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04-02-2012, 02:20 PM   #1
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Tokina 28mm diaphragm.

Hi, everyone.

So, I recently got an RMC Tokina 28mm 2.8, and there's something with it I find kind of strange...

So, let's say I take my Pentax-M 50mm 1.7 and start moving the aperture ring. When you see through the lens, you can clearly see the blades moving, closing and opening the aperture.

However, when I do this with my Tokina, there's no apparent blade movement at all. All I see is a perfect circle, not even some kind of polygon shape.

I've been wondering about this ever since I noticed the same behavior on a Canon Automatic lens.

Is there something wrong with my Tokina or this is just normal on some lenses? I've heard about jammed blades before, so I'm kind of concerned.

04-02-2012, 04:14 PM   #2
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What mount is the Tokina? If it's M42, does it have an M/A switch? If so, is the switch set to A? The switch needs to be at M for the iris to move when you twist the aperture ring.

Whatever the mount, is there an aperture-link pin or blade? If so, then when the aperture is set somewhere other than wide-open, does pushing the link cause the iris to move?

Canon FD lenses must be mounted for the aperture to move; there's an interlock. That drove me nutz when I was trying to count iris blades when selling FD lenses.
04-02-2012, 04:51 PM   #3
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Thanks for your reply, Rico.

Well, let's see... it is a PK mount. Yes, it has a little tab, I think it's called the diaphragm actuator. But no, the diaphragm doesn't move when I push the little tab.
04-02-2012, 05:12 PM   #4
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QuoteOriginally posted by J. Fabiano Quote
Well, let's see... it is a PK mount. Yes, it has a little tab, I think it's called the diaphragm actuator. But no, the diaphragm doesn't move when I push the little tab.
To quote a noted explorer: "Houston, we have a problem."

Does the aperture ring move too smoothly, without stopping at any detents? Then a wee tiny ball bearing may have slipped loose and the lens needs repair. If not, then the aperture blades may be stuck together, and the lens needs service. Sticky aperture blades can be cleaned. But opening the lens to get at the blades can let that wee tiny ball bearing escape into the wild, and replacing it is a non-trivial endeavor. What a pickle!

The options:

* Have it fixed. That'll cost more than it's worth.
* Try repairing it yourself. I'm not that brave.
* Only use the lens wide-open. I've done that.
* Toss it on your "to be fixed someday" pile.
* Sell it as-is and let somebody else suffer.
* Use it to burn ants or for target practice.

Good luck.

04-02-2012, 05:49 PM   #5
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Haha, thank you again.

Well, luckily, the lens came with a Pentax MX I bought last week. The lens was, somehow, not having influence on the price of the whole kit. I can now see why. I thought it was just a dumb seller giving it away.

Anyway, I'm going to try the risky solution. I'm good at taking things apart, so... hopefully, I'll be able to clean the blades with some alcohol.

So, just to make sure... it's impossible that this isn't like the Canon FD lenses with the interlock thingy, right?
04-02-2012, 06:31 PM   #6
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QuoteOriginally posted by J. Fabiano Quote
So, just to make sure... it's impossible that this isn't like the Canon FD lenses with the interlock thingy, right?
PK lenses don't have anything like the FD interlock. Have fun!
04-02-2012, 07:14 PM   #7
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Well... it's done.

That was kind of easy... although I didn't have to do much. I didn't even got to the blades. In the middle of the disassembly, they somehow snapped and started responding to the aperture ring.

Now I'm worried they'll get stuck again. But, oh well... time will tell.

04-02-2012, 08:04 PM   #8
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Aha, something was jammed too tight. Congratulations! I like fortuitous fixes. I've had 4 lenses with jammed apertures. Two I fixed (one accidentally, like yours) and two I FUBAR'd. Enjoy the lens!

04-02-2012, 08:25 PM   #9
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Thank you very much for the help!
05-11-2012, 12:16 AM   #10
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Similar issue with Tokina aperture:

LENS: 24mm F2.8 RMC II f stops: 2.8~16 s/n: 7900896, 52mm filter, marked "made in Japan", OM mount

Just got this, same as JF's but in the 24mm version. This mint copy looks 10/10, brand new, never used. Sad that the diaphragm acts like it is viscous damped. Slow to open and close. No sign of oil or dust on blades. Missing greatly my OM Zuiko 2 F2.8 that I bought new back in the 70's and had for decades. Was hoping that this Tokina would fill the void. Any suggestions here? If it can't be fixed looks like I'm in for hella ride to find another 24 Zuky. Good copies of those are over-$$.. Suggestions would be appreciated on repairing the Tokina or on best value 24m mm old glass..

OM film bodies, and as 42mm equiv on APS-C (full frame later).

Last edited by mudpipe; 05-11-2012 at 02:43 PM. Reason: tech spec correction
06-06-2012, 10:35 AM   #11
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Fabiano,

I have the same problem. I want to use this lens for reverse macro but blades don't move. How did you open the lens and make it work?
06-06-2012, 04:06 PM   #12
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I had exactly the same lens as in the pictures above and it developed sticky blades.

The rear element group unscrews easily and I used acetone to wipe the blades with qtips, flexing the blades a little to really wash out the oil as much as possible. Left it to dry and then screwed the rear group back in. Needed work every few months with lengthening time intervals between.

Eventually sold it with full disclosure as I had other 28mm lenses.

Probably needed to have a complete clean and re-lube. Stopped down a little the lens is very sharp and a good cheap performer.
06-15-2012, 10:23 PM   #13
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QuoteOriginally posted by Skywalker Quote
Fabiano,

I have the same problem. I want to use this lens for reverse macro but blades don't move. How did you open the lens and make it work?
Well, the first time I tried to fix it, which was the time I took those photos, I just unscrewed the back of the lens and while I was trying to remove the mount... *snap*, the blades started moving. Weird, I thought, and put the screws right back. I left the lens mounted on my K20D for a week without any kind of activity (which held the blades open while the lens was mounted) and discovered that the diaphragm was stuck again because of this. I reopened the back of the lens and finally discovered the issue (at least in my case).

I took some pictures to show you the very simple procedure.

1. These are the three screws you have to remove.


2.There is a little L shaped tab that goes through the mount, so try to remove it holding it the way you see in the picture. The first time I attempted this, somehow, the lens fixed itself without any direct intervention. I just couldn't figure out how to remove the mount, so I started jiggling it. I later discovered that the little tab I mentioned was holding it down.


3. Finally, you'll see what, at least in my case, was the source of the problem. My lens wasn't suffering from sticky blades. The problem was this little pin (circled in red), that protrudes from a triangular shaped piece of metal that is held by a screw. It was stuck and wasn't moving easily. To solve the problem I untightened the screw (just enough to free the movement) and put just a little bit of lubricant under it.


Reassembly is very straightforward. Just go over your steps in reverse order.

By the way, I'm J. Fabiano, I'm just posting with my new account.

Cheers! Good luck.
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