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09-10-2010, 04:06 PM   #1
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Pentax K7 metering mystery

Hi - I lurk here a lot - just don't post much.
Has anyone else experienced random exposures in manual mode?

On Labor Day I took my 1st waterfall pics at the Tahquamenon Falls in Michigan.
Al were taken with a K7, with the 18-55WR kit lens, on a tripod, in manual mode, with a 2-sec timer.
I took several hundred pictures, and the correct exposure was hit or miss to say the least.
I do not have a ND filter, so after 1st few tries, I set the exposure compensation to
-5.0 to see if that would bring the exposure in range. I'm not sure if that had any effect on the exposure - maybe someone can tell if it makes a difference.

I will try to upload 5 pictures to demonstrate the random exposures I got - same ISO, same aperture, different shutter speeds. Sometimes it would be overexposed, I would change the shutter speed, take a different pictures, and then come back to the same previous shutter speed, and get a totally different exposure. (Same exposure compensation by the way).

These numbers are taken right from Lightroom and there has been no PP done except conversion from RAW to JPEG.

1st picture - 1/6 sec @ f29
2nd - 1/8 sec @ f29
3rd - 1/8 sec @ f29
4th - 1/5 sec @ f29
5th - 1/2 sec @ f29

Any thoughts? -
Is the metering really in effect if it's in manual mode?
Could the exposure compensation I had set be screwing up the metering?
One more question if you have time - how do you upload photos with comments in between them?

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09-10-2010, 04:31 PM   #2
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Would it be that you have left bracketing on by mistake?
09-10-2010, 04:54 PM   #3
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Two things:

You could have "EV compensation" turned on in custom settings, which could screw you up.

Where were you metering in this scene? Were you using spot metering, center, or weighted? Spot metering can move your exposure all over the place, especially if you forget it is on and don't spot . I could see the contrast between the waterfall and the dark rocks throwing your exposure all over the place.
09-10-2010, 05:56 PM   #4
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Exit data from PhotoME shows Multi-segment metering mode - I don't believe I was using spot metering at all.
Auto EV compensation was set to off

Thanks for responding

09-10-2010, 06:09 PM   #5
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Also, just took some test shots in the kitchen -

Exposure compensation seems to have NO effect at all in manual mode -

Also tested spot metering and standard - no change in exposure in manual mode - at least indoors.

This wasn't the only series of photos this happened with - I have other examples from the same day where the same settings would give different exposure results.

Could it have anything to do with the lens not responding properly to the aperture settings of the camera? Does that ever happen?
09-10-2010, 06:48 PM   #6
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QuoteOriginally posted by dclauria Quote
Also, just took some test shots in the kitchen -

Exposure compensation seems to have NO effect at all in manual mode -

Also tested spot metering and standard - no change in exposure in manual mode - at least indoors.
Exposure compensation and metering mode don't affect the exposure in manual mode. That's why it is called manual mode -- you're manually controlling the exposure variables (shutter speed, aperture, and ISO sensitivity are all manually set).

You *can* still change the exposure compensation and metering mode in Manual exposure, but they don't affect the exposure level -- rather, they affect the level of the EV Bar graph. In other words, if you set the exposure compensation to -2EV and the metering mode to spot, that graph will be centered when the camera believes your manually selected shutter speed, aperture, and ISO sensitivity will result in a 2EV underexposure of the area under the spot metering circle. It's a guide to the photographer, that's all.

Just a guess, as I don't shoot in Manual mode very often myself, and haven't time to check your EXIF myself. Have you got Highlight or Shadow correction enabled, perhaps?
09-10-2010, 06:55 PM   #7
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Hi dclauria,

I'd check the lens. Random overexposure at small apertures can be caused by something sticky in the mechanical side of the aperture actuation.

The mechanical aperture lever at the back of the lens can be vulnerable to a number of things, dirt and fibers if stored or carried uncapped, and bending are two that come to mind as easy possibilities. If anything interferes with the lever moving quickly and easily, it can cause the aperture blades to not close down all the way, which can cause overexposure. The aperture lever must move very quickly to close down all the way to f29. If there's a sticky point past, say f16, then this fault wouldn't show up until you tried some shots past that point. If it was caused by some dirt or fibers, it's possible that this was dislodged sometime during the day, and the mechanism work correctly after that.

I'd also check the appearance of the aperture blades when stopped down. I've seen pictures of aperture blades that were not assembled correctly, and oil on the blades is a common problem with older lenses, but I imagine this can also happen with new lenses as we can't control the conditions under which they are stored before we own them. . .

I can relate a recent experience. I bought an open-box Sigma EX 180/3.5 APO DG Macro from Adorama at about the time the lens was discontinued. I assumed that the lens had been sold, or tested at the store, and was returned. This was a low risk purchase since Adorama has a very good return policy, and there was always the Sigma warranty to back everything up if there was a serious problem with the lens. . .

The lens had been taken out of its bag, but the hood had not, and everything looked new. My first few shots were alarmingly overexposed, and there was nothing that I could do to compensate for this. I was very disappointed, and started to examine the lens. I found the aperture lever very hard to move compared to my other lenses, and at closer examination, I saw that the lever was bent inward and was rubbing against the mount. I took a small screwdriver and gently bent the lever back to straight. I remounted the lens to the camera, took a few shots in varying lighting, and all was better in the world. . . I was spared the hassle of returning the lens, and was the happy new owner of a mint lens at 2/3 the market price. . .

Scott
09-10-2010, 08:07 PM   #8
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I also think it's the lens and not the camera. There is only a 2-stop difference in your exposure settings, but way more in actual exposure.

Also, I always thought the Pentax concept of "EV compensation" in manual mode (a contradiction of terms) by adjusting the meter "zero point" was just plain silly and awkward.

Uploading photos...(as attachments)..
Once you upload the attachments into the list, click the paperclip icon above the text box and insert all the images (or one at a time). Simply put an extra blank line between them and add text there.

09-10-2010, 08:39 PM   #9
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Thanks!

I bought the lens new with the K-7 in January, and have no problems most of the time.
These are probably the first shots I've taken with a high aperture setting.

I will look the lens over tomorrow - I too tend to think it's a problem with the lens at high aperture settings. I was happy to get some good shots - just took a lot more trial and error than I thought necessary.
09-10-2010, 09:15 PM   #10
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QuoteOriginally posted by dclauria Quote
Thanks!

I bought the lens new with the K-7 in January, and have no problems most of the time.
These are probably the first shots I've taken with a high aperture setting.

I will look the lens over tomorrow - I too tend to think it's a problem with the lens at high aperture settings. I was happy to get some good shots - just took a lot more trial and error than I thought necessary.

Of course you will still want to check out the lens, but if you want to do a lot of waterfall shots, you might invest in a neutral density (ND) filter so you can use a bit larger aperture to go with the long shutter speeds in daylight. If you use the really small apertures (say, f16-32) you can get the dreaded "diffraction" reduction of image quality. I doubt I would notice the difference, but peepers might.
09-10-2010, 10:19 PM   #11
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QuoteOriginally posted by SpecialK Quote
Also, I always thought the Pentax concept of "EV compensation" in manual mode (a contradiction of terms) by adjusting the meter "zero point" was just plain silly and awkward.
It's not just a Pentax concept. You'll find the exact same thing on Nikon cameras -- for example, check page 128 of the Nikon D3x manual:

http://www.nikonusa.com/pdf/manuals/noprint/D3X_noprint.pdf

Personally, I find it extremely logical, and potentially useful in some situations.
09-10-2010, 11:21 PM   #12
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QuoteOriginally posted by knoxploration Quote
Exposure compensation and metering mode don't affect the exposure in manual mode. That's why it is called manual mode -- you're manually controlling the exposure variables (shutter speed, aperture, and ISO sensitivity are all manually set).
Except when you use the green button, where Ev Comp will actually affect the exposure (along with metering mode and program line)...
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