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07-06-2010, 12:54 PM   #1
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K-mount Experts: How to convert a "KAF2" lens to "KF"?

I know, weird title.

What I'd like to do is take a modern, DA-series lens (the 10-17 fisheye, so no earlier version available) with the "crippled" KAF2 mount, and "do something to it" so that its aperture is locked to a certain, median value (say f/8), and yet still have it work on a modern Pentax DSLR (with AF, I know metering would be stop-down as if it were a non-A lens).
  • Now, before your grace me with a "Why even do this? Just use Av mode," I personally have a very good reason for wanting to do this. I need to insure that AEB will not vary the aperture based on metering while starting from a fixed ISO/ shutter speed (M mode) for 45 minutes (so AE-L isn't valid here) or more.

What I've discovered so far in tinkering is that if I tape over the "A" contact on the lens (or body) that will prevent the camera from even knowing it has an auto-aperture lens attached, so you can shoot freely (and it autofocuses) but the lens is completely stopped down to its limit as when unmounted. Nice if you want a constant f/22 lens...

(BTW, this is the site where I learned about taping the "A" contact... if you can read French.)

So, how to (hopefully reversibly) modify a modern DA lens so that it is "stuck" at some particular aperture, yet still works on a modern body (stop-down, I assume)? Any ideas, or am I just nuts?


Last edited by panoguy; 07-06-2010 at 01:04 PM.
07-06-2010, 01:18 PM   #2
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explain yourself or be judged just plain nuts

You need to explain what you are attempting to do, because the lens will work with aperture control on any pentax program series or newer body which uses the KA mount.

So unless you are intending to do something weird with the lens, like using it with a reversing ring or something, then I see no reason to modify anything.,

If you are using it with a reversing ring, then just get a K mount automatic extension tube and use that to activate the lever. You could even set up a screw thread to control it mounted on the extension tube.
07-06-2010, 01:57 PM   #3
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There's no such thing as a KF mount. There's no aperture ring on DA lenses, so it doesn't matter whether or not the camera knows the max aperture if it doesn't support scroll-wheel aperture adjustment. If you tape down the contacts, it'll shoot at F22, period. If you want to make the aperture larger, you'll have to jam the aperture control lever somehow.

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07-06-2010, 01:59 PM - 1 Like   #4
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Hi panoguy,

I have no idea why you'd want to do this, but if insulating the "A" pin defaults the lens to min aperture, then all you'd have to do is block the aperture lever from moving all the way to the right, essentially making the min aperture the size that you want. Tape might be sufficient, but I'm thinking that a solid piece would be more effective, and a wedge shaped piece that could not possibly fall into the slot would probably be the safest.

I am curious why you would want to do this though. . .

BTW, here's a link where you can learn more about the pins in the KAF mount -- but you'll also see that much of this has little bearing on Pentax DSLRs. One exception is the use of Sigma AF lenses with the F 1.7x AFA. The min/max aperture values for at least some Sigma models get messed up when using the AFA, so insulating pins can be a way to "fix" this, at least to some extent. Here's the link:

http://www.robertstech.com/matrix.htm

Scott


Last edited by snostorm; 07-06-2010 at 02:20 PM.
07-06-2010, 03:01 PM   #5
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QuoteOriginally posted by Adam Quote
There's no such thing as a KF mount.

well, actually there is !
Maybe we should dedicate a club for this Kf mount.
Lets call it "the limited Pentax AF club", featuring the only smc-PENTAX AF lens ever on the market
07-06-2010, 03:03 PM   #6
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QuoteOriginally posted by glasbak Quote
well, actually there is !
Maybe we should dedicate a club for this Kf mount.
Lets call it "the limited Pentax AF club", featuring the only smc-PENTAX AF lens ever on the market
Man, you're right! In any case, I'm sure the OP meant something different.

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07-06-2010, 03:04 PM   #7
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QuoteOriginally posted by panoguy Quote
  • Now, before your grace me with a "Why even do this? Just use Av mode," I personally have a very good reason for wanting to do this. I need to insure that AEB will not vary the aperture based on metering while starting from a fixed ISO/ shutter speed (M mode) for 45 minutes (so AE-L isn't valid here) or more.
It's a little unclear what you want, but unless you're trying to use it on a camera that doesn't allow you to control aperture in A mode there's probably a better way.

My best guess from what you wrote is that you want to set some aperture and ISO, and then use M mode and the green button to change the shutter speed. At least the K20D allows this. (Set custom function 26 "Green button in TAv & M" to "Tv shift", and it will change only the shutter speed. If you're worried about bumping the aperture, but some tape over that wheel after setting it.)
07-06-2010, 07:56 PM   #8
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QuoteOriginally posted by Adam Quote
Man, you're right! In any case, I'm sure the OP meant something different.
Well, actually... I meant KAF. (I'm not so immersed in the arcana to know that there was only 1 KF mount body and lens, and that it sucked - whoops!)

So, yes, I really *do* want to mechanically "fix" the aperture of a DA10-17mm fisheye to f/8, so that my K20D makes no attempt to change it. So, thank you, Snostorm, for your suggestion. Might be tricky, but could be the ticket! The only sticking point is the "crippled" part of the crippled KAF2 mount. Will the camera body still try to mechanically move the aperture lever? If it can't move it, will it think it's stopped down or wide open?( Might not matter, but I'd hate to hose the camera body...)

And for the curious, although I offered an explanation as to why in my initial post, here's the extended remix reasoning:
  • I simply want to use the AEB function to adjust only the shutter speed from a starting shutter speed that I define.

Why? Imagine making a panoramic image with multiple brackets. Huh, this side here is brighter than that side there... and when I put them all together the exposure of each bracket, relative to the others, is all over the place due to the meter reading different levels when panned across the scene - and the camera is adjusting both the aperture and shutter speed (seemingly randomly sometimes). Hey, stop that! Don't you know that adjusting the aperture messes with my depth-of-field??

So, yeah, first things first: let's get a consistent starting shutter speed. Let's try Tv mode and go to town... except that then both aperture and shutter speed will be adjusted (and pretty widely when you use 5 AEB at 2ev steps) based on the metering. No, no.. I want them consistent regardless of what is in front of the camera... I want to control the DOF and the exposure!

Aha, let's use M mode! That also lets us set the "starting" shutter speed regardless of what the meter sees (which we've wanted to do all along) and set the aperture. Whoops - same thing! Both will be adjusted as you pan to brighter/ darker areas of the scene.

Okay, let's use Av mode (so that damn aperture and my DOF is consistent at least) and then use AE-L after the first set of brackets in our pano. Great, the meter is no longer messing up the "not quite of my choosing" shutter speed... until the meter "times out" (and mine is set to the max metering time) and AE-L is cancelled. F-ing meter!

Wait a second, let's put this old lens on there with the aperture ring. Okay, now try AEB in M mode... set the shutter speed... HAHAHA the camera can't change the aperture no matter how smart it thinks it is! I get dead-solid consistent shutter speeds no matter where I point the lens, all bracketed from the speed of my choosing and the aperture is constant! Oh thank heaven, finally!!

That works on my SMC-A 50mm... buuuuut there's no 10mm fisheye with an aperture ring that I can move off the "A" mark! (Where's that weeping smily??)

I've long since given up thinking Pentax will change this, since I know they don't expect photographers to bracket and get the crazy notion of making panoramas (!), plus why would anyone set a shutter speed and aperture in M and expect to use them (and truly, how could they know I particularly don't want my exposure brackets to alter my DOF!??). However, I know I can pick up any Canon and not only will AEB in M mode only adjust the shutter speed (just like Av, except I set the speed), but (holy plastic inevitable!) if I choose "mirror lock-up" it actually locks up the mirror, takes all the brackets, then drops the mirror! (Wow, my Pentax makes me wait 2 seconds between each bracket while the mirror goes up and down, over and over!!) Too bad the only Canons that have more than 3-shot AEB cost $8000+. I really need 5-shot AEB, and ev-comp!

Ah well, maybe I can ask the forum cognoscenti about how to really cripple my "crippled KAF2 mount" lens. I probably won't have to explain all this and have them get confused over details that have nothing to do with the request...


Last edited by panoguy; 07-06-2010 at 08:28 PM.
07-06-2010, 08:17 PM   #9
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QuoteOriginally posted by Lowell Goudge Quote
So unless you are intending to do something weird with the lens, like using it with a reversing ring or something, then I see no reason to modify anything.,
Just as I thought. Nuts.
07-06-2010, 10:43 PM   #10
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QuoteOriginally posted by panoguy Quote
Aha, let's use M mode! That also lets us set the "starting" shutter speed regardless of what the meter sees (which we've wanted to do all along) and set the aperture. Whoops - same thing! Both will be adjusted as you pan to brighter/ darker areas of the scene.
Ummm, might want to test that again.
07-06-2010, 11:33 PM   #11
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Yeah, bracketing with my K-m has made me find out about that same thing. When you bracket in M mode (I don't remember other modes) for 1-stop increments it will change half a stop for speed and aperture, effectively changing the DoF. I think I get what you mean, it's a little annoying. The problem is, I think it's more a feature than a bug since they either haven't changed it or found out about the problem.

Let me see if I got what you're looking for:

You want to do 2 or more frame panoramas with a 5-bracket shots per frame (In what step amount? I'll asume 2/3 stop). In a "ideal world", the camera would do a 2/3 stop change to speed per shot and leave the aperture fixed. The camera in question (a K20D?) doesn't do that, but changes 1/3 stop in speed and aperture for each shot. So, what you need is a way to keep the aperture fixed and do the "ideal world" situation. Is that it?

Then, you have more problems where the AE-L is reset and exposure is redone. I didn't get this part, what happens? You want it to return to your metered exposure and start shooting the next frame without the camera changing the exposure, am I wrong?

If I got everything close to what you mean, and I think your best bet will need to be to work in Av mode and do the followitng: Mark your original speed and aperture values, shoot first frame and then fiddle a little with an exposure compensation before bracketing(if that's even possible) to your previous value and shoot the next frame.

I believe that to change and de-cripple the lens could be a mechanically destructive operation and you might (would?) even lose infinity focus since the FL is so short (I.E. add 1 cm of extention and you have a 1:1 fish-eye macro, which sounds interesting in its own way).
07-07-2010, 12:17 AM   #12
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Doesn't the Pentax mount version of the Samyang 8mm fisheye have an aperture ring just like the 85mm version? I haven't seen many sample photos from it, but they aren't too horrifically expensive.
07-07-2010, 03:17 AM - 1 Like   #13
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QuoteOriginally posted by panoguy Quote
  • I simply want to use the AEB function to adjust only the shutter speed from a starting shutter speed that I define.

[....]

Aha, let's use M mode! That also lets us set the "starting" shutter speed regardless of what the meter sees (which we've wanted to do all along) and set the aperture. Whoops - same thing! Both will be adjusted as you pan to brighter/ darker areas of the scene.
Ok, in my first reply I didn't understand AEB (I read it as "auto exposure button" and took that to mean green button), but if you follow my instructions in it at least the K20D really does only vary the shutter speed for bracketing in M mode. (But you do have to set custom function 26 "Green button in TAv & M mode" to 2 "Tv shift", if your camera doesn't have this setting I guess you need a new camera.)

I don't have your particular lens, but I would expect all DA lenses to behave the same.

QuoteOriginally posted by panoguy Quote
Wait a second, let's put this old lens on there with the aperture ring. Okay, now try AEB in M mode... set the shutter speed... HAHAHA the camera can't change the aperture no matter how smart it thinks it is! I get dead-solid consistent shutter speeds no matter where I point the lens, all bracketed from the speed of my choosing and the aperture is constant! Oh thank heaven, finally!!

That works on my SMC-A 50mm... buuuuut there's no 10mm fisheye with an aperture ring that I can move off the "A" mark! (Where's that weeping smily??)
As Steinback says, the Samyang 8mm has an aperture ring. (And gives the same field of view as your DA at 10mm, with a slightly different projection.)

And if none of this gives you satisfaction, just use M mode and change the shutter speed yourself, it's not that bad.
07-07-2010, 03:54 AM   #14
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Are you really shooting panoramas with the DA 10-17? I'd like to see them.
07-07-2010, 04:42 AM   #15
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QuoteOriginally posted by panoguy Quote
Just as I thought. Nuts.
In reading your posts, I think you need to go back and read the manual some more.

you can set the green button to change shutter, aperture or track the MTF curve, it's a option

You say you want to do panorama shots I would not use a fisheye because youwill not be able to stitch them due to distortion, but aside from that,. panorama's should be shot in full manual not AE or with changed settings between shots as you get lines at the stitch points.

In Manual mode or Av mode you can set aperture and it will stay put!

I think you have missed something in the manuals
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