset by the camera? The comment that it is normal behaviour for a cheap consumer zoom, but most disappointing behaviour for a professional grade zoom is without any basis.

Yes, I would expect the lens to refocus as the camera detects a changed contrast point - the camera has no way of knowing if you're aiming at the same point, or something else altogether. It only detects the edges/contrast - which would change as the aspect ratio changes... In fact, I'm very confused as to why anyone would think this would not be the case.



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03-19-2008, 04:57 PM   #10
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Zooms that change focus while zooming are called varifocal zooms and are not considered true zooms. They are cheaper to make than true zooms which are called parfocal zooms and do not change focus when zoomed

Its certainly an inconvenience to have to refocus, when you change focal length, in the same way that it is an inconvenience to have to change aperture (for manual flash. dof, shutter speed etc.) whilst zooming. Being able to maintain constant focus and constant aperture whilst zooming are important requirements fro professional zooms - having to change settings between shots will mean missed shots and therefore missed revenue for a Pro.
03-19-2008, 05:29 PM   #11
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I believe many are well aware of the difference between a varifocal and parfocal zoom. Whether it is one or the other, from my experience, there is little practical difference in today's context considering that zoom lenses today focus very quickly. The issue is whether it performs where it matters most, namely optical quality. I'll hazard a guess the 50-135 is a parfocal zoom, but does it make a big difference? Probably not.
03-19-2008, 06:59 PM   #12
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QuoteOriginally posted by creampuff Quote
I'll hazard a guess the 50-135 is a parfocal zoom, but does it make a big difference? Probably not.
If you believe the 50-135 is a parfocal, then the focus should not change while zooming. Its varifocals that do change the focus point.
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