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09-15-2018, 03:39 PM   #1
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Limiting the Widest Aperture on K Mount?

Ok, here's another weird Bruce question.

I have a couple of K Mount lenses (such as the Takumar 135/2.5). In Av mode everything works except the aperture, it is fixed at f2.5, in order to use any other aperture you must be in Manual mode. At f2.5 the quality suffers CA and some softness, however even one stop down to f2.8 and wow... very different story, sharpnessville and CA ghost town! The sad thing is that to use this aperture at f2.8 you need to be in Manual mode and constant green buttoning. Which is not so bad for a posed portrait shot etc but as a walkaround lens where lighting constantly changes this becomes a little hampering.

I had a brain fart. Can you adjust the lens somehow so that the widest it can now go is f2.8 instead of f2.5, or some other aperture, so that it can never actually open fully wide anymore, and thus when in Av mode things would work normally at that aperture (ie it's now performing at 2.8 instead of 2.5)?

It just strikes me that there are a few great K Mount lenses that at their widest aperture are not at their best, but stopping down just even one to two stops creates a very powerful image with minimal loss to bokeh etc, however every day use is thwarted by being only Manual mode...


Cheers!

Bruce

09-15-2018, 04:05 PM   #2
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The F or FA series 135/2.8 might be more to your liking. They are also smaller, lighter, and autofocus....

But is stop down metering in M mode really that hard? I rather like my K135/2.5
09-15-2018, 04:19 PM   #3
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Just disable the wide open aperture by wedging the aperture actuator in the closed position so it behaves like an m42 lens or otherwise disable the open aperture feature of the lens. Set the ring to your desired aperture. It will meter whatever stopped down aperture you selected. It may have some issues with accuracy but should do fairly well.
09-15-2018, 04:22 PM   #4
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QuoteOriginally posted by Sandy Hancock Quote
The F or FA series 135/2.8 might be more to your liking. They are also smaller, lighter, and autofocus....

But is stop down metering in M mode really that hard? I rather like my K135/2.5
If you're anyone else other than me then no... prolly not

Thanks for the heads up on the F/FA 135's.

Long term I will likely grab a Samyang 135/2 to replace my Samyang 85 that has now left the building (). If it's anything like the 85/1.4 then it is sharp even from f2, which will make it a killer lens (which from photos I have seen seems to be true).

But it's quite a bit pricier than the 85/1.4, it'll be 2019-2020 before I likely acquire it, and hence this thread, just seeing if for me I can mod this Tak 135 to being even more Bruce friendly.

---------- Post added 09-16-18 at 09:57 AM ----------

QuoteOriginally posted by UncleVanya Quote
Just disable the wide open aperture by wedging the aperture actuator in the closed position so it behaves like an m42 lens or otherwise disable the open aperture feature of the lens. Set the ring to your desired aperture. It will meter whatever stopped down aperture you selected. It may have some issues with accuracy but should do fairly well.
Ok... thanks for your swift reply but you forget it's me you're talking too here, so everything you said whilst sounds like it might be a solution for me I don't understand at all what's really being asked of me

Can you hold my hand more and point to a place where I can see pics and greater descriptions of what you're talking about here? It sounds as if I could set the Tak to 2.8 instead of 2.5 and get it to work in Av mode at this aperture? But it might have slight metering issues, but that doesn't phase me so much as I shoot RAW and the K-1 has such good dynamic recovering ability

09-15-2018, 07:10 PM   #5
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You should be fine unlocking and turning the lens a bit so that it can be used in Av mode and setting the desired f stop with the aperture ring.

09-15-2018, 07:16 PM   #6
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QuoteOriginally posted by torashi Quote
You should be fine unlocking and turning the lens a bit so that it can be used in Av mode and setting the desired f stop with the aperture ring.
How to do this!?!
09-15-2018, 07:34 PM   #7
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The easiest way to temporarily test your theory is to unmount the lens a little. Mount the lens, set the aperture ring to f22, press the locking button and unmount the lens slowly until you see the aperture blades close fully. Now you can control the aperture with the ring. Most lenses are still attached OK like this, but it's obviously a little risky. You can drill a second mounting position into the mount, so the lens locks this way. The mount is probably plated brass and not too hard to work with.

You can remove the aperture lever on this lens but it looks like work. You'd remove the mount and the light shield around the rear element. Then you can see a single screw that the lever pivots on. There's also a spring for the return action, remove that. Obviously cutting or damaging the lever permanently is completely unconscionable.

09-15-2018, 07:44 PM   #8
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You could always take the aperture ring off and see where the physical stops are and solder or glue (depending on the material of the ring) more material so the ring stops a bit earlier.
09-15-2018, 08:01 PM   #9
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I appreciate the replies, but I fear I have asked something that is beyond my technical ability. I need pictures, steps by step instructions
09-15-2018, 09:04 PM   #10
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Imitate a A lens either with paint or tape by recreating the pattern of another f2.5 lens on the lens mount. The metering for small apertures like f8 or smaller will be wrong but for nearly open like f2.8 it should be fine (or lets say the metering will be right but your lens won't stop down to the correct F-stop)
09-15-2018, 09:20 PM   #11
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Just use the green button. These cures are worse than the disease.

Last edited by Sandy Hancock; 09-15-2018 at 09:35 PM.
09-15-2018, 09:28 PM   #12
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QuoteOriginally posted by BruceBanner Quote
I appreciate the replies, but I fear I have asked something that is beyond my technical ability. I need pictures, steps by step instructions
I guess it's exactly like most Pentax PK mount lenses. I think you can find YouTube videos, and a step by step guide in the right section of the Lenses subforum.
You can either remove/unlatch the spring (that's what keeps the iris open), or give up the AV and just use the DOF preview switch (that allows to preview the picture in the same conditions it will be impressed on the sensor, but you still need to work in M).
Unfortunately not all cameras have this function.
If you glue the ring you'd still have the lens wide open, and no AV mode...


Another option would be to imitate the functions of a PKA lens with conductive painting or adhesive copper foil.
As you've already been told, the exposure will be off the more you stop down... and it's an exercise in patience and precision. Not easier than removing the spring, IMHO.
The only advantage is that you can't mess with the innards of the lens, and it's not difficult to reverse to its original state.
If you open the back of your lens, use the right size/type of screwdriver.

If you hate the Green button just forget about it, stop down one click, set AV, and compensate with the +/- function.
F/2.5 to f/2.8 is one third of a stop, I'd forget about it and just shoot.
If you stop down a little more, compensate accordingly. I usually go for the green button once or twice a session, and compensate when needed with the ISO setting. Raw files can often be saved in PP even when I forget the green button procedure when illumination levels change a lot.

Last edited by cyberjunkie; 09-15-2018 at 09:53 PM.
09-15-2018, 09:28 PM - 1 Like   #13
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Get a Samyang 135 f2 and you'll be fine.
09-15-2018, 09:47 PM   #14
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Try wedging a small folded piece of ink jet/copy paper in to push the aperture lever on the lens to the position where it closes down the iris.
09-15-2018, 10:17 PM   #15
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QuoteOriginally posted by biz-engineer Quote
Get a Samyang 135 f2 and you'll be fine.
Oh yeah, I'd be fine with a top end SUV instead of my 20 y.o. Mazda 4WD pick-up!
It goes without saying...
The Samyang is the Ferrari of 135mm primes, if you need/want a PKA lens.
Of course you can tell the owner of a old Cinquecento that a Ferrari is way better, but I doubt of the usefulness of the information
Personally I've been looking for a second-hand Samyang for long. Couldn't find a single one that was decently priced and shipped to my location.
Now it's possible to find them at a reduced price (around 450 USD), so I considered it as self-gifted Xmas present... but I just found a brand new MC APO Telezenitar for a dirty cheap price, and decided that I will give it a try as "sharp 135mm".
It's brand new and "made in Russia"... so no collector's remorse: I might glue a PK ring and drill a retaining hole to turn it into a PK.
It would still work in AV, as the OP desires.

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