[TDN]

Hi,

I recently got a Lens Turbo II to use with my Sony NEX-6, in the hopes of getting some true wide-angle out of my Pentax primes. Unfortunately, both of the Pentax 28mm lenses (M and K) I have exhibited rear-element-collision with the adapter, so no infinity focus and the risk of damaging the lens as well as the adapter.

However, I do also have an Auto Chinon 28mm f2.8 lens I got with a camera set. This one had the same problem, but to a much lesser degree than the Pentax lenses. The collision between the elements happened just barely before infinity focus was reached.
My only use for this lens would be with the lens turbo (The better Pentax 28mm's fit my K50, but aren't really wide-angle on APS-C), and I can't resell it because it has oily aperture blades, even after I did an amateur repair. So, I decided to experiment with the lens a little to see if I can't make it usable on the Lens Turbo, with succes!

I'm sharing the process on here, since it might be applicable to other older lenses as well. I wouldn't try this with a modern and/or expensive lens.

DISCLAIMER: TRY AT YOUR OWN RISK. Don't hold me responsible if you mess up expensive glass

I've attached illustrations for all of the steps.

0. Before you do anything, put your lens on the Lens Turbo II, and put the rear cap on the complete assembly of adapter + lens, so you can put it upright on your work surface.

1. Open up the front of the lens by screwing off the vanity plate. Usually this is done using a rubber thingy to create friction with the plate, and it simply screws off counterclockwise. In other cases a spanner wrench specifically for lens repair is needed, but not with this lens.

2. Take off the metal flange covering the focus mechanism. In this case it was done with 2 tiny screws. (1.5mm flathead)

3. Next we're going to loosen the focus ring. It's a good idea to mark the current position of the ring using a small scratch/punch at this time. No one will ever see this unless they open up the lens themselves. Once you've marked it, partially loosen the 3 screws holding the focus ring in position, but don't take them out! Once you can turn the ring loosely without the focus mechanism adjusting, you're good. It doesn't take much, maybe 1 full turn for each screw. For this lens I used a PH00 screwdriver.

4. Now you can put the loose focus ring in the correct position. Be sure to check that everything still lines up after the next step as well.

5. You can now use your screwdriver to softly turn the focus mechanism to the point where the lens elements collide. You can distinctly hear the difference between the lens "ticking" to max focus, and the soft "thud" of the glass elements colliding. Once you're certain you've turned it to where the elements collide, turn the focus mechanism back a tiny bit, so the elements no longer collide. Then you can tighten the 3 screws again (make sure the infinity mark on the ring in step 4 still lines up!).

That's it, you're as good as done! Before you re-assemble the lens in reverse order, you can do a quick test on the camera to see whether the focus is alright.

You now have a lens that technically doesn't really focus entirely to infinity, but at least it handles as if it would, and it can't damage the glass of the Lens Turbo II or the lens.
Whether you can actually get far-off objects/landscapes in focus will depend on how much you had to adjust the focus mechanism.
Since the difference was very small on my lens, I honestly don't really notice it, especially when stopped down.
The good thing about this is that it's fully reversible if you marked the original position of the focus mechanism in step 3. So if you ever do get that Pentax full frame, you can adjust the lens right back and toss your LT2 .

I'm pretty happy with the results. I'll see if I can take some sample pictures later.

Tom

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