Screw-drive auto-focus from the inside

Just1MoreDave · 06-18-2009, 11:19 AM

Ever wondered where that screw-drive shaft goes when it disappears into your lens? I have an FA 28-105mm f4-5.6 Power Zoom with a sticking, uneven-feel focus problem, so I took it apart. And here is the rest of the focus mechanism:

This is from the front. The shaft is in the center of the photo, going under the wires. The metal gear in the lower center does all the work, fitting into teeth on the interior of the focus ring to focus the lens. The brass gear turns white plastic reduction gears, then they spin a slotted wheel in a sensor. That keeps track of how far the lens has turned.

The reduction gears are where my problem is. They won't turn smoothly, though I think it's better now. I tried light focus grease as a last resort, seen in the photo. It feels like a broken gear tooth but nothing appears to be broken or missing. I'm going to put it back together and if that goes OK, just use the lens.

BTW, the lens is quite heavy, 515 grams, and the other two non-powerzoom versions are 255 grams and 305 grams. I figured I would rip out some power zoom crap and lighten it up. It turns out power zoom crap adds hardly anything to the weight on this lens, maybe 50 grams tops. The weight is all in the four concentric thick metal barrels and the 13 optical elements. The exterior is all plastic, but the optical section is serious.

Just1MoreDave · 06-20-2009, 07:45 PM

Originally Posted by Just1MoreDave

I'm going to put it back together and if that goes OK, just use the lens.

Just got it together and working again, and I did improve it a lot. I'll have to use this lens some more. It's going to have to be really great to make the cut as a travel lens, though.

PentaxPoke · 06-20-2009, 09:05 PM

Thanks for posting that. I have wondered what the mechanism looks like.

Class A · 07-28-2009, 06:52 AM

Originally Posted by Just1MoreDave

The brass gear turns white plastic reduction gears, then they spin a slotted wheel in a sensor. That keeps track of how far the lens has turned.

I didn't know that there is actually a sensor for checking the focus travel.
Cool.

Do all screw-driven AF lenses have such sensor?

*Ash* · 07-28-2009, 07:03 AM

Cool post.
Interesting mechanism.
Seems simple enough, but I'm glad there are engineers...

Just1MoreDave · 07-28-2009, 09:15 AM

Originally Posted by Class A

I didn't know that there is actually a sensor for checking the focus travel.
Cool.
Do all screw-driven AF lenses have such sensor?

I was wondering about that. I took apart my Pentax-F 28mm f2.8 and there is no sensor. It might be a feature added to FA lenses, or power zoom lenses. The power zoom tracking features might require the focus to adjust along with the zoom.

If everyone can disassemble their autofocus lenses, we could get a database of their internal construction.

Class A · 07-28-2009, 04:56 PM

Originally Posted by Just1MoreDave

It might be a feature added to FA lenses, or power zoom lenses.

I guess it is associated with non-parfocal power zooms as you don't want the lens to change the focal length without keeping the focus constant.

Originally Posted by Just1MoreDave

If everyone can disassemble their autofocus lenses, we could get a database of their internal construction.

Yes, and everyone please send their parts which they cannot reassemble anymore to Dave.