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03-10-2017, 06:21 AM   #1
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Underexposed viewfinder, normal LiveView

This seems to be related to several other posts, but I haven't seen one that is the same. Since most of the others are older (in Internet years), I am posting a new question.
I have a K50 that I purchased about 2.5 years ago. It has been working fine up until about a month ago. Pictures taken through the viewfinder are severely underexposed while LiveView is fine. In looking at some of the other debugging suggestions, I found that aperture is wide open when the viewfinder is used and fully stopped down when the LiveView is used. This could explain why the viewfinder is underexposed (too much light hitting the exposure sensor). Since the LiveView takes fine pictures and the aperture is correctly adjusted, I don't think the aperture control solenoid is the issue.
Thought?
Thanks.

03-10-2017, 07:49 AM   #2
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It is normal when using the viewfinder for the camera to meter for exposure with the lens wide open. It will only stop down when you press the shutter button fully.

Does this happen with more than one lens? What lens are you using ?

Is the camera held to your eye during the metering/exposure ? ie is any light getting in through the viewfinder eyepiece?

Does the camera calculate a different exposure when using the viewfinder compared to LV ? Or does it calculate the same exposure but fails to deliver it?

Does this happen in all exposure modes ?
03-10-2017, 08:51 AM   #3
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What type of lens? M, K? Da?
03-10-2017, 09:53 AM   #4
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Welcome to the Pentax Forums!

The questions above are good and through magic, I can answer at least one of them...I believe you are using an A-series or newer lens with electrical contacts on the mount face. As noted above, it is normal for your lens to be wide open when using the viewfinder. It is also not unusual for the lens to be stopped down when using live view, though how much it stops down depends on the scene brightness. The lens will stay open except for bright conditions where it will stop down to reduce the light striking the sensor. The only other time when it should stop down is when doing DOF preview or when actually doing an exposure. The aperture will stop down to the appropriate setting at exposure time regardless of where it is when viewing.

So, why the dark frames when using other than live view? The two views use different meter systems, but they usually have fairly close agreement. To figure it out, the first step is usually to determine if the issue is metering or exposure control...
  • Camera on tripod or counter top and pointed at a well-lit subject
  • Turn OFF auto-ISO and use fixed ISO
  • Use multi-segment (matrix) or center-weighted average metering
  • Switch to M mode
  • Visually confirm that the aperture is wide open. If it is not, there is an issue with either the lens or the camera aperture control.
  • Set your exposure using the optical viewfinder metering and an aperture of f/8
  • Do the exposure
  • Switch to live view
  • Without changing any settings or subject framing position, do a second exposure
  • Review both images. The exposure should be identical.
  • Check that live view metering indicates a setting close to that of the optical viewfinder
If either of the last two points fail, there may a issue with exposure control (aperture, shutter, or ISO) that is likely independent of the which view is used. (These things tend to be intermittent.) If exposures are the same, switch back to optical viewfinder and do 20 exposures using the same settings. All 20 images should have identical exposure. If not, there is an exposure control issue.


Steve


Last edited by stevebrot; 03-10-2017 at 10:17 AM.
03-10-2017, 10:15 AM   #5
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To test for an aperture control problem (lens or body):
  • With lens removed from the camera, manually flick the actuator lever on the lens rear while watching the aperture through the rear of the lens. Movement should be snappy with no sticking or hesitation.
  • With lens on camera:
    • M mode, shutter at 2 seconds
    • Lens at maximum aperture
    • Press shutter while looking looking at the front of the lens
    • The lens should remain full open
    • Set the aperture one stop narrower (the sequence is f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22) and repeat.
    • Continue through the sequence to the narrowest aperture. There should be a step-wise decrease in aperture opening as you run through the sequence with the aperture returning to full open at end of exposure.
An aperture controller problem on the camera will show itself with the aperture closing beyond the set aperture, usually far beyond.


Steve
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