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07-20-2013, 08:51 PM   #1
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Having a little trouble with a new (to me) lens

I just purchased 2 used manual lenses for use on my K-30. I've read the instructions here in the forums for using manual lenses with newer dSLR bodies, and have no problems using one of the lenses, but the second just won't seem to meter correctly.

These are the 2 lenses:
  • Takumar (Bayonet) 1:2.5 135mm
  • Vivitar 28mm 1:2.8 (marked also with "Auto" & "VMC")

For reference, the Vivitar is designated K22 in Robin Parmar's The Great Vivitar 28mm Bestiary. The information on this lens is less than complete.

The Takumar works beautifully. I'm using manual mode, compose & focus, press the green button to meter/set shutter speed, then shoot. Works just like it's supposed to. The Vivitar is giving me a problem with metering. When I follow the same procedure that works with the Takumar lens, the lens just doesn't "click" to meter, so my images are usually very underexposed.

Has anyone else ever had a similar issue with a particular lens and found the fix?

I'm attaching photos I took of the lens with my mobile phone (I apologize for the quality!) for a little more reference.

Attached Images
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SPH-L710  Photo 
View Picture EXIF
SPH-L710  Photo 

Last edited by T.P.S.; 07-20-2013 at 08:57 PM.
07-21-2013, 12:15 AM   #2
Ole
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Let's see a picture of the rear of the lens with the stop down lever. The lever could be bent or missing entirely.
07-21-2013, 12:41 AM   #3
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QuoteOriginally posted by Ole Quote
Let's see a picture of the rear of the lens with the stop down lever. The lever could be bent or missing entirely.
To the untrained eye, it looks straight. Let me know what you think.

Thank you.
Attached Images
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PENTAX K-30  Photo 
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PENTAX K-30  Photo 
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PENTAX K-30  Photo 
07-21-2013, 01:07 AM   #4
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Perhaps this is the trouble?

I did another web search and found another post here in the forum - here.

Is it possible I just need to sandpaper the contact portion of my anodized mount to allow my green button to work?

07-21-2013, 01:16 AM   #5
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Your user name made me feel like I had to respond, huge MMA fan here. But yes I think sanding the mount portion that comes in contact with the camera body contacts is going to be the trick. Otherwise the aperture will stay wide open and the exposure problems will be the result. The anodized coating can be stubborn and I would recommend taping off the opening for the glass element so as to prevent dust from getting in.
07-21-2013, 03:09 AM   #6
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If the lens doesn't shut down wouldn't that lead to over exposed shots rather than underexposed?
07-21-2013, 03:37 AM   #7
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Could be a mute point, but you have not said that you have moved the aperture ring to f22 and checked with the aperture lever that the iris is moving free and easy. The lens is made by Cosina, (pretty sure because of the anodised lens mount) they have a tendency when they get old for the focus grease to dry out and infect the leafs of the iris.
07-21-2013, 04:35 AM   #8
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QuoteOriginally posted by UFCfanWI Quote
Perhaps this is the trouble?

I did another web search and found another post here in the forum - here.

Is it possible I just need to sandpaper the contact portion of my anodized mount to allow my green button to work?
Yes, you need to establish an electrical short of the contacts of the camera mount. Otherwise the camera doesn't detect that a lens is mounted.

07-21-2013, 04:45 AM   #9
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Or you can just tape some conductive foil on it? Less of a chance to damage the lens, and you can test it to see how it works out, if thats the problem or if its something else
Keep in mind that metering with these type of lenses is never 100% and you need to learn how to use Av (possibly with auto ISO) and M (with green button), and estimate a correction
07-21-2013, 09:40 AM   #10
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Your Vivitar does not have contacts and requires that you be in M mode for correct stop-down metering. I do not have a K-30 and cannot comment about the conductive vs. non-conductive base and metering on that camera. I know that it makes no difference on my K10D as long as the lens is detected as lacking the A-contacts (F-- in the display).

Sooo...first part of lecture over, I would ask a few questions...
  • When you press the green button, do you hear the stop-down mechanism?
  • When you press the green button, do you see the viewfinder dim as the lens stops down for the meter measurement?
  • After you press the green button, does the shutter speed in the viewfinder display indicate a reasonable exposure for the aperture setting on the lens? Compare with a known-good lens.
  • Do you have your eye to the eyepiece when you press the green button?
If you don't hear the mechanism, there is something wrong with your camera or configuration or the camera "thinks" the lens is A-contact with ring in the "A" position. If you don't hear it with your Vivitar, try again with your Takumar (Bayonet) as a double check.

Likewise, if the aperture ring on your Vivitar is set for, say f/11, the viewfinder should get noticeably dim when the green button is pushed. If you hear the mechanism, but the viewfinder does not dim, there is a problem with the coupling. I would not expect this to result in underexposure, however.

That last point is a bit of a stretch, but with stop-down metering and smaller apertures, extraneous light through the eyepiece can be significant. On my camera, I have noticed up to two full stops underexposure when shooting on tripod in bright light. The solution is to shade the eyepiece with your fingertip when taking the meter reading.

One last consideration that is also a bit of a stretch is that it is rather easy to exceed the limits of the metering system in stop-down mode. It is hard to explain, but with the lens stopped down in dim light the amount of light actually reaching the sensor may be less than the low end of the range (0 EV(100) for the K-30)*. This is true regardless of your ISO setting. It is also likely that the meter is not fully linear as the light approaches the lower limits of its sensitivity. At least that is how it is on my camera. Do your testing using a well-lit subject.


Steve


*Here is a real world case:
  • Room light is somewhat dim
  • Desired taking aperture is f/8
  • A sensitive hand-held meter (say a Gossen Luna Pro) indicates 1/32s at f/8 for ISO 1600
  • When the lens is stopped down to f/8 (green button pushed), the light reaching the meter cells is only 0 EV(100) and the camera's meter is at the ragged edge of its linear sensitivity
This was common knowledge 40+ years ago when stopped down metering was the norm for many TTL metering SLRs. There was usually a table in the owner's manual that showed the useable range of aperture and shutter speed combinations at ASA(ISO) 100.

Last edited by stevebrot; 07-21-2013 at 10:21 AM. Reason: Huge section that was not correct for OPs gear
07-21-2013, 10:25 AM   #11
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QuoteOriginally posted by Na Horuk Quote
you need to learn how to use Av (possibly with auto ISO)
I don't own the K-30, but it is my understanding that Av mode with a K-mount lens will result in the exposure being made "wide-open" regardless of aperture setting. Auto ISO will help, but only to a point if the light is bright.


Steve

Last edited by stevebrot; 07-21-2013 at 10:36 AM.
07-21-2013, 10:36 AM   #12
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Oh...and it just occurred to me. It is possible that the lens is stopping down to a small aperture regardless of the ring setting. Does the aperture mechanism on the Vivitar actually work properly? By that, I mean that with the lens off the camera, moving the aperture ring from large to small and back again results in a step-wise increase in opening size. Repeat the same but use the actuator lever to do the stop down.
07-21-2013, 11:08 PM   #13
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Thank you to all who replied! I'm sorry I didn't reply sooner, but I've been away from the computer all day.

littledrawe - I, also, am a huge MMA fan. Hence, the username.

A little earlier this evening, I took Na Horuk's advice and tried a strip of aluminum foil between the electrical contacts on the body of the camera & the lens. I didn't want to go through the trouble of sanding the lens mount if this wasn't going to work. After two attempts to get the foil in place, I was ready to test. I turned on the camera, it prompted me for focal length and I entered the proper 28mm setting. I was then greeted by the information screen with F-- in place of my aperture reading. I selected M mode, set the focus mode switch to MF, then composed and focused my test shot. At this point I held my breath and depressed the green button... I heard the wondrous sound of - as stevebrot put it - the stop-down mechanism! Then, I exhaled.

Because the foil is a bit cumbersome, I went ahead and went through the trouble of sanding the section of the lens mount that makes contact with the electrical contacts of the body and it works beautifully. After finding the post I linked to in my earlier reply, and knowing the Takumar 135mm lens was a clean metal mount, it only seemed logical for this to be the answer. It made sense because the Takumar would work, then I wouldn't make a single setting change (other than focal length input) when switching the lenses, but the Vivitar just wouldn't meter. I'm very happy it worked.

Again, thank you so much to everyone who took the time to respond to a relative newbie to the forum. It is very much appreciated!
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