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03-22-2012, 03:29 PM   #16
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QuoteOriginally posted by Lowell Goudge Quote
use aperture only to offset shutter speed, not ISO, you are changing too many variables at once.
Indeed.

Furthermore, it seems that the program line that determines correct exposure in the camera's metering system may be sensitive to the lens used, the lens aperture used, and not have a simple 'one step of shutter speed equals one step in aperture' relationship to shutter speed, particularly at the highest shutter speeds.

This is illustrated (for example) by the exposure program line for the K10D:



03-22-2012, 07:07 PM   #17
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There *is* a problem with the shutter

The TV shutter test indeed revealed that the first curtain is too slow:

Pentax K-r with Pentax D FA 50mm Macro. M, 1/6000s f/2.8 ISO 3200.



Note that the TV is scanning top-down. Though it's a bit subtle, the right side (the higher part of the band) is wider than the left side (the lower part of the band). I am planning to have the shutter replaced at some point after I get my K-5.

--DragonLord

Last edited by bwDraco; 07-20-2012 at 08:29 PM.
03-23-2012, 05:18 AM   #18
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just a simple caution here, a slightly slow curtain will not necessairly cause the problem you seem to be having.

You might wish to do a more controlled test, over the entire range of shutter speeds, with your lens.

I would do this in several ways just to make sure.

first, I would start with the lens at F8 and set exposure so that you are about 3/4 of the way to the right on the histogram with your target (this shouldbe about 180-190 greyscale on thehistogram). Then without changing shutter speed, take shots from F8 up to F22. Look at how greyscale values change with aperture. with your shutter setting and exposure you should find an almost straight line at 45 greyscale per full stop. and should bottom out at about 45 greyscale and F22.

then repeat the test, this time starting at F8 and 3/4 to the left of the histogram (i.e. about stops up on the shutter speed from the last test), and workfrom F8 down to F2.8. this will give you an indication of how linear and correct your aperture is. on the assumption that the shutter is not changing frame to frame in its performance.

once you have confirmed the aperture of your lens is working correctly, you can use the lens to really check shutter speed over the entire range., but beware, my experience is that maximum aperture is never right on the money, and neither is minimum.

the reason I am still suggesting you check the accuracy across the entire range, is to make absolutely sure that the shutter is really out of wack. Replacing the shutter will cost you upwards of $300, I know, I had one replaced in my K10D and that was 3 years ago!
03-23-2012, 06:17 AM   #19
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In this case, I will probably use the K-r (as a backup or secondary camera once I get my K-5) until the shutter problem becomes unacceptable. I rarely shoot at shutter speeds above 1/1250, so it's not a serious problem at this time. But a problem of this nature will get worse with further usage, so it will inevitably fail at some point in the future.

--DragonLord

03-23-2012, 06:25 AM   #20
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QuoteOriginally posted by DragonLord Quote
In this case, I will probably use the K-r (as a backup or secondary camera once I get my K-5) until the shutter problem becomes unacceptable. I rarely shoot at shutter speeds above 1/1250, so it's not a serious problem at this time. But a problem of this nature will get worse with further usage, so it will inevitably fail at some point in the future.

--DragonLord
when my K10 died, one curtain was being dragged by the other, leaving a dark band on the frame, that got wider and wider as speed increased above sync speed, below sync speed it was fine.

as I said, it cost me just over $300 3 years ago, but they also replaced the mirror assembly, and lens mount, plus as part of the teardown did a full cleaning. I still have the camera and it has become a perminant M42 body but to be honest, I have not checked lately on any of my cameras how accurate the shutter is at very high speeds, although I have shot at over 1/2000 lately but did not notice anything unusual.

I might just run some checks to see how accurate the shutters actually are on perceived properly functioning cameras

one additional thought crossed my mind. You might just want to check the camera with a Non A lens, and use the lens aperture clicks to check exposure. just as a thought, maybe the aperture mechanism, either camera or lens, not the shutter is defective.
03-23-2012, 06:43 AM   #21
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QuoteOriginally posted by Lowell Goudge Quote
one additional thought crossed my mind. You might just want to check the camera with a Non A lens, and use the lens aperture clicks to check exposure. just as a thought, maybe the aperture mechanism, either camera or lens, not the shutter is defective.
Taking the D FA 50mm Macro lens off the A position in manual mode and testing with 1/6000s f/2.8, 1/3200 f/4, and 1/1600 f/5.6 still produces underexposure at 1/6000s f/2.8. The problem is not in aperture control.

--DragonLord
03-23-2012, 06:47 AM   #22
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QuoteOriginally posted by DragonLord Quote
Taking the D FA 50mm Macro lens off the A position in manual mode and testing with 1/6000s f/2.8, 1/3200 f/4, and 1/1600 f/5.6 still produces underexposure at 1/6000s f/2.8. The problem is not in aperture control.

--DragonLord
no, but it might be maximum aperture is over stated.

I will look at my bodies to see how good they are at the limit. for me if the change is less than 1/2 stop, then it is probably within spec at that end. WHat I noted in you r first data is that things didn't really seem to track fully I think because there were too many variables being changed at once.

edit note, maybe you have a problem. just ran my K5 using my 135/2.5 started at 1/250 and F16, and worked down to F2,5 and 1/8000. although I have not yet measured greyscale, all histograms peaks on the LCD are dead center of the histogram

Last edited by Lowell Goudge; 03-23-2012 at 06:55 AM.
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