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10-05-2011, 12:52 PM   #1
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SMC 50mm F1.7 lens under exposing on a K200D

I have a K200D and have been using a number of lenses including my old SMC Pentax-M 50mm F1.7 lens from my old ME. I've taken some cracking shots with this combination having set up the K200D to handle the lens and the aperture ring. I typically set the scene, focus and press the green button to set the exposure. This has worked well for me until recently...

Now, about half the photos I take with this lens are under-exposing. Could this be sticking blades within the lens? How could I prove this? Could it be a setting on the camera regarding the green button?

Has anyone else been through this? If so, what approaches did you take to resolve this?

Thanks

10-05-2011, 01:36 PM   #2
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This is a known issue with non-A contact lenses on your camera and others of that generation. My K10D is a prime example. The simple answer is found in the user manual. They basically say that stop-down metering will give unpredictable results. To give a more complete description:
  • The underexposure problem is more severe at apertures wider than f/4
  • The amount of underexposure varies by aperture and by the lens being used
  • There are issues at narrow apertures as well, but that is related to meter low-light sensitivity
  • The problem is related to logic that is applied to the meter output to offset the characteristics of the stock focus screens.
  • Newer bodies (K-7, K-5, K-r) turn off the offending logic when in M-mode with a non-A contact lens
The solution? Meter, shoot, chimp the histogram, adjust, reshoot.


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10-05-2011, 01:39 PM   #3
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Sticking blades would cause over-exposure. The blades are held wide-open when focusing, then allowed to close when you press the shutter button. If they are slow, they'll be open further than you think.

I can't think of a good simple cause for underexposure. Is it several shots in a row, or alternating between good and bad exposures? Is it a certain aperture range that fails?
10-05-2011, 01:39 PM   #4
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QuoteOriginally posted by savcom Quote
Could this be sticking blades within the lens? How could I prove this?
You can test the aperture function by simply flipping the actuator lever looking through the lens with the lens off the camera. The action should be snappy. If not, it will be readily apparent.

However...as noted above, sticky blades cause overexposure...


Steve

10-05-2011, 01:54 PM   #5
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QuoteOriginally posted by Just1MoreDave Quote
Sticking blades would cause over-exposure. The blades are held wide-open when focusing, then allowed to close when you press the shutter button. If they are slow, they'll be open further than you think.
That's what I would have thought. But then I thought the process through. If the blades are slow when pressing the green button, then, as you say, they will not stop down to match the aperture setting on the ring and would be open further than I expect. In this case, wouldn't MORE light be let in while the camera is metering as the blades would not actually be at their assigned F-stop? And if more light gets in in this mode, the meter would then adjust the exposure to compensate - resulting in a shorter exposure time and an underexposed photo?

Or am I missing something?

I'm going to try a comparison between that lens and my M 40mm F2.8 and see if there is a difference.

Martin
10-05-2011, 02:04 PM   #6
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I have had different results with manual metered lenses. Generally, I found that the more you stopped down the lens, the more the camera overexposed the scene. I posted a thread about this some time ago with example pictures. I'll link it below. Others that contributed to the thread seemed to have similar experience to mine. I'm surprised to read that you are seeing the opposite (e.g., underexposure the more you stop down).

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/34265-takumar-...w-f-stops.html
10-05-2011, 02:12 PM   #7
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One way to tell whether this is the well-known problem of inconsistent exposure with manual lens it to swap the focus screen of the camera body (K10D or K20D) with a focus screen for the *ist DS (e.g. LL-60 or LI-60).

Some related threads:

Thread 1.

Thread 2.

Thread 3.
10-05-2011, 08:08 PM   #8
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QuoteOriginally posted by savcom Quote
That's what I would have thought. But then I thought the process through. If the blades are slow when pressing the green button, then, as you say, they will not stop down to match the aperture setting on the ring and would be open further than I expect. In this case, wouldn't MORE light be let in while the camera is metering as the blades would not actually be at their assigned F-stop? And if more light gets in in this mode, the meter would then adjust the exposure to compensate - resulting in a shorter exposure time and an underexposed photo?

Or am I missing something?

I'm going to try a comparison between that lens and my M 40mm F2.8 and see if there is a difference.

Martin
I think that you are likely to hold the green button for a longer time interval than the camera uses between shutter button press and opening the shutter. For underexposure, they would have to go slow and stay too far open when they have lots of time to do that, then in a short interval, close further/faster.

I have a theory which I have seen on one lens. The screws holding this lever (arrow) might be loose:



Or maybe something else deep in the aperture blade mechanism with a lot of wear and slop. The blades somehow can overshoot, close too much, when they are asked to close quickly. When you press the green button, they have time to settle back into the right (or close) position. If you hold the green button, it measures light continuously, so you aren't seeing the "overshot" reading, only the "settled" reading. When you shoot, things happen fast and there's no time to settle. It's unusual but still my only theory.

10-06-2011, 12:53 PM   #9
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One way to test the sticky blade theory is to use DOF preview rather than green button to check exposure before shooting. Hold the DOF preview as long as it takes for the blades to fully close and manually set the shutter speed to give a zero meter reading. then shoot and see if it,s still off. I take it, BTW, we aren't talking about something like the slight difference that always exists between center weighted metering (which fully manual lenses use) and matrix metering (which you might otherwise be using), or something really basic like shooting a backlit scene and expecting it to be exposed for the foreground.

But FWIW, the K200D normally produces quite good exposures with manual lenses, unlike some other models that used a different focus screen.
10-10-2011, 12:34 PM   #10
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QuoteOriginally posted by Marc Sabatella Quote
One way to test the sticky blade theory is to use DOF preview rather than green button to check exposure before shooting.
Thanks. I'll give that a whirl. I'd never noticed this on my K100 when I used the lens on that camera.

QuoteOriginally posted by Marc Sabatella Quote
I take it, BTW, we aren't talking about something like the slight difference that always exists between center weighted metering (which fully manual lenses use) and matrix metering (which you might otherwise be using), or something really basic like shooting a backlit scene and expecting it to be exposed for the foreground.
You take it correctly.

QuoteOriginally posted by Marc Sabatella Quote
But FWIW, the K200D normally produces quite good exposures with manual lenses, unlike some other models that used a different focus screen.
They're not bad, when they're right.

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10-10-2011, 12:42 PM   #11
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Actually - there *is* something I can try. I have a lens reversing ring that I use for macro shots. If I shoot (say) ten shots of a subject at a given F-stop with the lens reversed then there would be no blade movement when exposing the shot, so the exposures would all be with the aperture blades in steady state. That should answer the question! I'll try that when I've next got time.

Any suggestions for a procedure here? Tempted just to use the green button for each exposure and to log exposure times for each shot and see what comes back. I could then assess the different brightness levels too to see what changes I need to make in post processing to bring them all back to the same brightness.

Martin
10-10-2011, 03:50 PM   #12
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QuoteOriginally posted by SOldBear Quote
One way to tell whether this is the well-known problem of inconsistent exposure with manual lens it to swap the focus screen of the camera body (K10D or K20D) with a focus screen for the *ist DS (e.g. LL-60 or LI-60).
Bear, leaving aside the metering, what does the focussing screen from an *iST look like when looking thru the VF when fitted to a 10d/20d ? Is it brighter than the stock 10d/20d screen ? I was just wondering if this LL-60 was any brighter to look through than a stock K-7 screen. I've tried split screens but didnt get on with them.
10-10-2011, 03:58 PM   #13
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QuoteOriginally posted by Squier Quote
Bear, leaving aside the metering, what does the focussing screen from an *iST look like when looking thru the VF when fitted to a 10d/20d ? Is it brighter than the stock 10d/20d screen ? I was just wondering if this LL-60 was any brighter to look through than a stock K-7 screen. I've tried split screens but didnt get on with them.
In term of brightness, I can't tell the difference between the stock K10D screen (LI-80 I think) and the LL-60 screen. If you imply whether it's easier to focus manually with the LL-60, the answer is no.

Also, note that the screen in K-7/K-5 has different dimensions from the one in the K10D/K20D.
10-10-2011, 04:08 PM   #14
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Thanks bear - that kinda settles my wondering. I dont mind the stock screen, but they always seem a little dull to me. My old K10, K200D, and even my Canon 450D and 40D were all the same.

I just fancied a brighter screen without going to a split type. I found the Katz Eye to be way over priced for what little advantage it gave in brightness. Add the fact that then realised i didnt like split screens

Plus i guess one would lose the viewable focus points
10-10-2011, 08:19 PM   #15
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QuoteOriginally posted by savcom Quote
Actually - there *is* something I can try. I have a lens reversing ring that I use for macro shots. If I shoot (say) ten shots of a subject at a given F-stop with the lens reversed then there would be no blade movement when exposing the shot, so the exposures would all be with the aperture blades in steady state. That should answer the question! I'll try that when I've next got time.

Any suggestions for a procedure here? Tempted just to use the green button for each exposure and to log exposure times for each shot and see what comes back. I could then assess the different brightness levels too to see what changes I need to make in post processing to bring them all back to the same brightness.

Martin
That should eliminate blade movement from consideration. I would definitely keep some kind of records too.

QuoteOriginally posted by Squier Quote
Plus i guess one would lose the viewable focus points
I don't think that's true with Pentax. I still have focus points with a Katzeye screen.
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