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04-18-2017, 03:29 AM   #1
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Cosina extension tubes

Hello,

I have a set of Cosina extension tubes that seems to have all the electronic contacts for aperture control and autofocus, but nothing works! Anybody knows why? I have tested it on a K3-ii and the K-7. I supposed that it would be ok with all those contacts, but in practice it does not work. It is also strange that on the smallest tube it says "manual focus", while the biggest one says "PKA/AF".

I have also noticed that extension tubes with all the automatic functions are scarce for Pentax, the only ones I have seen are quite expensive and from the brands Caruba and Kooka. Anyone who has experience with use of aperture control and autofocus on those?

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04-18-2017, 05:37 AM - 1 Like   #2
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These are the same as the jessup extension tubes.

Jessops Auto Extension Tubes PK-A reviews - Pentax Camera Accessory Review Database

See the reviews for comments re the black painted mount and electrical connection.
04-18-2017, 05:51 AM   #3
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Ok, yes I now see I have aperture control. But on the topscreen and in the visor this was not indicated, that was my confusion. It is only on the backscreen visible which aperture is choosen. Topscreen and visor say F-- . But at least this works, and I now indeed see the contacts for autofocus are lacking, thx (the three rings have the same contacts)!

This must be the one that also will work with autofocus (7 contact points, 2 contacts on the ring in the middle and the drive shaft a the bottom):

https://www.cameranu.nl/pictures/1442838977_541/caruba-extension-tube-25mm-pentax-chroom.jpg
04-18-2017, 07:51 AM   #4
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If you see "F--" then aperture control is not working. That indicates that the camera cannot see the lens. I believe the screen will show the aperture selected but if it is showing "F--" it will just operate wide open. But I might be wrong on that.

First make sure your lens in 'A' mode. And perhaps try the lens alone first to make sure it works correctly. Test with each tube separately to see if there is any difference.
Then test all of the contacts for continuity. I had a similar tele-converter at one time that looked fine but some of the wires connecting the contacts inside were broken.

There is no auto-focus on this tube as already noted.

04-18-2017, 09:03 AM   #5
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QuoteOriginally posted by marcusBMG Quote
These are the same as the jessup extension tubes.

Jessops Auto Extension Tubes PK-A reviews - Pentax Camera Accessory Review Database

See the reviews for comments re the black painted mount and electrical connection.
While doing some research it appears Jessups is a distributor for Kenko and the Jessups tubes are re-badged Kenko tubes.

QuoteOriginally posted by jatrax Quote
If you see "F--" then aperture control is not working. That indicates that the camera cannot see the lens. I believe the screen will show the aperture selected but if it is showing "F--" it will just operate wide open. But I might be wrong on that.

First make sure your lens in 'A' mode. And perhaps try the lens alone first to make sure it works correctly. Test with each tube separately to see if there is any difference.
Then test all of the contacts for continuity. I had a similar tele-converter at one time that looked fine but some of the wires connecting the contacts inside were broken.

There is no auto-focus on this tube as already noted.
Right, check each tube individually - one or more may have faulty connections. Or when combined the pins may not line up enough to make contact. If you have a multimeter or continuity checker you can check each individual pin.

I seem to recall in another thread the Kenko tube connections had a tendency to go off and it was a simple matter of reconnecting the spring connectors.

Last edited by Not a Number; 04-18-2017 at 09:10 AM.
04-18-2017, 02:21 PM   #6
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QuoteOriginally posted by Not a Number Quote
While doing some research it appears Jessups is a distributor for Kenko and the Jessups tubes are re-badged Kenko tubes.



Right, check each tube individually - one or more may have faulty connections. Or when combined the pins may not line up enough to make contact. If you have a multimeter or continuity checker you can check each individual pin.

I seem to recall in another thread the Kenko tube connections had a tendency to go off and it was a simple matter of reconnecting the spring connectors.
In Av mode on the K7 I get longer shutterspeeds with smaller apertures. With lenses without aperture ring or lens with aperture ring on A position. So it works ok, but still I get the F-- sign on topdisplay and in visor.
04-18-2017, 03:16 PM   #7
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QuoteOriginally posted by JPfromHolland Quote
In Av mode on the K7 I get longer shutterspeeds with smaller apertures. With lenses without aperture ring or lens with aperture ring on A position. So it works ok, but still I get the F-- sign on topdisplay and in visor.
Do you get the F-- with the lens only? If the camera shows an aperture number then at the very least the "A" contact on one or more of the tubes is not working. If you get the F-- with an "A", FA, DA, DFA lens (set to "A" if there is a ring) without tubes on your body then the contacts on you camera are at fault.

Try cleaning the contacts on the camera and the tubes.

Extension tubes cut down the amount of light reaching the sensor - the so-called "bellows extension exposure factor". This would change the exposure with tubes vs no tubes in Av mode. Try different combination of tubes. You probably get different exposures per combination.
04-18-2017, 03:26 PM   #8
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I will experiment a bit more, and try to clean better. It is not the camera or the lenses. All ise normal when I put only a lens on the camera. But why would only the backscreen show the aperture? Exposure is correct also, so there is not really a problem apart from how aperture is not shown on the visor and topdisplay.

04-18-2017, 03:36 PM - 2 Likes   #9
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QuoteOriginally posted by JPfromHolland Quote
But why would only the backscreen show the aperture?
i should have explained a little more.
Your extension tubes have only 6 contacts. Your DSLR has 7 - there is an additional data contact - the one at the ~ half past position. This data contact needs to be connecting for your camera to show the f number in the top lcd screen (why does it show on the rear screen anyway? no idea it's just the idiosyncracies of the firmware). It is prevented from connecting by the insulating black paint on the extension tubes mount.
Make a connection with eg a bit of foil (see here), mount an "A" lens set on Auto aperture (A on the aperture ring), and the f number should show on both rear and top screens.
04-20-2017, 02:19 AM   #10
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Thanks a lot MarcusBMG, I was able to scratch away the black paint on each ring at the place of the 7th contact, and now I get the aperture indication on each screen without a problem.
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