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02-16-2013, 01:08 PM   #1
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Tamron SP 24-135mm AF Front Element Removal

This is not a specific Pentax question as much as it is about a lens which mounts to a Pentax. I recently purchased a Tamron SP 24-135mm lens. It is working fine and may quickly become my go to lens. However, two weeks ago i was shooting at a "Color Me Rad" marathon where people are covered in copious quantities of colored chalk. The Tamron lens is not weather proof and as it turns out is far from chalk proof. After the shoot I cleaned my gear and saw a large amount of chalk between the first and second element.


No worries, I am a smart guy, i have taken a lens apart before so I proceeded to dissect the front element. Easy, simple operation. I cleaned the two elements and assembled the glass in the same order I pulled it apart. Here in lies my question. I noticed the front element is held in place by three screws. These screws sit in slotted grooves which allow the front element to rotate about 10 degrees. The front element sits on a cam. By twisting the lens within these slotted grooves the front element can be adjusted forward or backward a very small distance when the three screws are loosened.

What does this micro adjustment used for? What does it effect?

I was careful not to move this adjustment when the glass was assembled again and everything seems to work fine. I have gotten several different answers to this question which are at times completely opposite in their functions.

pre cleaning photo:

Sweet Baby Blue Hair | Flickr - Photo Sharing!

Post cleaning photo:

Crap On My Desk | Flickr - Photo Sharing!

02-16-2013, 09:40 PM   #2
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Location: Aurora, CO
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My guess is infinity focus, easy to test. Check to see if you can focus to infinity at either end of the zoom. I think the adjustment is supposed to leave a little room for the lens to overshoot infinity and back up.
02-17-2013, 03:47 PM   #3
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Another person had suggested infinite focus while a third had suggest near focus. In order to find out what it does I may need to intentionally align the front element out of it's original position and see what changes.

Thank you for the input.

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