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04-19-2009, 06:50 AM   #1
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K20D and FA50 continuous-shoot exposure problem?

Hi, i recently purchased a Pentax FA50 F1.4 for my K20D, it's my first prime and first non-kit lens!! And i have been amazed at its low-light performance without flash compared to my kit lenses - well worth the money. However i seem to occasionally have a problem with exposure in continuous-shoot mode: in certain low-light scenes, my K20D always meters the first shot correct, but if i hold the trigger down then subsequent shots end up over-exposed. E.g. in TAv mode if the first shot was at 200ISO, then subsequent shots get evaluated to be between 200 and 640ISO. I have never seen this with my kit lenses, and i always do continuous shoots.

This is how i take a shot to reproduce the issue:
- Choose a scene in room lighting that has a dark background (e.g. dark blue curtain) but with a small white object in the middle (e.g. a white medicine bottle label sitting on top of a black stand) as a focus point, in room lighting.
- Set the camera to AF-C, with focus to occur only via AF button (not on trigger half-press), and set to HI continuous shooting mode. Also set metering mode to multi-segment. Bracketing and extended bracketing are off.
- Focus with AF button first, then release AF button.
- Press and hold trigger half-way and note exposure settings. Can be in any mode i think but in this case i'll use TAv, so note the ISO and get it to be around 200. Also note that light level is constant and ISO reading is constant.
- Press and hold trigger all the way down to take say 5 shots.
- The first shot is always exposed correct (e.g. 200ISO) but subsequent shots can be brighter than the first (e.g. at 400ISO), even though the camera hasn't moved and the light level hasn't changed.

The exposure issue doesn't occur with AE-lock of course, and it also doesn't seem to occur in spot-metering but does occur in centre-weighted metering. The scene must have a small bright object right in the middle of the frame compared to the rest of the frame for the issue to occur - that is the pattern i have found so far anyway. Has anyone seen anything like this? Could i have a faulty lens in some way? Could the aperture be so big that when the re-metering occurs the aperture is still in motion from the previous shot thus affecting metering? Multiple single shots are never a problem.

04-19-2009, 10:55 AM   #2
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It is possible that the lens has a slow aperture blade problem. This is not common, and also not unknown. It is a mechanical device, and the machinery may not be as fast as it should. The only solution would be to have the lens repaired.
04-19-2009, 11:29 AM   #3
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QuoteOriginally posted by Canada_Rockies Quote
It is possible that the lens has a slow aperture blade problem. This is not common, and also not unknown. It is a mechanical device, and the machinery may not be as fast as it should. The only solution would be to have the lens repaired.
I had a similar thought. Is the problem different with the lens stopped down as apposed to wide open?

DAZ

If you look at an out of focus highlight is it round?
04-19-2009, 05:08 PM   #4
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QuoteOriginally posted by DAZ Quote
I had a similar thought. Is the problem different with the lens stopped down as apposed to wide open?

DAZ

If you look at an out of focus highlight is it round?
Yes - if the lens is wide open the aperture blades do not move. The out of focus highlights will generally reflect the number of blades in the aperture - octagonal for 8 blades for example - but sometimes the blades are deliberately made with a curve so that the highlights will be out of focus circles.

04-22-2009, 06:18 AM   #5
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As Albert noted, the aperture blades could be the problem, though I doubt that. Take the lens and close the paerture manually down to F716 or f/22, The push the aperture lever and open the aperture fully with it. If you then let go the aperture lever, the aperture should close instantly. If that works, it is quite probable not the lens's aperture.

I suspect two other possible sources of the problem:
1. The K20 sometimes has erratic exposure when shooting series of images. I experience that myself from time to time, with one or two shots out of a series of 5 or so being overexposed (mostly, underexpsoure is rarer). But this has not the strong systematic, you observe

2. it could be a problem with your test target. You see, a small bright object in front of a big dark background would usually lead to overexposure, if not corrected. If the bright object has a certain size, it simply might be (only a possibility), that it's just big enough to unbalance the camera logic. The camera does not "know" how to correctly expose this scene and gets erratic.

3. if you handhold your camera while doing this experiment, even the tiny movements all of us make all the time, might be enough in such a demanding situation, to alter the exposure considerably. If of once your first shot is tightly centered on the bright target, you get the correct exposure (assuming center weighted metering), but if you just sway a bit up or down or sideways, the bright target would be slightly off center and the camera will take more of the dark background into account. The result would be overexposure.

However a few questions remain:
- can you repeat the experiment with the kit lens or some other lens? And do you get the same result. If yes, you can rule out the lens (which I personally would expect).
- is it the same with matrix metering? Is it the same with fixed ISO but in Av or Tv mode?

You see, that you can narrow down the source of the problem by eliminating all the different variables one after the other.

Ben
04-22-2009, 08:23 AM   #6
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Thanks for all your replies. It had given me ideas on running other tests which i have now completed, nearly 1000 photos later!

The main thing that was confusing me was that i could only see the issue with my new lens, and could not reproduce the issue with the kit lens that came with my K20D (DA 18-55 II). I also didn't initially notice the problem with my Sigma 70-300 lens (the kit lens that came with my very old MZ-6 ), but finally did see a similar issue with the 70-300 after further tests at different positions and distances from my test scenario - on TAv, the worst i could do is the first shot being 1000ISO and subsequent shots then being at 1250ISO. If i stop shooting and then take another shot it's always at 1000ISO again. It's the first indication that the issue might not be lens-specific, though the effect is nowhere near as strong.

QuoteOriginally posted by DAZ Quote
I had a similar thought. Is the problem different with the lens stopped down as apposed to wide open?

DAZ

If you look at an out of focus highlight is it round?
The problem seems to occur at all apertures, i tested at 1.4, 5.6 and 22. Out of focus highlights are round at F1.4, and octagonal if stopped down.

QuoteOriginally posted by Ben_Edict Quote
As Albert noted, the aperture blades could be the problem, though I doubt that. Take the lens and close the paerture manually down to F716 or f/22, The push the aperture lever and open the aperture fully with it. If you then let go the aperture lever, the aperture should close instantly. If that works, it is quite probable not the lens's aperture.

I suspect two other possible sources of the problem:
1. The K20 sometimes has erratic exposure when shooting series of images. I experience that myself from time to time, with one or two shots out of a series of 5 or so being overexposed (mostly, underexpsoure is rarer). But this has not the strong systematic, you observe

2. it could be a problem with your test target. You see, a small bright object in front of a big dark background would usually lead to overexposure, if not corrected. If the bright object has a certain size, it simply might be (only a possibility), that it's just big enough to unbalance the camera logic. The camera does not "know" how to correctly expose this scene and gets erratic.

3. if you handhold your camera while doing this experiment, even the tiny movements all of us make all the time, might be enough in such a demanding situation, to alter the exposure considerably. If of once your first shot is tightly centered on the bright target, you get the correct exposure (assuming center weighted metering), but if you just sway a bit up or down or sideways, the bright target would be slightly off center and the camera will take more of the dark background into account. The result would be overexposure.

However a few questions remain:
- can you repeat the experiment with the kit lens or some other lens? And do you get the same result. If yes, you can rule out the lens (which I personally would expect).
- is it the same with matrix metering? Is it the same with fixed ISO but in Av or Tv mode?

You see, that you can narrow down the source of the problem by eliminating all the different variables one after the other.

Ben
The aperture seems to close instantly if i release the aperture lever.

Re point 1, the effect is only this strong on this particular test subject, and is the reason i decided to shoot so many shots in the past week in different scenarios - i'm yet to see the issue with this lens in any other scenario when shooting more normal shots while out and about, which is reinforcing that it might just be something about this particular scene type.

Re point 3, i did hand-hold for the test, but checked that the successive images were static enough. It's a good point though - i will repeat one of the tests with a tripod to rule out hand-shake.

With my FA50, i've now tested it in Av, TAv, Tv, and P, and the issue occurs in all those modes. In M, if i shoot a single shot with the exposure level correct, i sometimes see that immediately after i take the shot the exposure meter changes to 1EV underexposed, before quickly returning to normal exposure level.

The issue occurs with matrix metering and centre-weighted metering. The issue doesn't occur in spot metering.
04-22-2009, 08:57 AM   #7
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QuoteOriginally posted by defiant Quote
The aperture seems to close instantly if i release the aperture lever.

Re point 1, the effect is only this strong on this particular test subject, and is the reason i decided to shoot so many shots in the past week in different scenarios - i'm yet to see the issue with this lens in any other scenario when shooting more normal shots while out and about, which is reinforcing that it might just be something about this particular scene type.

Re point 3, i did hand-hold for the test, but checked that the successive images were static enough. It's a good point though - i will repeat one of the tests with a tripod to rule out hand-shake.

With my FA50, i've now tested it in Av, TAv, Tv, and P, and the issue occurs in all those modes. In M, if i shoot a single shot with the exposure level correct, i sometimes see that immediately after i take the shot the exposure meter changes to 1EV underexposed, before quickly returning to normal exposure level.

The issue occurs with matrix metering and centre-weighted metering. The issue doesn't occur in spot metering.
Thank you for your insights. It is a very inzteresting phenomenom. I would think, after reading your additional infos on the tests you did, that it is mainly a problem related to the test target, with the bright center target being small. Whether it is a bit of handshake or whether ithe camera meter is just at a limit to decided between different settings is hard to say, both would be possible, I guess.

Ben
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