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07-26-2012, 05:48 AM   #1
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Use of a close-focusing zoom lens

I recently posted a thread in the Beginners Forum on the behavior of a Vivitar 75-150 mm close-focusing lens, but did not receive a technical explanation to my question. One respondent said he had similar behavior with one of his lenses, but did not know why. I'm trying again in this SLR Lens Forum where I believe the lens experts hang out.

I recently acquired this lens, which is the two-touch type, i.e there is one ring for changing the focal length and another for changing the focusing distance. I found that when I set the focal length ring to "close focusing" I have to move the lens to about 12 inches from the subject to get a sharp image. I cannot tweak the sharpness by moving the focusing ring. Even if I move the ring from 5 ft. to infinity it has no effect on the sharpness. It seems that setting the lens to "close focusing" decouples the focusing mechanism and one just has to move the camera to the point of sharpest image. This doesn't make sense to me because moving the focusing ring greatly changes the length of the lens and the lens elements physically move, yet this does not affect the focus. How can lens elements change position without altering the focus? Please enlighten me.

07-26-2012, 05:59 AM   #2
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Closer?

Hello P Soo, Welcome to the Forum!
Perhaps I am misunderstanding your post, do you mean that when you change the lens to the "Close Focus" range, you cannot focus FARTHER than 12 inches?
If that's the case, you now should be able to focus CLOSER than 12 inches. That's the way macros work. It decouples the long-distance range.
Try this; With the lens in macro mode, move to within 12 inches of a subect, then move a bit closer. NOW the focus adjustment should work. It will take a long throw (turn) to obtain proper focus, most macros have this feature also, very fine increments of movement. Your focal range (in macro mode) is probably 2-3 inches to 12 inches, and no more.
Let us know if this works!
Ron
07-26-2012, 06:13 AM   #3
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I do not have the 75-150, but I do have the series 1 70-210F3.5 macro (version 1) which has a mechanical switch for close focusing

In this mode, the optical elements are changed in their relationship, and the close focusing is achieved by varying the focal length of the lens, not by changing the focus.

I believe you have the same thing with your lens,

generally maximum magnification is achieved at the shortest focal length with this type of arrangement.
07-26-2012, 06:47 AM   #4
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Hi Lowell. Thanks for your response. I'm a little dense - it comes with olde age you know. Your answer doesn't seem to fully satisfy me. I can agree that setting the lens to "close-focusing" will indeed realign the lens elements to allow focusing at a short distance. In my lens I have to move to about 12 inches to get a sharp image. At any other distance the image becomes blurred. My confusion arises because if I hold my lens at the 12 inch distance, and move the distance ring on the lens from 5 ft. to infinity, the lens elements move to other locations as the length of the lens changes, but the size and sharpness of the image remains the same. How can you keep the camera stationary, move the lens elements, and maintain a sharp image. I would think that whenever you change the relative locations of the lens elements the sharpness of the image must change. Am I missing something?

07-26-2012, 07:15 AM - 1 Like   #5
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QuoteOriginally posted by rbefly Quote
Hello P Soo, Welcome to the Forum!
Perhaps I am misunderstanding your post, do you mean that when you change the lens to the "Close Focus" range, you cannot focus FARTHER than 12 inches?
If that's the case, you now should be able to focus CLOSER than 12 inches. That's the way macros work. It decouples the long-distance range.
Try this; With the lens in macro mode, move to within 12 inches of a subect, then move a bit closer. NOW the focus adjustment should work. It will take a long throw (turn) to obtain proper focus, most macros have this feature also, very fine increments of movement. Your focal range (in macro mode) is probably 2-3 inches to 12 inches, and no more.
Let us know if this works!
Ron
Hi Ron. I just read your reply after I sent a follow up comment to Lowell. I tried your suggestion and I believe you removed my dilemma. I think Lowell also gave me a valid explanation but I misunderstood what he was saying. Following your advice I set my lens to "close focusing" and moved the front of the lens to 12 inches to get a sharp image. Moving away a little from this distance gives a blurred image. On careful examination I did indeed find that I could sharpen the image but it takes a very large rotation of the focusing ring to achieve this. That is why I initially thought the focusing ring did not work. It does work but it is extremely insensitive and takes a lot of rotation. If I move the camera too far from 12 inches I cannot achieve focus. Thus it seems that 12 inches is the best working distance, and I should move the camera to achieve a sharp image and not rely on the insensitive focusing ring.

Problem solved. Thanks Ron and Lowell.
07-26-2012, 09:12 AM   #6
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Glad To Hear It!

Hello P Soo,
Glad to hear my advice helps. Lowell has actual experience with the Vivitar macro/standard focusing system, my suggestion was based on a couple of zooms I own that have a "Macro" mode. One is the Pentax F 35-70mm zoom with macro function.
Yes, the amount of rotation needed on the focusing ring (in macro mode) is huge, but necessary; When you're that close to the subject the depth of field is measured in fractions of an inch. Even at f/22, you might have an range of sharp focus that's an inch. My Vivitar Series 1 90mm Macro has a focus throw of 270 degrees, 3/4 of a full turn. Even with that slow of a movement, I still sometimes go zipping by the focus point!
Which is why most macro shooters use manual focus (although their lens may have A/F available). Also, a tripod is pretty much standard equipment; once you get a sharp focus lock, any movement of the human body could take you that tiny bit out-of-focus. It's not exactly "camera shake" in the usual sense. You're not vibrating up-and-down or side-to-side, you're actually getting slightly closer or farther from the subject as you fire the shutter.
But, once you get everything right, the results can be fantastic!
Good Luck,
Ron
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