Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
10-27-2014, 02:52 AM   #1
New Member




Join Date: Oct 2014
Posts: 4
K-r ignoring aperture setting?

Hello!

I have a Pentax K-r, and a while ago, I found that the metering would often be way off; perhaps -5 stops? I'm using the stock 18-55mm lens that came with the body.

I'm almost always shooting in aperture priority, and after a lot of experimenting, I eventually got to the root of the problem.

I discovered that if I was shooting at the smallest aperture, the metering would be ok, but it was when I opened the aperture up, the metering went crazy and my pictures would be massively underexposed.

Ultimately, the problem was this: when I selected f/40, I could see the aperture narrow right down to a pinhole as expected, but when I selected ~f/4.0... the aperture would narrow right down to a pinhole!! Whatever aperture I select; the shots get taken at f/40.

I've just updated to the latest firmware (the one I was running was horribly out of date), but the problem persists. Has anyone experienced this problem before and found a solution?

I don't know if this is related, but around the same time this problem started, I was trying out some old K mount lenses from a much older Pentax.

Thanks for any advice.

10-27-2014, 04:15 AM   #2
Pentaxian
Na Horuk's Avatar

Join Date: Feb 2012
Location: Slovenia, probably
Photos: Gallery | Albums
Posts: 10,317
Hello, welcome!

So the problem isn't metering. Its probably a lens problem. Can you test with another lens, to see if it is the lens or camera's fault?
What I suggest is the standard stuff:
Make sure your +/- is set to 0, and that you don't have any special jpeg options enabled. For the tests, use matrix or center weighted metering, not spot metering.
Remove battery, charge it, leave it out of the camera overnight
Remove any filters you might have on your lens. Are you using any special parts, like an off-brand focus screen?
Dismount lens and make sure the lens and camera contacts are clean and free of dust, grime.
Mount another lens, use M mode and press green button for metering. Does the lens stop down to the selected aperture? (of course, if the lens is m42 or really basic K-mount, the camera cannot affect the aperture, so do this test with a lens that supports camera aperture stop-down)

Btw, is the lens still in warranty? Because I suspect there is a problem with its aperture selection inside, and you cannot fix that yourself. The good news is that the 18-55mm is rather cheap in the used market, and pretty much any Pentax lens you buy will perform optically as good or better.
10-27-2014, 01:26 PM   #3
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 26,181
QuoteOriginally posted by Na Horuk Quote
So the problem isn't metering. Its probably a lens problem.
...or a mechanical problem with the body. IIRC, there have been reported aperture actuator problems with the K-r similar to those on the K-30.

Here is a quick test to see if the aperture actuator is doing what it is supposed to:
  • M-mode
  • Aperture set to full-open
  • Shutter speed set to 2 seconds
  • While looking into the lens from the front, actuate the shutter
  • Confirm that the aperture continues to stay open and does NOT stop down
If the lens stops down when it is supposed to stay fully open, there is a fault in the mechanism that controls the aperture coupling.


Steve
10-27-2014, 04:32 PM   #4
New Member




Join Date: Oct 2014
Posts: 4
Original Poster
Thank you both for the input.

The lens and body are well beyond warranty support unfortunately!

Steve, I tried the test you described, and can confirm that the aperture does not stay fully open.

It makes me think that although the hardware is functional (the body can still manipulate the aperture), the software has become corrupt because it's not controlling the actuator correctly? Or perhaps the actuator is just not sensing it's own position properly, and could do with a clean?

I'll try some more poking around.

10-27-2014, 05:20 PM   #5
Pentaxian
fgaudet's Avatar

Join Date: Oct 2013
Location: Ontario
Posts: 726
I'd blame the lens. I am not sure but isn't the smallest aperture opening on the 18-55 supposed to be f/22 to f/38?

Maybe try cleaning the lens contacts. Does the same behaviour happens with other lenses?
10-27-2014, 05:40 PM   #6
New Member




Join Date: Oct 2014
Posts: 4
Original Poster
I haven't been able to try any other lenses unfortunately. As Na Horuk says, it should be cheap to find a similar replacement, so maybe I'll try that.

What makes you think it could be the lens (or the contacts)? The body is evidently capable of manipulating the aperture - it just doesn't seem capable of manipulating it correctly.

Is the lens responsible for reporting the position of the aperture back to the body? Could that feedback mechanism be an issue?
10-27-2014, 07:41 PM   #7
Pentaxian
fgaudet's Avatar

Join Date: Oct 2013
Location: Ontario
Posts: 726
Dirty or malfunctioning contacts will not report the lens capabilities correctly to the camera and may fail to report the precise aperture opening. Also, a very slightly bent aperture lever on the lens can have a huge impact.
10-27-2014, 07:56 PM   #8
New Member




Join Date: Oct 2014
Posts: 4
Original Poster
The body seems to know the range of apertures that should be allowed, and altering the focal length seems to correctly update the effective aperture that's displayed on the LCD display. So based on that, I was assuming that the contacts and associated electronics were ok. Is that a safe assumption to make?

Also, regarding the bent aperture level: Visually, it looks ok, but based on the fact that the aperture is in a normally fully-open state when not in use, and the body only stops it down during a shot, I assumed that mechanism is not at fault.

10-27-2014, 10:23 PM - 1 Like   #9
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 26,181
QuoteOriginally posted by as2003 Quote
It makes me think that although the hardware is functional (the body can still manipulate the aperture), the software has become corrupt because it's not controlling the actuator correctly? Or perhaps the actuator is just not sensing it's own position properly, and could do with a clean?
The problem is with the electronics that control the stepping motor.


Steve

---------- Post added 10-27-14 at 10:29 PM ----------

QuoteOriginally posted by fgaudet Quote
I'd blame the lens. I am not sure but isn't the smallest aperture opening on the 18-55 supposed to be f/22 to f/38?

Maybe try cleaning the lens contacts. Does the same behaviour happens with other lenses?
This problem is common on the K-30 and has been reported on the K-r. The fact that the lens is stopping down in M-mode when it is supposed to stay open indicates that the actuator is malfunctioning. The max/min apertures are being communicated properly as is the presence of the "A" contacts. This can be confirmed by simply using the aperture e-dial to set to max/min.


Steve
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
actuator, aperture, camera, contacts, f/40, k-mount, k-r, k-r ignoring aperture, lens, metering, pentax, pentax help, photography, steve, troubleshooting
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Pentax K-r JPEG image style setting dmnf Pentax K-r 20 01-08-2014 03:55 PM
K-r's Best AUTO setting for non-photographers dmnf Pentax K-r 14 06-25-2013 11:22 AM
Weird aperture behaviour with K-r and Cosina 28mm 2.8 MF jonpv Troubleshooting and Beginner Help 5 04-09-2013 01:49 PM
K-5 reporting wrong aperture for A setting atekant Troubleshooting and Beginner Help 8 10-17-2012 12:29 PM
K-r setting vn33 Pentax K-r 2 07-28-2011 01:46 AM



All times are GMT -7. The time now is 01:00 PM. | See also: NikonForums.com, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top