Pentax-F 50/1.7 doesn't show as A when using A-type teleconverter

asharpe · 10-29-2014, 08:51 PM

Hi folks,

Here's a puzzler. I just converted a Focal 2x A-type teleconverter to an extension tube (extremely easy, by the way, no hammer required), and I tried it on all my A lenses: Kit DA 18-55, Pentax-F 35-70, Takumar-F 70-200, and a Pentax-F 50/1.7. All work perfectly as A lenses when directly attached to the camera (the aperture can be changed by the camera). All work as A lenses with the teleconverter EXCEPT the 50/1.7! I don't understand. The contacts on the Pentax-F 50 are the same as the Pentax-F 35-70 (not surprisingly), yet I only get an F-- when I mount the 50 on the teleconverter, and I get full aperture control when I mount the 35-70. I did, of course, make certain that the lens was on A, and simply mounted it as such on the camera directly to verify that I could indeed set the aperture. But not on the A-type teleconverter. Is there anything special about the 50?

The teleconverter, when looking at the lens mounting side, has 6 pins in a row. The first 3 pins corresponding to the 3 grouped pins on the lenses, then an unused pin (lowered), then two more pins, one of which corresponds to one additional pin on the lens, with the final pin on both the teleconverter and the lens simply touching the mount (and not each other).

As a last point, the extension tube worked beautifully. There is no vignetting, nor did I expect it (though there were some folks here who said there would be). In fact, there might be if I was shooting wide open, but this is for macro, so why would I be shooting wide open? Anyway, it works great, and for $10, I cannot complain. Except about the 50/1.7 not working, that is.

**jatrax** · 10-29-2014, 09:11 PM

Try buffing the contacts with a micro-fiber cloth.

Try wiggling the F 50mm a bit to make sure the contacts are seated properly.

Something is slightly different and it must be very slight as your other F lenses work. Might be just a bit of tarnish on the F 50 that combined with the extra electrical path added by the extension tube causes a failure.

asharpe · 10-29-2014, 09:52 PM

Originally posted by jatrax

Try buffing the contacts with a micro-fiber cloth.
Try wiggling the F 50mm a bit to make sure the contacts are seated properly.
Something is slightly different and it must be very slight as your other F lenses work. Might be just a bit of tarnish on the F 50 that combined with the extra electrical path added by the extension tube causes a failure.

All excellent suggestions, thanks. But cleaning the contacts off the teleconverter, lens and body, and applying a small amount of pressure to ensure contact did nothing. Recall that the lens works perfectly on the body, too.

stevebrot · 10-29-2014, 10:01 PM

If you have a continuity tester or ohmmeter, you can test the "A" contact to see if it is working. With the aperture ring at the A position there should be continuity between the mount face and the center of the "A" contact.

If there is no continuity, examine the "A" contact on the lens carefully. There are two types. One has a pin that is physically raised/lowered when the aperture ring is moved onto and off from the "A" position. The other type has an internal switch. With the former type the possibility exists that the pin may not make positive contact with the corresponding surface on the TC.

Steve

asharpe · 10-29-2014, 10:07 PM

Originally posted by stevebrot

If you have a continuity tester or ohmmeter, you can test the "A" contact to see if it is working. With the aperture ring at the A position there should be continuity between the mount face and the center of the "A" contact.
Steve

I do have a multimeter, so I will test this. But since the lens works fine on the body, isn't a moot point? Anyway, I'll check it, why not... Ok, I checked it. The contact is definitely grounded on the A setting, and not grounded when the ring is not on A. And as far as I can tell, the contact on the lens does not move when the position on the aperture ring is set (or not set) on A. So, I assume it is an internal switch, and not a physical movement of the contact.\

I suppose it is possible that the contact on the lens is simply not long enough to be grounded on the teleconverter. I'll try a piece of tin foil to validate this theory...Well, I tried, but the foil is difficult to position accurately. But nothing seems to have changed; the lens still shows F--.

**jatrax** · 10-30-2014, 12:36 PM

Did you try the multimeter with the lens connected to the TC but not the camera? Lens works on camera, TC works on camera, Lens and TC do not work together. Indicates the contacts between lens and TC are not touching, or the resistance in the combined electrical path is enough to reduce current below that required by the camera.

Test conductivity with lens alone in ohms not just yes/no, then test again with the TC connected and see how much the resistance has increased. If it measurable then you may have tarnish or a poor connection. If it is 100% then there is no connection.

stevebrot · 10-30-2014, 02:12 PM

Originally posted by asharpe

But since the lens works fine on the body, isn't a moot point?

No, test the TC mount with the lens attached.

Steve

asharpe · 10-31-2014, 12:46 AM

Well, I have tested the teleconverter contacts with the lenses attached, and not surprisingly, the 50 does not short the A pin to ground when attached. However, I cannot tell the difference between it and the other lenses that work, so it must be an extremely small difference. I guess I'll have to keep looking at it until I figure it out. One possibility I see is that the A contact on the teleconverter is indeed slightly lower than the rest. But taking the mount off revealed that they are all on rather delicate springs. I may try to slightly stretch the A spring, but this is probably not worth the danger of completely screwing it up.

Not a Number · 10-31-2014, 02:52 AM

The "A" contact on the female side is supposed to be slightly lower that the rest of the contacts. The "A" pin on the lens probably extends to the minimum length within specifications. And the "A" pin on the female side of the teleconverter is at or near the maximum depth within specifications. Put the two together and contact isn't made.

Stretching the spring probably won't help. If the pins and springs are like the Kiron and Vivitar TCs that I have the springs and pins are all the same. Try switching pins and springs.

Adjusting the "A" pin on the lens may be possible but it would depend on how the mechanism is designed.

Otherwise just use a piece of foil to short the "A" pin out when using this lens.

10-29-2014, 08:51 PM	#1
asharpe Senior Member Join Date: Sep 2007 Location: Palo Alto, CA Posts: 221	Pentax-F 50/1.7 doesn't show as A when using A-type teleconverter Hi folks, Here's a puzzler. I just converted a Focal 2x A-type teleconverter to an extension tube (extremely easy, by the way, no hammer required), and I tried it on all my A lenses: Kit DA 18-55, Pentax-F 35-70, Takumar-F 70-200, and a Pentax-F 50/1.7. All work perfectly as A lenses when directly attached to the camera (the aperture can be changed by the camera). All work as A lenses with the teleconverter EXCEPT the 50/1.7! I don't understand. The contacts on the Pentax-F 50 are the same as the Pentax-F 35-70 (not surprisingly), yet I only get an F-- when I mount the 50 on the teleconverter, and I get full aperture control when I mount the 35-70. I did, of course, make certain that the lens was on A, and simply mounted it as such on the camera directly to verify that I could indeed set the aperture. But not on the A-type teleconverter. Is there anything special about the 50? The teleconverter, when looking at the lens mounting side, has 6 pins in a row. The first 3 pins corresponding to the 3 grouped pins on the lenses, then an unused pin (lowered), then two more pins, one of which corresponds to one additional pin on the lens, with the final pin on both the teleconverter and the lens simply touching the mount (and not each other). As a last point, the extension tube worked beautifully. There is no vignetting, nor did I expect it (though there were some folks here who said there would be). In fact, there might be if I was shooting wide open, but this is for macro, so why would I be shooting wide open? Anyway, it works great, and for $10, I cannot complain. Except about the 50/1.7 not working, that is.

10-29-2014, 10:07 PM	#5
asharpe Senior Member Join Date: Sep 2007 Location: Palo Alto, CA Posts: 221 Original Poster	Originally posted by stevebrot If you have a continuity tester or ohmmeter, you can test the "A" contact to see if it is working. With the aperture ring at the A position there should be continuity between the mount face and the center of the "A" contact. Steve I do have a multimeter, so I will test this. But since the lens works fine on the body, isn't a moot point? Anyway, I'll check it, why not... Ok, I checked it. The contact is definitely grounded on the A setting, and not grounded when the ring is not on A. And as far as I can tell, the contact on the lens does not move when the position on the aperture ring is set (or not set) on A. So, I assume it is an internal switch, and not a physical movement of the contact.\ I suppose it is possible that the contact on the lens is simply not long enough to be grounded on the teleconverter. I'll try a piece of tin foil to validate this theory...Well, I tried, but the foil is difficult to position accurately. But nothing seems to have changed; the lens still shows F--. Last edited by asharpe; 10-29-2014 at 10:19 PM.

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10-29-2014, 09:11 PM	#2
jatrax Site Supporter Join Date: May 2010 Location: Washington Cascades Photos: Gallery \| Albums Posts: 12,991	Try buffing the contacts with a micro-fiber cloth. Try wiggling the F 50mm a bit to make sure the contacts are seated properly. Something is slightly different and it must be very slight as your other F lenses work. Might be just a bit of tarnish on the F 50 that combined with the extra electrical path added by the extension tube causes a failure.

10-29-2014, 09:52 PM	#3
asharpe Senior Member Join Date: Sep 2007 Location: Palo Alto, CA Posts: 221 Original Poster	Originally posted by jatrax Try buffing the contacts with a micro-fiber cloth. Try wiggling the F 50mm a bit to make sure the contacts are seated properly. Something is slightly different and it must be very slight as your other F lenses work. Might be just a bit of tarnish on the F 50 that combined with the extra electrical path added by the extension tube causes a failure. All excellent suggestions, thanks. But cleaning the contacts off the teleconverter, lens and body, and applying a small amount of pressure to ensure contact did nothing. Recall that the lens works perfectly on the body, too.

10-29-2014, 10:01 PM	#4
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	If you have a continuity tester or ohmmeter, you can test the "A" contact to see if it is working. With the aperture ring at the A position there should be continuity between the mount face and the center of the "A" contact. If there is no continuity, examine the "A" contact on the lens carefully. There are two types. One has a pin that is physically raised/lowered when the aperture ring is moved onto and off from the "A" position. The other type has an internal switch. With the former type the possibility exists that the pin may not make positive contact with the corresponding surface on the TC. Steve

10-30-2014, 12:36 PM	#6
jatrax Site Supporter Join Date: May 2010 Location: Washington Cascades Photos: Gallery \| Albums Posts: 12,991	Did you try the multimeter with the lens connected to the TC but not the camera? Lens works on camera, TC works on camera, Lens and TC do not work together. Indicates the contacts between lens and TC are not touching, or the resistance in the combined electrical path is enough to reduce current below that required by the camera. Test conductivity with lens alone in ohms not just yes/no, then test again with the TC connected and see how much the resistance has increased. If it measurable then you may have tarnish or a poor connection. If it is 100% then there is no connection.

10-30-2014, 02:12 PM	#7
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by asharpe But since the lens works fine on the body, isn't a moot point? No, test the TC mount with the lens attached. Steve

10-31-2014, 12:46 AM	#8
asharpe Senior Member Join Date: Sep 2007 Location: Palo Alto, CA Posts: 221 Original Poster	Well, I have tested the teleconverter contacts with the lenses attached, and not surprisingly, the 50 does not short the A pin to ground when attached. However, I cannot tell the difference between it and the other lenses that work, so it must be an extremely small difference. I guess I'll have to keep looking at it until I figure it out. One possibility I see is that the A contact on the teleconverter is indeed slightly lower than the rest. But taking the mount off revealed that they are all on rather delicate springs. I may try to slightly stretch the A spring, but this is probably not worth the danger of completely screwing it up.

10-31-2014, 02:52 AM	#9
Not a Number Moderator Join Date: Mar 2012 Location: Venice, CA Posts: 10,526	The "A" contact on the female side is supposed to be slightly lower that the rest of the contacts. The "A" pin on the lens probably extends to the minimum length within specifications. And the "A" pin on the female side of the teleconverter is at or near the maximum depth within specifications. Put the two together and contact isn't made. Stretching the spring probably won't help. If the pins and springs are like the Kiron and Vivitar TCs that I have the springs and pins are all the same. Try switching pins and springs. Adjusting the "A" pin on the lens may be possible but it would depend on how the mechanism is designed. Otherwise just use a piece of foil to short the "A" pin out when using this lens.