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07-19-2010, 10:34 PM   #1
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T mount - KA mount electrical

Having perused Features and Operation of the Ka Mount, where the pin connections used by the KA mount electronics are listed, along with how the signaling works, I'm wondering if anyone has tried the following:

Take a t screw mount adaptor for pentax K. Turn the bayonet lugs and the base off in a lathe. Screw the KA mount from, say, a broken KA mount lens, or a cheap teleconverter into the t-mount.

To what end, you ask? Well, I have a t-mount catadioptric lens with a fixed aperture of f/8. It seems to me that if I could signal the camera that the lens's aperture is always at f/8, I could use (at least) aperture priority mode on the camera with this lens. I realize I can accomplish the same thing with the ae/L button in manual mode (I have an *ist DL) but there's a convenience factor here.

On a related note, I notice that my new SMC-M 50mm f1.7 lens, while it has the aperture lever, lacks the contacts on its mount to signal the body with its important characteristics, as well as lacking the A position on the aperture ring. Has anyone tried replacing the mount on this lens with a KA mount, wired to give the camera the proper electrical signals?

I'm looking for "Tried that. Smoked the electronics in the body." and other sorts of information.

Thanks!

-JRS

07-20-2010, 01:56 AM   #2
Ole
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Since your lens has a fixed f-stop you don't need to signal anything to the camera. Just put the lens on and start shooting in Av mode. The camera doesn't need to know what f-stop the lens is set at.

On your second question: Adding the contacts to an M-lens will not make it work as an A-lens, if that's what you are after. There is more to it: The shooting aperture is set by the stop-down lever, and the movement of that lever on an M-lens is not calibrated to facilitate setting the aperture with the required accuracy.

Last edited by Ole; 07-20-2010 at 02:02 AM.
07-20-2010, 02:20 AM   #3
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Not an article- moved.

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07-20-2010, 05:33 AM   #4
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QuoteOriginally posted by Ubergeek72 Quote
Having perused Features and Operation of the Ka Mount, where the pin connections used by the KA mount electronics are listed, along with how the signaling works, I'm wondering if anyone has tried the following:

Take a t screw mount adaptor for pentax K. Turn the bayonet lugs and the base off in a lathe. Screw the KA mount from, say, a broken KA mount lens, or a cheap teleconverter into the t-mount.

To what end, you ask? Well, I have a t-mount catadioptric lens with a fixed aperture of f/8. It seems to me that if I could signal the camera that the lens's aperture is always at f/8, I could use (at least) aperture priority mode on the camera with this lens. I realize I can accomplish the same thing with the ae/L button in manual mode (I have an *ist DL) but there's a convenience factor here.

On a related note, I notice that my new SMC-M 50mm f1.7 lens, while it has the aperture lever, lacks the contacts on its mount to signal the body with its important characteristics, as well as lacking the A position on the aperture ring. Has anyone tried replacing the mount on this lens with a KA mount, wired to give the camera the proper electrical signals?

I'm looking for "Tried that. Smoked the electronics in the body." and other sorts of information.

Thanks!

-JRS
A note or two

the contacts of the aperture reading are all pretty well bullet proof to what you attach., after all they are all shorted to the lens mount when you put a K mount lens on them.

With respect to aperture coding, on a T mount, the only function I can see is to be able to achieve TTL flash. You need to code the apertures by drilling a patten to match the lens characteristics in the mount, making open circuits where necessary.

You also need to add a pin (spring loaded) to make the connection for the third pin lication from the lens locking pin, that contacts the recessed pin on the body, this is the A setting.

once coded, if you set the lens aperture in the camera and the lens to the same, it may meter with TTL, but I am not sure how well it will perform.

As for converting a K to KA mount, I have done it but since the aperture opening mechanism on K mount lenses is different, and changes diameter linearly with lever moption, not area, exposure control is really bad, worse than with a K mount on a K 10 in manual mode. so I gave up

07-20-2010, 10:33 PM   #5
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Thanks, Ole and Lowell. That clarified things a lot (and saved me a ton of work.)

-JRS
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