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08-16-2010, 05:59 PM   #1
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Focusing at 18mm different than at 200mm

I noticed that on my Tamron 18-200mm lens that when I focus on an object at 18mm, it will be in focus then when I zoom in on that object the object is out of focus, even though the focus distance has remained the same. For example, when I focus in on a car that is 25 feet away at 18mm the focus ring will show 75 feet, when I zoom in on that same car to 200mm the focus ring will show the correct 25 feet. So I guess my question is, is it normal for the focus distance to be different at 18mm than at 200mm?

08-16-2010, 06:07 PM   #2
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That lens, as with most modern zooms, is varifocal. Zooming it changes the focus distance.
08-16-2010, 06:29 PM   #3
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I have the same lens and i find the 200mm a bit soft for me..
08-16-2010, 07:34 PM   #4
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This is a classical problem with vari-focal zooms, and goes back many years - I recall the same problem from my film days. A trick I use is to use was focus at full zoom, and then zoom back.

The physicists on this forum will explain it better - but I believe it relates to the area of the image projected onto the focus sensor and the amount of contrast. Zooming in will provide stronger contrast differences which are more accurately focused on.

Cheers

08-16-2010, 08:09 PM   #5
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QuoteOriginally posted by redeleon Quote
I have the same lens and i find the 200mm a bit soft for me..
Is it normal for the focus ring to have a "mechanical" feel and noise when manual focusing?

QuoteOriginally posted by MoiVous Quote
This is a classical problem with vari-focal zooms, and goes back many years - I recall the same problem from my film days. A trick I use is to use was focus at full zoom, and then zoom back.

The physicists on this forum will explain it better - but I believe it relates to the area of the image projected onto the focus sensor and the amount of contrast. Zooming in will provide stronger contrast differences which are more accurately focused on.

Cheers
Yeah, that's what I've been doing and it's annoying to have to do that all the time, but it's good to know that's normal
08-16-2010, 08:43 PM   #6
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QuoteOriginally posted by KxBlaze Quote
I noticed that on my Tamron 18-200mm lens that when I focus on an object at 18mm, it will be in focus then when I zoom in on that object the object is out of focus, even though the focus distance has remained the same. For example, when I focus in on a car that is 25 feet away at 18mm the focus ring will show 75 feet, when I zoom in on that same car to 200mm the focus ring will show the correct 25 feet. So I guess my question is, is it normal for the focus distance to be different at 18mm than at 200mm?
The lens is a varifocal lens, to some of us old-timers it's not really a zoom lens. Back in the days of the dinosaur, they were two distinct breeds. With manual focus, it was really a lot better to have a zoom (focus does not change while zooming) to a varifocal, which gives the effect you have noticed. A "true" zoom lens is much more difficult and complicated to build. It shows in the prices, and we are not talking small change difference.

Another poster mentions zooming to the longest focal length and focusing, then zoom back. This works more or less on varifocal lenses because of the increased depth of field at shorter focal lengths.

Most modern "zoom" lenses are varifocal lenses. With autofocus, it really does not make much difference, but with a manual focus lens it can be a royal pain.
08-16-2010, 10:16 PM   #7
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QuoteOriginally posted by Canada_Rockies Quote
The lens is a varifocal lens, to some of us old-timers it's not really a zoom lens. Back in the days of the dinosaur, they were two distinct breeds. With manual focus, it was really a lot better to have a zoom (focus does not change while zooming) to a varifocal, which gives the effect you have noticed. A "true" zoom lens is much more difficult and complicated to build. It shows in the prices, and we are not talking small change difference.

Another poster mentions zooming to the longest focal length and focusing, then zoom back. This works more or less on varifocal lenses because of the increased depth of field at shorter focal lengths.

Most modern "zoom" lenses are varifocal lenses. With autofocus, it really does not make much difference, but with a manual focus lens it can be a royal pain.
That's why the Vivitar 70-210 series I was so extremely expensive at it's time (and in the present day still an extremely good M-lens, it is even f1:3.5 over it's full focal length)

I didn't had the money at that time to buy the series I and settled for a cheaper version of the lens with the same focus problem. But i was able to find a very nice one (with original box) some time ago and still am using it as my preferred lens for bird hunting.
08-16-2010, 11:20 PM   #8
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QuoteOriginally posted by KxBlaze Quote
Is it normal for the focus ring to have a "mechanical" feel and noise when manual focusing?


Yeah, that's what I've been doing and it's annoying to have to do that all the time, but it's good to know that's normal
yep..
the noise is normal...

08-17-2010, 08:29 AM   #9
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I remembered doing this zoom in/focus trick on video - to ensure that when I completed a zoom function that the final image was sharp. For some reason, I didn't trust my eyes for doing this same thing with still cameras.

Does anyone know the varifocal status of the SDM zooms from pentax? They seem like true zooms but I wonder if someone has done testing.
08-17-2010, 09:00 AM   #10
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Even on a parfocal zoom (focus point doesn't change as you zoom), focusing at the wide end and zooming in all the way can result in an out-of-focus subject. That's because the big depth of field at, say, 18mm allows a wide range of focus distances to look sharp. Zooming in to, say, 200mm magnifies the error, and can show that you were never precisely focused on your subject to begin with.
08-17-2010, 09:23 AM   #11
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QuoteOriginally posted by KxBlaze Quote
Is it normal for the focus ring to have a "mechanical" feel and noise when manual focusing?
If you mean resistance to being turned and a whining noise, no, that's not good. Some lenses have a built in clutch mechanism, but on ones that don't, you need to disengage the AF with the AF/M switch before manually focusing.
08-17-2010, 09:26 AM   #12
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QuoteQuote:
when I focus in on a car that is 25 feet away at 18mm the focus ring will show 75 feet
Regardless of anything else, that does not seem right.
08-17-2010, 11:51 AM   #13
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QuoteOriginally posted by aerodave Quote
Even on a parfocal zoom (focus point doesn't change as you zoom), focusing at the wide end and zooming in all the way can result in an out-of-focus subject. That's because the big depth of field at, say, 18mm allows a wide range of focus distances to look sharp. Zooming in to, say, 200mm magnifies the error, and can show that you were never precisely focused on your subject to begin with.
My recollection is that even in the days when we talked about "true zooms" it was still often a matter of degree. Even parfocal zooms often had some small variance.
08-17-2010, 01:19 PM   #14
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QuoteOriginally posted by MoiVous Quote
I recall the same problem from my film days. A trick I use is to use was focus at full zoom, and then zoom back.
QuoteOriginally posted by aerodave Quote
...focusing at the wide end and zooming in all the way can result in an out-of-focus subject... Zooming in to, say, 200mm magnifies the error...
MoiVous' technique is exactly the opposite of aerodave's. And this article says not to bother, either way.

But the question arises: How to clearly distinguish the parfocals from the varifocals? Other than by screwing with the focus, I mean.
08-17-2010, 02:33 PM   #15
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QuoteOriginally posted by RioRico Quote
MoiVous' technique is exactly the opposite of aerodave's. And this article says not to bother, either way.

But the question arises: How to clearly distinguish the parfocals from the varifocals? Other than by screwing with the focus, I mean.
Set the camera on a tripod with a suitable subject. My favourite is a bookcase full of (what else?) photo books I have collected over the years. Focus ever so carefully at long end, then take photos at short (and intermediates if you like). Use AF if you are more comfortable with that for the first shot. View the results on your computer and compare. This is far more difficult with a one touch zoom than the two ring types.
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