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04-20-2009, 01:09 PM   #1
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K to A conversion: auto exposure

Is it possible to convert a manual K lens into A?

Sure I'm not planning to use aperture dial on the camera

All I want to do is: to use Av, Sv or any other modes besides M on the camera to get exposure automatically,
without pressing green (or AE-L) button, whenever I change the diafragm on the lens.

Any diagram showing where should I drill holes and make dents (fill the holes with epoxy or some other insulator)?

Or is it ok if I change the camera adapter of the K lens with one from A lens which already possesses electric contacts?

04-20-2009, 01:33 PM   #2
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I've never heard of that being done successfully. Electrically it's simple, but mechanically it's complicated.

The potentially very hard part is making the body control the aperture correctly. The KA mount spec specifies that the movement of the lever is proportional to the area of the diaphragm opening; for a pre-A lens it's typically proportional to the diameter (but really, could be anything).


Details at the K mount page:

Features and Operation of the Ka Mount
04-20-2009, 02:37 PM   #3
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I tried this on an XR rikenin 50mm F2, and while it "functioned" the apature control was no where close to being useful.

I wanted to do it to get flash metering on the K10D with P-TTL but it is so non linear it is not worth it.
04-20-2009, 03:48 PM   #4
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QuoteOriginally posted by troyz Quote
I've never heard of that being done successfully. Electrically it's simple, but mechanically it's complicated.

The potentially very hard part is making the body control the aperture correctly. The KA mount spec specifies that the movement of the lever is proportional to the area of the diaphragm opening; for a pre-A lens it's typically proportional to the diameter (but really, could be anything).


Details at the K mount page:

Features and Operation of the Ka Mount
Troy is correct regarding how the aperture lever works for the KA mount. As for pre-KA...the assumption is that the coupler in the body will be at full travel before the shutter opens. Lens manufacturers used a couple of different approaches to satisfy this standard. With some, the lever movement is proportional to the area of the aperture opening (non-linear), with others it is directly proportional to the blade movement (linear), and on still others the lever travel itself varies according to stop (hardly moves at all at maximum aperture, full travel at minimum). As a result, you cannot reliably control the aperture of pre-KA lenses from the camera.

Steve

04-21-2009, 11:23 AM   #5
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QuoteOriginally posted by troyz Quote
I've never heard of that being done successfully. Electrically it's simple, but mechanically it's complicated.

The potentially very hard part is making the body control the aperture correctly. The KA mount spec specifies that the movement of the lever is proportional to the area of the diaphragm opening; for a pre-A lens it's typically proportional to the diameter (but really, could be anything).


Details at the K mount page:

Features and Operation of the Ka Mount

According to that bdimitrov link:

By the conducting/non-conducting pattern of the lens electric contacts,
knowing the narrowest aperture and the number of f-stops,
the body could calculate the widest aperture, and then the manually-selected f-stop.
Yes, but only for prime lenses!

So, theoretically it's possible. Is there anybody who test it practically,
to explain tricks?

Last edited by Adnan EROL; 04-21-2009 at 11:39 AM.
04-21-2009, 11:38 AM   #6
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QuoteOriginally posted by Lowell Goudge Quote
I tried this on an XR rikenin 50mm F2, and while it "functioned" the apature control was no where close to being useful.

I wanted to do it to get flash metering on the K10D with P-TTL but it is so non linear it is not worth it.
what exactly "functioned"?
I'm not expecting to control the aperture reliably from the camera.
Did you see the selected f-stop in the viewfinder?

Did you get "auto exposure" without pressing the green button next to shutter?

Last edited by Adnan EROL; 04-21-2009 at 11:44 AM.
04-21-2009, 12:33 PM   #7
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QuoteOriginally posted by Adnan EROL Quote
what exactly "functioned"?
apature controlled by camera (but not correct exposure) and fstop in view finder, all auto modes on the camera, and P-TTL flash (again not correct exposure)
QuoteQuote:
I'm not expecting to control the aperture reliably from the camera.
the camera controlls apature reliably, i.e. it makes the same exposure msitake every time
QuoteQuote:
Did you see the selected f-stop in the viewfinder?
yes see above
QuoteQuote:
Did you get "auto exposure" without pressing the green button next to shutter?
you got auto exposure and all auto modes on the camera (again, and I can't repeat this enough, the exposure was wrong) pressing green button in manual set exposure without stopping down the lens to measure, and just used the apature information.

See the attached post.
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/4893-replacing-lens-mounts.html
I deleted the curve from the post so I have added it back here

Note that a greyscale of 45 is about 1 f-stop when between 25 and 225 grey scale. it is very linear, i have mapped this out
Attached Images
 
04-21-2009, 01:58 PM   #8
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Dear Lowell Goudge,

I had searched the forum before writing in new thread, and I couldn't find your post. I think I didn't choose the right words, anyway.

It would be so nice to read your message, if repeated words were deleted automatically in the posts, as repeated words are detected with wavy red lines in Microsoft Word when spelling checker is on.

Thanks for sharing. Hope to find a solution, one day...

04-22-2009, 08:51 AM   #9
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I've got a message from Bojidar Dimitrov,
as Troy,Lowell and Steve, he says the very same thing:
:-(

> No, you cannot convert a K lens into an A one by simply drilling holes.
> There is a fundamental difference in the way the aperture works on K
> lenses.
>
> the problem is that the aperture lever on the lens moves proprtionately
>to the aperture area in the K lenses and to the aperture diameter in the
> A lenses.

Last edited by Adnan EROL; 04-22-2009 at 01:09 PM.
04-22-2009, 12:35 PM   #10
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QuoteOriginally posted by Adnan EROL Quote
According to that bdimitrov link:

By the conducting/non-conducting pattern of the lens electric contacts,
knowing the narrowest aperture and the number of f-stops,
the body could calculate the widest aperture, and then the manually-selected f-stop.
Yes, but only for prime lenses!

So, theoretically it's possible. Is there anybody who test it practically,
to explain tricks?
Yes I have done that to my poor old 15/3.5 K. Transplanted a complete A-bayonett from a lowly Pentax zoom (35-70 or so) with the same max. aperture. But it proved to be completely useless. The camera heavily realies on calculating the correct exposure for an apaerture. And this calculation goes completely wrong, when the aperture mechanism does not behave as calculated - and K/M lenses don't behave like A and later lenses (Steve, troy, Lowell - all are completely right about that).

The smaller the aperture gets the more the expsoure was off. And the same applies for P-TTL. So after some testing I apologized to my 15mm and gave it back his old original bayonet flange. Much easier to use it in M mode with the green button.

Ben
04-23-2009, 05:54 AM   #11
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It is practical to fool the camera into thinking the K lens is an A lens by shorting out the A pin on the camera mount. Then the K or other manual lens can be used in strictly manual mode (manually set the aperture). Use the e-wheel to tell the camera the lens is wide open so it won't try to close the lens' aperture or do any calculations during metering. Then metering in Av mode, flash etc, all work properly.

The K lens will work in manual mode if the interaction between its aperture lever and the camera is defeated. This can be accomplished a few ways: don't twist the K lens fully when mounting it (drilling another depression in the lens mount to accept the locking pin will remove security concerns), or, remove the len's lever, or (I haven't tried this) prevent the lens' lever from moving with wire, tape, whatever.


Sticky-backed foil is not necessary-just squish some conductive foil between the A pin and the mounting plate.

As pointed out by others, it is not possible to do a complete K->A conversion because the aperture lever on the K moves in proportion to F-number, while it moves in proportion to lens aperture area for an A lens (ie. Square Root (F-number)).
Pentax K mount - Wikipedia, the free encyclopedia

Iowa Dave

PS the DATA pin must also be shorted for the K100D camera. The Data pin is the one farthest from the A pin....I just remove finish from the lens base to short out the DATA pin.

Last edited by newarts; 04-23-2009 at 06:07 AM.
04-23-2009, 04:21 PM   #12
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QuoteOriginally posted by newarts Quote
It is practical to fool the camera into thinking the K lens is an A lens by shorting out the A pin on the camera mount. Then the K or other manual lens can be used in strictly manual mode (manually set the aperture). Use the e-wheel to tell the camera the lens is wide open so it won't try to close the lens' aperture or do any calculations during metering. Then metering in Av mode, flash etc, all work properly.

The K lens will work in manual mode if the interaction between its aperture lever and the camera is defeated. This can be accomplished a few ways: don't twist the K lens fully when mounting it (drilling another depression in the lens mount to accept the locking pin will remove security concerns), or, remove the len's lever, or (I haven't tried this) prevent the lens' lever from moving with wire, tape, whatever.
Dear Iowa Dave,

It sounds good to fool the camera.
Altough I had sold all my M42 screw lenses because I hate to look into darkness through the view finder when stop down; I'll give it a try, when I receive the lens I have ordered. Sometimes it's better than pressing two buttons (green button and then shutter release) for one shoot.

I like the idea drilling another depression in the lens mount to accept the locking pin. So, while I use it as manual mostly, now and then I can twist the lens a bit more to get a lighter brighter view finder when stop down, and start using green button again which is still there.

And also it will help to cease my passion of drilling the lens

Thank you.
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