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05-07-2011, 06:46 AM   #1
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Image over-exposure using Av on a K-m

I've been using my K-m for over two years now and been very happy with it. I normally use P mode because the end results come out just fine for me.

Recently there have been a couple of situations where I required an greater depth of field. Dialing in a small aperture such a f16 and firing the shutter resulted in images that were far too bright, even though the information recorded by the camera could be interrelated to "correctly" exposed shots at different aperture/shutter combinations. Trying different lenses made no difference and produced similar results. This has made me draw the conclusion that the camera must have some sort of mechanical problem as the brighter the subject is and the more I stop the lens down the greater the image's overexposure becomes.

At the moment I'm loathed to part with the camera and send it away to be checked so was wondering if there were any tests I could conduct to establish the problem's cause.

I will also say that similar things occur when using the camera in M and Tv modes. As I've said P mode works fine.

05-07-2011, 07:00 AM   #2
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Welcome aboard. Sounds like it's time to learn what your meter is really telling you..

https://www.pentaxforums.com/forums/pentax-beginners-corner-q/141538-metering-av-tv-mode.html

Be sure to follow any links I give in the thread.

05-07-2011, 07:00 AM - 1 Like   #3
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Put it on manual mode, choose a 10 second exposure, f/16. Press the shutter and look at the aperture when it closes. Is it really f/16?
05-07-2011, 08:04 AM   #4
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Thanks JeffJS, I'm going to have to read through the threads you offered a couple of times for it to sink in and really understand what's going on. I thought Av would be fairly straightforward, looks like I'm well off the path.

Thanks Aegon, that was such a simple suggestion and I never even considered it. I am now able to prove that the camera is capable of stopping a lens down to its smallest aperture and those in between.



05-07-2011, 08:42 AM   #5
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If you have any questions, feel free to ask.

05-09-2011, 10:01 AM   #6
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I'm still puzzled as to why my K-m wonít record images at similar EV levels in Av mode when selecting smaller f-stops, so today I ran a simple test.
I put the camera on a sturdy tripod and aimed it at an 18% grey card on the floor. The target was lit by natural light from a cloudy sky and the camera set on auto white balance using multi-segment AE. The lens used was a SMC Pentax-DA 1:2.4 70mm Limited.

Firstly, seven exposures were taken at ISO 200 commencing at f2.8 and thereafter in one-stop increments to f22. This was repeated at ISO 800. As expected with a fixed exposure value the EXIF data recorded shutter speeds that matched aperture/shutter pairings. However, the actual images increased in brightness as the lens was stopped down. This was verified by the histogram curves which for ISO 200, when looked at in PSE7, gave pixel peaks at 82 on the 0 - 255 scale for f2.8, moving gradually to 174 at f22. When the histograms for the ISO 800 setting were viewed they ranged from 75 at f2.8 to 231 at f22!

Iím no camera technician but these findings are telling me something is wrong with my camera. Has this problem always been there or is it something that has occurred recently? Unfortunately, I have tended to stay with P mode as, like I mentioned at the beginning of this post, I was very happy with results from this setting. These findings have now made me doubt the reliability of the camera and I expect it will cost more to put right than itís worth, which is a bit gutting. Any ideas guys as it looks like a K-5 will have to be funded much sooner than planned.
05-09-2011, 03:47 PM   #7
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Just for grins, did you try it with spot metering? I assume from your description, your ISO is Fixed at whatever value you chose, correct? If your lighting isn't changing, the exposures should be about the same. Spot metering would insure the most repeatability in exposure.

05-09-2011, 04:06 PM   #8
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I own a K-m and have not found the metering to be a problem. Try bracketing some shoots to see happens. Some lighting conditions need you to override the meter via the EV button to correct the the exposures to get what the eye has seen. I find the K-m works well for me in most of the picture taking conditions I use it in.

05-10-2011, 03:02 AM   #9
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Hi JeffJS. Yes, I fixed my chosen ISO and waited for uniform light, i.e. cloudy/overcast, expecting exposures to be close enough that a little variation wouldnít matter. What I got was well and truly wide of the mark!

Because you suggested spot metering I fired off a few more frames, this time with a 35mm Macro Ltd and got the same Av results - smaller apertures producing increasingly overexposed images. Also tried two extremes in Tv mode and similarly a slower shutter speed (smaller aperture) produced an image brighter than a faster one (wider aperture). In addition I put the camera in M mode metering with 1/30sec @ f9 and 1/125sec @ f4.5 here again the slower speed was brighter than the faster one. So why does P mode seem to give the more reliable result?

Thanks stevbike for your suggestion. I do use EV bracketing, which is extremely useful, but when I know what I want in Av, Tv or M mode I now find I cannot guarantee results without a lot of experimenting. Quite often the opportunities won't wait for you.

Just wish I knew if this was a mechanical or programming malfunction.
05-10-2011, 03:52 AM   #10
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RE your M mode settings. Fixed ISO...

IF: (for example)

f2 @ 1/1000 then f4 @ 1/500; f5.6 @ 1/250; f8 @ 1/125; f11 @ 1/60(ish); f16 @ 1/30; f22 @ 1/15

Should all give you the same exposure. Something is definitely screwy. The attachments below were taken with my K5 and an FA43. The spot metered point is the gray bar in the middle of the screen. Taken at your 1/30sec @ f9 and 1/125sec @ f4.5 settings in M mode (the first two) at ISO 400 to center the meter, Going to Av mode gives me the same shutter speeds for each respective aperture (exifs should be there). In case the EXIFs don't show, the first one is at f4.5 1/125 the second at f9 1/30 in M mode. The third is at f9 in Av and the camera chose 1/30 and the last one is at f4.5 in Av where the camera chose 1/125.

The idea you get satisfactory results in P mode (which I don't use and don't really know how to), suggests, the Meter is working. I know that sometimes on my K10d (same generation) the exposures would vary sometimes but not as consistently as you are seeing.

Just a stabs in the dark at this point, do you by any chance have EV Comp turned on (set to a value other than 0)? Maybe post some examples with EXIFs in tact.. Try updating the firmware if you haven't already. Try metering and locking the exposure (AE-L) before pressing the shutter button. If you have AE-L when AF is locked (pg 131 in your manual) turn it off. All of the above can be achieved by doing a complete reset (Pg 179,180).

Attached Images
View Picture EXIF
PENTAX K-5  Photo       
05-10-2011, 12:45 PM   #11
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Hi woodyneale,

Let me suggest another possibility -- sticky aperture blades. If the aperture blades are slow to close, this can cause the same problem.

The most common causes are oil on the blades or a mechanical problem with the actuating lever. Consider that the aperture closes down a small fraction of a second before the shutter fires. If it is slow to close, it still might be in the process of doing this when the exposure is taken. The smaller the aperture chosen, the larger the difference will be.

I ran into this when I bought an open-box Sigma 180 Macro. Of course, I tested it wide open to see if there were any focus issues, and then stopped down to very small apertures because I'd be using them to get greater DOF for macros. Wide open everything was good, but the small aperture shots were massively overexposed, and I was bummed.

I did the slow shutter speed/look into the lens thing and saw that the aperture blades did indeed close down, but when I dismounted the lens and worked the aperture lever manually, the blades were slow to close, not instantaneous (snappy) like it's supposed to be. I finally noticed that the aperture lever was not centered in its slot, and was, in fact, bent towards the center of the lens. It was rubbing on part of the lens mount. I gently bent it back and centered it in the slot with a couple of jeweler's screwdrivers, and the blades snapped like they were supposed to. The exposure problems disappeared, and I had a brand new lens for about half the going price of the lens on the used market -- This was when the lens had just been discontinued, and the demand was very high.

If this had been a used lens, this would have been one of the first things I checked, but being new, I had assumed that it would function properly mechanically. Apparently, this was a return or a lens that had been tried at the store, and they didn't find what was causing the problem.

I've also had a lens where the blades were slow, but there was no apparent damage. Working the lever a number of times cured it, and when I turned the lens up over a sheet of paper, a small bug and some assorted bug parts fell out. Apparently it had gotten trapped when I last capped the back of the lens, and ended up in the mechanism. Who would've thought. . .

Another cause could be a weak or damaged return spring for the lever. If you have another lens, compare the spring tensions of the two.

For oil on the blades, dismount the lens and inspect the blades from both sides, shining a bright light into the lens. If you see any shiny or dark areas on any blades, this might be the problem, and you'd need a CLA (Clean Lubricate Adjust) servicing of the lens by a competent lens tech. If any of the blades do not close completely, then it could be a bent blade, but realize that most lenses don't stop down exactly symmetrically, so small variations wouldn't indicate a problem -- you're looking for a cause of slow actuation.

Just a shot in the dark. . .

Scott
05-10-2011, 12:49 PM   #12
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QuoteOriginally posted by snostorm Quote
Hi woodyneale,

Let me suggest another possibility -- sticky aperture blades. If the aperture blades are slow to close, this can cause the same problem.


Scott
I thought of that too but he has tried this with more than one lens. The DA70 and the DA35 f2.8. One thing that hasn't been determined though (stated) is does the camera have the original Focus Screen?

05-10-2011, 04:50 PM   #13
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QuoteOriginally posted by JeffJS Quote
I thought of that too but he has tried this with more than one lens. The DA70 and the DA35 f2.8. One thing that hasn't been determined though (stated) is does the camera have the original Focus Screen?

Hi Jeff,

While it's unlikely that two lenses could both have similar problems, it's not inconceivable. For example, if he had swapped lenses back and forth on a windy beach, they both could have gotten sand in the mechanism. If he stored them in the same place, they may both have been subjected to conditions that could promote oil on the blades. I learned early on as a motorcycle race tuner a very long time ago not to assume anything when troubleshooting a weird problem. That's why I took the time to describe my experience with the Sigma -- and the bug lens.

Strange things happen, and the more puzzling they get, the more you have to make sure you've covered all the bases.

Scott
05-10-2011, 05:06 PM - 1 Like   #14
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I think woody did a fairly good set of tests. -something is wrong,
maybe not the lens side, but the camera side of the stop down lever.
Like snostorm said , this has to be snappy and accurate

I add: p mode might not stop down so much, so works OK same as sluggish lens often works OK down to f8
05-10-2011, 05:21 PM   #15
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QuoteOriginally posted by snostorm Quote
Hi Jeff,

While it's unlikely that two lenses could both have similar problems, it's not inconceivable. For example, if he had swapped lenses back and forth on a windy beach, they both could have gotten sand in the mechanism. If he stored them in the same place, they may both have been subjected to conditions that could promote oil on the blades. I learned early on as a motorcycle race tuner a very long time ago not to assume anything when troubleshooting a weird problem. That's why I took the time to describe my experience with the Sigma -- and the bug lens.

Strange things happen, and the more puzzling they get, the more you have to make sure you've covered all the bases.

Scott
I'm not saying it's impossible or even inconceivable. I've definitely had 2 lenses in my bag at one time that had slow apertures for various reasons. Those were 30 year old lenses however and I have no idea of their history.

I can't get past the fact here though that P mode gives him satisfactory results (and I think that's what has thrown Him for a loop). My only camera right now is the K5 but if I go to P mode, take a picture, and then set the same settings in one of the other modes, I get the same exposure, or very close to it. Locking the exposure in the semi auto modes and stepping up or down from there, gives me the same exposure.

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