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03-31-2010, 09:53 AM   #1
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PK lens mod to work in Av mode

Yesterday i went to a local shop to primt some pictures, and i found myself buyng an old MF tokina 35-70 f3.5 PK lens for about 20$.
When i went home i notice that the diaphragm coupling lever of this particular lens was easy to remove without damaging the lens.

Now when i close the aperture ring the VF gets darker as it should do.
I've taked a couple of shot with it using Av mode just to see how the camera reacts, i was really surprised as now i think it can be used in Av mode and not only wide open.
Here's the shoot

@f3.5


@f5.6


@f8


@f11

iso was set to 1600 and spot metering was selected.

Then i take my precious M 50 1.4 and sadly discovered that remove the annoying lever was not as easy as on the tokina lense.
Later i found that if i don't fully close the lense when i mount it i can use the aperture ring as if i was using the modified tokina.

Do you think it's safe to remove (even if i have to cut it) the last part of the lever so i can use Av mode with PK lens?
Can it influence the IQ/Operation of the lens?



03-31-2010, 10:55 AM   #2
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This is one of 3 options I have seen for getting Av Mode to work on K mounts.

one mod, is to drill a second lens locking pin hole about 3/8 of an inch away from the first, so the aperture activation lever does not hit the camera operating lever at all.

the second is to add a spring contact in the A position (3rd from th elocking pin) and drill the aperture pattern, this works but since the aperture movement of the K mounts is diameter, not area linear with movement of the lever, the metering is off by -1 stop wide open, + 2 stops at about F11 (I tried this on a rikenon 50mm F2)

the last is as you suggest, cutting or removing the activation lever.

Of the 3, the first one I describe is the least offensive with respect to the resale of the lens , the second makes the lens value almost zero, and the third is close behind, because cutting the activation lever off the lens, while not interfering with the optics (assuming you don't scratch anything or fill the lens with filings) also hurts the resale value.
04-01-2010, 12:23 PM   #3
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thank you for the info, the second option made me so curious that i had to buy a modded lens as you've described in the 2nd option, and seems to work quite well (matrix metering seem to be thrusted and p-ttl support is back and working).
As you have said pheraps this kind of mod hurt the price of the lens quite a bit as i paid 16$ for it and it was quite cheaper than the same unmodded lens.
If someone is interested i can make some quick test
04-01-2010, 02:27 PM   #4
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Mat

early on I modified a rikenon 50mmF2

while it worked, exposure was not linear and had as muc error as the K10D with green button on a K mount lens.

that was my motivation, to get better metering, but the K series lenses, as opposed to A series do not really have a defined aperture movement with lever, so you take your chances with metering and control of the aperture

04-01-2010, 03:22 PM   #5
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has you have said metering gives the exact same value using spot or center weighted as when you use the green button but now matrix metering and P-TTL can be used, not bad at all!

I understand the differences in the lever movement it's a major problem sice you may have mismatched aperture between body and lens, but if you convert the mount and "cut" the lever itself, then you can manually input the same aperture on body and lens and they will match.
Since you already have a modded lens, you can try to remove or cut the lever.
04-01-2010, 04:17 PM   #6
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QuoteOriginally posted by matam Quote
has you have said metering gives the exact same value using spot or center weighted as when you use the green button but now matrix metering and P-TTL can be used, not bad at all!

I understand the differences in the lever movement it's a major problem sice you may have mismatched aperture between body and lens, but if you convert the mount and "cut" the lever itself, then you can manually input the same aperture on body and lens and they will match.
Since you already have a modded lens, you can try to remove or cut the lever.
I don't really want to mod the lens because it still works on my two MF film bodies.

Also If I moded the lens except for P-TTL flash it has little gain over an M42 lens or simoply cutrting the lever off.

I don't see it as prectical really.

the reason I made the Av Mode and auto position was to have both open aperture focusing and metering with auto aperture.
04-05-2010, 07:07 AM   #7
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First let me say sorry to bumb this up.
Then let me show how matrix metering worked: (judge exposure not the crappy composition )




Camera was deliberatley set in Av mode, lens closed to f22 and the body handled the aperture itself; only resize was done in PP.
I'm quite confident if i had used spot or center weighted metering the result would be an underexposed picture for the swan and probably overexposure for the 2nd shoot (is it a crown?)
I seriously think matrix metering works and i'm quite confused as why pentax doesen't gives us an option for making this possible without modding the mount (it colud be done with a firmware upgrade).
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