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05-15-2019, 12:31 PM   #1
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Weird lens behaviour on Pentax K Mount (film SLR)

I have come across a really weird thing with a lens and camera and its driving me insane as it makes no sense at all. I'll try and explain and if anyone has any ideas they would be most gratefully received.

OK heres the issue -

I have two non Pentax K mount lenses. A Sigma 40-80 and a Chinon 28-50. When I mount these on my trusty KX and K1000 the meters both agree with the reading taken with a Pentax 50mm 1.7 and agree with an Olympus OM-1 and a Canon AE-1 so we have unanimous vote from all cameras with a range of lenses as to the correct aperture and shutter. Alls good it would seem.

Now I mount the Pentax 50mm 1.7 on a Chinon CE5 and it also agrees (its about 1/2 a stop off but there is a small variation between all of the cameras tested above and I could put that down to different patterns of light meter, sensitivity, differences in lens design and slight differences in focal length of various 50mm lens. So whats the problem ? - All cameras agree with their respective 50mm lens so life is good. Heres the problems - when I mount the Chinon or Sigma lens on the Chinon CE5 it reports a totally different aperture/speed and its out by as much as 4 stops in bright light (though it comes within a 1/2 stop on a dim light) - its out in the down
direction (it thinks its dimmer than it is).

Its not the tab in the camera as the Chinon responds perfectly well with any other lens and is always in the mix compared to the others. So the issue is I have two lenses which work perfectly on two different Pentax cameras and yet create weird meter readings on a non-Pentax. I can see no reason for this although the view through these two lenses is quite dim even when they are wide open.

So is it that the lenses are rubbish or is there some other possible issue that is perhaps unique to the Pentax K mount ? Are there possible compatibility issues for instance ? The lenses all have their aperture tabs working as they work fine on two Pentax cameras. Its just doing me head in trying work out whats going wrong here. It can't be the camera as it responds perfectly well with any other lens and it can't be the lens because they work perfectly on the two Pentax bodies.

I could say -oh who cares anyway - just sell the lenses and move on but (a) I don't want to sell someone a pup (b) I am curious as to why, how this is happening and (c) its could something to watch out for with other lenses when they are coupled to the Pentax cameras which are my real interest in all of this.

05-15-2019, 02:23 PM   #2
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This does sound rather strange. The meter coupling mechanism on the CE-5 is functionally the same as in your two Pentax bodies. What may be different is the size, orientation, and state of repair of the body's aperture ring position coupling tab and how well it mates to the corresponding tab on your lenses. It may be that the lens-side "tab" is slipping past the body-side "tab". This is not a good thing in that there may be potential for damage when attempting to dismount.


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05-15-2019, 04:02 PM   #3
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If I am understanding this correctly - the lenses on the Chinon body overexpose? And only when in bright light? It sounds like they are not stopping down. Set the body to a long shutter speed and a narrow aperture and look from the front. Does it stop down all the way as expected? Do the same on the Pentax bodies where it works - is the lens stopping down more on the Pentax?
05-15-2019, 10:53 PM   #4
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Are you talking about metering or actual exposure.

It sounds like the aperture tab is not engaging on the Chinon body.

I am not talking about the operating lever, but the tab that takes the aperture setting to couple it to the light met r

05-16-2019, 01:02 PM   #5
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Hi Guys,

The aperture tab is properly engaging on the Chinon I know this because (a) when you shift aperture the meter is responsive in the correct manner e.g. it responds one stop at a time albeit on what seems the wrong value and (b) by putting the camera on brief and locking it with a cable release I can look inside through the shutter curtains and see that as you twist the aperture the follower tab on the camera moves with it. It can't be the aperture tab having an issue as its perfectly happy with any other lens (except the Sigma).

The iris on both the Sigma and the Chinon lens is responsive in all situations - it stops down correctly and activates perfectly on shutter release - looking down the lens you can see it stopping correctly when the stop down lever is pressed so the lens seems on its own terms ok and they both work fine with either of my Pentax K bodies.

You can see what I am baffled here - everything functions as it should but the readings are wrong but only in a combination of Sigma/Chinon Lens on Chinon Body . Its obvioulsy some sort of compatibility quirk but I am damned if I can see how this is even possible on a purely mechanical coupling.

Anyway to see whats REALLY happening I have put a roll of HP5 through the camera tonite - one set of shots on AUTO and Manual mode with one of the pesky lenses and one set of shots with a Pentax 50mm f1.7 and same again some on AUTO some on Manual and I will have a look at what comes out the bag on these and see if its REALLY ok. The thought occurred to me that the lenses may actually be VERY slow and their F stop markings are way out but that makes no sense. I'll let the film roll decide the outcome.
05-16-2019, 02:07 PM   #6
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QuoteOriginally posted by Lowell Goudge Quote
Are you talking about metering or actual exposure.
QuoteOriginally posted by Astro-Baby Quote
The aperture tab is properly engaging on the Chinon I know this because (a) when you shift aperture the meter is responsive in the correct manner e.g. it responds one stop at a time albeit on what seems the wrong value and (b) by putting the camera on brief and locking it with a cable release I can look inside through the shutter curtains and see that as you twist the aperture the follower tab on the camera moves with it. It can't be the aperture tab having an issue as its perfectly happy with any other lens (except the Sigma).
...The Sigma and the Chinon?

Please explain what is meant by "wrong value"; do you mean a different shutter speed than with your Pentax-M 50/1.7 at the same aperture setting and ISO? Assuming this is the case, let's recap:
  • Pentax-M 50/1.7 provides comparable meter readings on all three K-mount cameras
  • Sigma zoom provides similar readings on the Pentax bodies as the Pentax-M
  • Chinon zoom provides similar readings on the Pentax bodies as the Pentax-M
  • Both zooms on the Chinon body are at odds with their readings on the Pentax bodies
The description points back to the Chinon body. Moving on, lets look at the assumptions regarding the Chinon body:
  • The ISO on the Chinon is set to the same value for all three lenses
  • The Exposure comp is set to the same value for all three lenses
  • All three lenses are fully mounted with retention pin engaged
  • All three lenses show consistent* measured EV at all aperture ring positions on the Chinon body
  • The aperture is actually fully open on the two weird lenses when the meter readings are observed
  • The light on the subject was the same for all three lenses
If all the above check out, the only things left are:
  • Aperture ring position coupling. Just because it engages properly with one lens does not mean it is doing so with others.**
  • Meter linearity (all readings are done wide open, but the zooms admit less light)
Do you have an M42 lens and adapter to test that last point?


Steve

* When the aperture ring is moved one full stop narrower, we expect a corresponding one stop slower shutter speed at all full-stop aperture increments.

** My Zenit Automat does this. The follower engages properly with the Helios 44K-4 that shipped with the body, but not with my other K-mount lenses.
05-17-2019, 10:25 AM   #7
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Steve,

I'll try and answer the questions and thank you for your help and interest...

First off yes the Sigma AND the Chinon lens both exhibit the same weird behaviour - but with every other K mount lens I have - Hanimex, Pentax, Hoya all cameras are perfectly well behaved including the Chinon.

The 'wrong value' - I can see where that may have been vague. OK - if I mount up a 50mm f1.7, Film Speed set to ASA 400 for all cameras and shutter speed set to 500th with a good bright sun I would expect to see the meter in almost any camera give me f16 or thereabouts (i.e. it may vary by 1/2 a stop either way and with a big telephoto on it may vary quite a bit) in this instance all cameras are set for ASA 400, no exposure compensation and shutter at 500th. With the 50mm (or indeed any lens except the Chinon and the Sigma) all cameras report - yup - f16 (the K1000 sometimes can't seem to make its mind up and oscillates between f11 and f16 but thats ok. All other cameras in the collection will go at f16 with a similar 1/2 stop difference which as I said above could be a number of factors between different designs and meter sensitivity etc . So in that config I would expect a 28-50mm to be there or thereabouts - nope - when you put that on the meter will insist on f5.6 at most but ONLY when that lens is on the Chinon body. So its very specific - if I put that lens on a Pentax body the meter will show f16 is good (which it would be) - put it on the Chinon body and it reckons f5.6 which is clearly wrong for the light. By the way the light is consistent and and all tests are done in a matter of minutes on a clear sky so theres no real variation in light - certainly not 4 stops worth

So yes - your second point about consistency the answer is yes - the Chinon body is configured exactly the same (i.e. ISO value set, exp comp set at zero, pin engaged, apertures are fully open, the target is a radio mast about 5 miles away which gives a picture in the bottom half of the viewfinder of dark(ish) grass and trees and the top half being sunny sky (sun behind me giving solid illumination).

The aperture ring tab/follower is working fine on all lenses at least so far as I can tell - if I put the camera on brief and keep it there with a cable release I can look inside and although once a lens is mounted I can't physically see the tab I can see the ring that the tabs mounted on moving as the aperture shifts and the meter in the Chinon is completely consistent - i.e. it will suggest f5.6 at 500th but if I drop the speed down a notch to 250th the meter will centre up on f8 of thereabouts - so its consistent but madly wrong.

Perhaps it is just a weird incompatibility is just I struggle to see how it can be - its pretty much just a mechanical linkage.

Unfortunately I don't have an M42 adapter
05-17-2019, 12:27 PM   #8
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QuoteOriginally posted by Astro-Baby Quote
but ONLY when that lens is on the Chinon body
Yep...That body does not like those lenses.

QuoteOriginally posted by Astro-Baby Quote
if I put the camera on brief and keep it there with a cable release I can look inside and although once a lens is mounted I can't physically see the tab I can see the ring that the tabs mounted on moving as the aperture shifts and the meter in the Chinon is completely consistent - i.e. it will suggest f5.6 at 500th but if I drop the speed down a notch to 250th the meter will centre up on f8 of thereabouts - so its consistent but madly wrong.
You may be able to see the body-side tab from inside the mirror box, but what you don't see is how well it is engaged. With no lens on the bodies, compare the appearance of the Chinon's tab with that on either of your Pentax bodies. Now compare the lens-side tab on the zooms to that on the M 50/1.7. Is the body-side tab on the Chinon crooked, scratched, or bent? How about the tabs on the lenses. You may need a magnifying glass.

Now, put all three lenses at maximum aperture with none mounted to the Chinon body. Starting with the M 50/1.7, mount the lenses to the Chinon body, taking care to leave the aperture ring at the maximum aperture position and put the zooms at the their minimum zoom setting with all three lenses focused to infinity. Record the suggested shutter speed and whether the aperture number in the viewfinder agrees with the lens setting. At that position, the body-side follower should be at the same position (close to maximum deflection) for all three lenses regardless of which are mounted and should result in the very similar metered EV* for all three lenses. For convenience, you might want to put the shutter dial in the Auto position. Meter against an evenly-lit blank wall or something similar. Don't be concerned about compliance to expected Sunny-16 values.

Repeat for the two zooms at their maximum focal length. I would expect the Chinon zoom to require 2/3 stop slower shutter speed assuming it is a 28-50/3.5-4.5. This last was sort of a "dumb-Steve" moment as I realized that both were probably variable aperture zooms and as such should be evaluated at their shortest focal length. This may well be the smoking gun.

If the tab on the body is other than properly aligned, the tab in the lens may slip fully or partially past it.


Steve

* EV is simply a numeric representation of the exposure provided for particular combination of shutter time and lens aperture. The Wikipedia entry on Exposure Value has an equivalence table (LINK). Also helpful is Ken Storch's table of equivalent exposure stops (shutter, aperture, and ISO), (LINK)


Last edited by stevebrot; 05-17-2019 at 12:43 PM.
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