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Some may have heard of the aluminium-foil trick which lets the camera "think" it has a f/1.2 lens mounted. And I think this trick gets far to less attention.
This simple and very cheap "cheat" saves you a lot of money because you don't have to buy an A-lens to have one.

You'll get multi-segment metering, all exposure modes (but Tv, Sv and P need special attention - more later on), aperture data in your exifs and working P-TTL - with every lens!
(not sure about M42 lenses but I'm pretty certain it even works with those)

How it's done:
short-circuit the camera's A-pin with the bayonet - you can either tape some tin-foil onto the pin (which I did) or you can remove the mount plate and stick it to the inside (which I also tried but I could not remove this damn thing - if anyone knows how to remove it please tell me).
The camera will now display 1.2 instead of F-- when you mount a non-A lens.
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Getting the camera to use the correct aperture is very easy if you have a f/3.5 (f/6.7 for K7 owners) lens or faster.
Simply compensate for the difference between f/1.2 and the maximum aperture of your lens.
A f/3.5 lens would mean you need a 3 stop exposure value (EV) compensation (1.2-1.7, 1.7-2.4, 2.4-3.5 --> 3 stops), a f/2.8 lens 2 1/2 stops and so on. So set your compensation accordingly and your exposure should be correct (my Pentax 2.5/135 only needs 1-1.5 so you have to test for yourself).
The reason I write f/3.5 lens or faster is that every (?) Pentax DSLR only allows for -3EV compensation (with EV steps setting set to 1/2).
Excluded are the lucky K7 owners because the K7 allows up to -5EV compensation meaning you could also use a f/6.7 lens on it.

But if you are fast calculating there is a (admittedly clumsy) solution for you to use lenses slower than f/3.5 (and f/6.7 for the K7): leave the EV compensation where it was and use the camera's "wrong" f-stops instead.
Calculations for a f/5.6 lens: if you want to set your aperture to f/8 turn the lens' aperture ring to f/8 (obviously) and set the aperture to f/1.7 on the camera - exposure should be right - both equals one stop of light.
Yes, it's a compromise and more complicated than green button-metering but you still have multi segment-metering, all exposure modes, aperture data in your exifs and working P-TTL.

In short - how you use it:
Set the aperture ring to your desired aperture
Set EV compensation as described above
Match the aperture on the lens' ring and camera body
----> semi A-mode!
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To get another benifit of A-lenses - automatic aperture setting - you have to modify the lens' aperture lever. I glued some plastic to the lever but you could use pretty much everything that has some stiffness and isn't too thick.
But this is neither destructive (unless you are manually incapable) nor extremely complicated. You now have to compensate for the difference in stops your lens has to f/1.2 (exactly like the EV compensation just physical).
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For a f/2.8 lens this would be 2,5 stops. That is to say the aperture lever must be a at the position of f/6.7 when it is really at f/2.8.

The downside is that you lose these stops at the small apertures end - so instead of f/22 you now can only stop down to f/22 minus how much you compensated for. For this f/2.8 lens the smallest aperture possible would be f/9.5 then. You can technically still stop down to smaller apertures but the lever will be pushed open - overexposure is the result.
Strangely on my 3.5/28 I can still use apertures as small as f/11 although following my own logic f/6.7 would be the limit. On my 2.5/135 I can still go to f/13.

Luckily using smaller apertures is still possible - you just have to unmount the lens to the point where the aperture lever is no longer coupled with the body. Now you can use the lens like a M42 one (but with A-functions remaining).

Also I have to warn you not to set any aperture larger than physically possible - this could damage the aperture lever actuator (there is a rated break point so you are not really destroying your camera).
So using my f/2.8 lens example don't set anything bigger than f/2.8.

Compared to manually setting the aperture ring exposure is a little bit off - but the strongest difference I had was about 0.4EV so not really anything to worry about.

Again - how you use it:
Set the aperture ring to its smallest aperture
Set EV compensation as described above
Modify the aperture lever to compensate for the difference in "speed"
Set tha aperture on the camera just like with any A-lens
-----> A-mode!

Using P, Tv, Sv and Green mode (but who is using Green-Mode, anyway) is kind of tricky because the camera can choose apertures that are faster than possible for your lens so you have to manually correct your exposure until the camera chooses apertures that are actually available or simply use Av, TAv, M, B, X and User-Mode instead.

Conclusion:
alumimium-foil to the mount gives you a semi A-lens
alumimium-foil to the mount and a modified aperture lever gives you a fully working A-lens with limitations at smaller apertures.

Update:
Unfortunately it didn't work out as I thought it would. But let me explain: I removed these silly plastic things and "modified" them to be true A-lenses instead. I followed the codes that can be found on this Mark Robert's site and placed some insulators on the lens mount accordingly.
The good news is that this works perfectly without drilling holes in the lens' mount - insulating with thin tape is enough. The middle ring of the lens mount is slightly recessed so the tape if thin enough won't wear up because of friction.

The bad news is that these people who said this doesn't work because of different aperture mechanisms on pre-A lenses were right. I now can set the aperture on the body but the lens won't be
stopped down enough to reach the set aperture.

On the 28 and 135 I could fix this by going back to f/1.2 simulation (that means A-pin connects to the lens but no insulators on the lens' mount. The 1.4/50 I just aquired sadly can't be used that way: the difference between f/1.2 and 1.4 is just too small so the problem of too little stopping down remains. I'm either using it like a M42 lens - slightly unmounted - or wide open only right now.

However on M42 lenses this trick will still work perfectly and you would get half A-lens functionality (half because it's manual aperture action).

To prove it works:
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[Spock]

The other way is to modify your non-A lens. or better still, have it modified in this way by a professional camera repairman. I had this done to a Pentax 75-150 F4 zoom some years ago and was very satisfied with the result.

[Egg Salad]

I have already seen this site before but to be honest I didn't really understand what it says.
I thought these insulator placements are binary codes so they were of little use.
But I think I now understand. this also explains very random transmitted aperture data I get with the Vivitar TC: I got every aperture from f/1.2 to f/5.6 - because the tin foil didn't completely touch the mount.

Knowing this there are still some things I don't understand: Why doesn't both my F 1.7/50 and DA 18-55 have these insulators but another data pin instead?

If I interpret his table right this would mean for my 2.5/135 with an aperture range of f/2.5-f/32 the insulator position would be 0 1 1 * 1 1 (?)

I'll definately try this and if it works I'll sure update my text.

[aka]

Wait, really?

All you have to do to make a -M lens work like a -A lens is:
1) Drill a couple appropriately spaced holes holes in the mount and fill them with epoxy to indicate aperture range. (per Pentax Lens Modifications for Matrix Metering - Mark Roberts Photography)

2) Tell the camera it is an "A" lens (and in A mode) by either adding a protuberance for a contact on the lens mount (in place of the retractable pin on the lens) or directly grounding the "A" pin contact on the body.

Presumably, you would install the lens stopped all the way down.

And that's it- no modification of the aperture lever or anything else?

Incredible. I'm surprised nobody's selling a $50 kit consisting a $1 drill bit, $.50 of epoxy and a stick of gum (for the foil).

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Last edited by aka; 10-04-2009 at 09:27 PM.

[Egg Salad]

Basically, yes.
Actually for A-lens benefits you only have to short-circuit the A-pin to the lens' mount (must be conductive, though - so you had to remove the paint from a M42 lens).
The other things I described are only necessary for easier useabilty.
Glueing these "fins" to the lever doesn't make these lenses prettier but who cares if you get automatic aperture action on a pre-A lens for near to nothing.
But like I wrote this doesn't work over the full aperture range and probably only for lenses slower than f/2 so this isn't for anyone. Surprisingly on the 28 and 135 this technique was pretty reliable in terms of exposure - the difference between manual aperture and these plastic things was around 0.3 EV max at medium apertures.

Anyways, now I'm back to manual aperture action - and this does work 100% correctly (if you don't forget to set the right aperture on the lens).

So, to answer your question:
yup, it's that easy and you don't even have to drill any holes in your lens.

[aka]

I drilled indents and filled them with epoxy and drilled the "A" hole all the way through to make a spring-loaded aperture pin and got it to all work on an old broken sears lens. I'm now converting a $15 Beck 135mm lens which is proving much harder just due to how it is built. I think the "M" lenses are more like the sears lenses and rather easy to convert. I will try with a smc-M 1.4 that's coming in the mail.

[Seppo]

You can also use catch in focus after modification.