[LarsW]

The lens is a RMC Tokina 135mm 1:2.8 (manual focus and aperture)
My problem is that the lens often stays wide open when i shoot, no matter where i set the aperture ring. I get heavily overexposed photos with horrible CA because it's wide open.

* When not mounted on camera the aperture closes and opens ok when i turn the aperture ring.
* When i press the green button (on my K-5) to measure exposure i can see the aperture stops down.
* If i take a long exposure, let's say 1 second, i can see that the aperture stops down on exposure when i look into the lens. So it seems to be a problem only on normal and short exposure times.

See attached image, i suspect the problem can be with in the the little thing pointed out (don't know what it's called). I suppose the camera moves this to stop down on exposure. But it can't do it quick enough? When i move it by hand i notice it doesn't move just as easy as on my other lenses.

Is it possible to fix this?

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Last edited by LarsW; 05-12-2016 at 09:31 AM.

[Philoslothical]

With the lens off the camera, if you stop it down to f./22 and flick the lever a few times with your finger, is the action of it snappy? If the diaphragm is sticking, as is not uncommon with old lenses, that could well be the problem. Can you see any oil on the aperture blades?

This issue matches your description, in that the aperture may be slower to stop down but not completely stuck, so it does continue to work (albeit, too slowly) for long exposures.

[LarsW]

Thx for reply. When i do the test you described, it doesn't look like the lever or the aperture blades move "snappy", but still the lever itself doesn't move nearly as smoothly as with my other better lenses. I can't see any oil on the blades, but maybe there is some invisible oil that cause "sticking"? The lens was very cheap, so i think i will take the risk and try myself to do something about it.

[Philoslothical]

It could be sticky blades, or it could be damage to the mechanism itself (perhaps it was dropped at some point). I think you've narrowed it down though.

[LarsW]

I unscrewed some parts at the back to try to see if could see something wrong with the mechanism. Screws and springs inside look ok to me (with my limited knowledge). A little oil was on mechanics inside. I screwed back everything. Still same problem. Now i consider to remove that lever i pointed out in my first post, this will give me a darker viewfinder (dof preview all the time), but giving correct aperture.

Anybody tried this?

[gp1806]

You should not remove the lever, but rather force it to the stopped down position. Remove the lens mount, as you did previously, and attach a small spring from your junk drawer and force the lever all the way to make this lens a full manual aperture, then replace the lens mount. If you don't force the lever to it's full travel position, and just cut it off, it will only allow wide open apertures, and you can do that right now without modification. You could also search here for tips on cleaning the aperture blades and the lens could work as intended. Just something to think about.

[LarsW]

Thanks for tips! I will learn more about the cleaning, and new try this weekend to fix it.

[ChrisTrevino]

I don't know why no one asked yet but. . . .

You are shooting in Manual Mode right? Not Av or some other mode?

Older lenses, specifically ones without the electronics only work wide open unless you are shooting in Manual. So in order to adjust the aperture via the ring you HAVE to shoot in M.

[LarsW]

You are shooting in Manual Mode right? Not Av or some other mode?

Yes, I am in M mode, using the green button to measure exposure. It works fine on my Pentax M50/f1.7.

This weekend i will try some more surgery.

[ChrisTrevino]

Yes, I am in M mode, using the green button to measure exposure. It works fine on my Pentax M50/f1.7.

Just making sure, was one of the things I didn't know in the beginning and was like "WTF why do all my shots have the same DoF!" Someone had to tell me my old lenses would only work in Manual as well.

If you are doing everything YOU are supposed to be doing then I'd go with the lens & blade cleaning. You can also compare the level to others to see if it's bent.

[LarsW]

Thanks Chris! I will look closer at the lever too. I found a youtube clip describing a similar problem:

You Tube

You Tube

He says that aperture "should snap quickly back into place" and it really doesn't, it takes almost half a second or so. Difference is just that i can see no oil on the blades. Maybe an old weak spring inside causes this if it's not oil.

[stevebrot]

Maybe an old weak spring inside causes this if it's not oil.

...or gummy old lubricant in the works. The general solution is to partially disassemble the lens and flood the mechanism with lighter fluid (naphtha) followed by lubrication (very sparing) with something like watch oil applied with a toothpick. The trick is to avoid fouling the aperture blades themselves while applying the oil.

As noted above, the aperture mechanism should be "snappy", both to open and to close. Consider that your camera in burst mode will be doing this several times per second and that will give you an idea of how snappy it should be.


Steve

BTW...you may have gotten your lens on-the-cheap, but the RMC Tokina line were well respected back in the day and equivalent to SMC Pentax optics of similar focal length. If you have the inclination, tools, and time, it is worth salvaging.

[LarsW]

... the RMC Tokina line were well respected back in the day ...

Yes indeed, the few times i've had it working at f/5.6 to f/16 it is definitely sharper than the Pentax 50-200 WR at 135mm. So it is worth taking the time with it! Thanks for help