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02-26-2010, 10:00 AM   #1
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Differences in exposure in Av with 'A' setting?

I've noticed that I didn't get consistent exposure in Av with some of my lenses (A lenses used on the A setting, and the Pentax F 35-105mm). Usually, what happens is a leap in exposure from max. aperture (where pics look darker) to the next aperture (where pics look brighter) followed by another similar leap after f8 or f11 (depending on the lens). It's not a matter of stuck aperture, as I initially thought, since in M mode exposure is very consistent with all lenses.

Are you familiar with this phenomenon? Could it be a camera defect or does it have to do with the number of contacts that a given lens has/ allows?

Thank you.

02-26-2010, 10:10 AM   #2
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aperture changes along with the focal length. It should more or less be the same if your using 2 same length lenses, but otherwise it's different.

Here's one link that more or less explains it. There are plenty more if you use Google.
02-26-2010, 10:26 AM   #3
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Thank you for the link, but I can't see its relevance to my problem. I wasn't talking about inconsistency among different lenses, but among different apertures of the same lens (while keeping the focal length constant). This happens with almost all my 'A' lenses...
02-26-2010, 10:43 AM   #4
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Sorry for my lack of understanding your first post. In my opinion that shouldn't happen
I don't know what to say then. All my cameras pretty much meter evenly, with the same lenses.

02-26-2010, 10:46 AM   #5
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Have you been using spot or area meetering?
02-26-2010, 10:47 AM   #6
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Can you explain in more detail what you are seeing? Are you saying you are taking several pictures in a row of the same scene, but the exposure chanes between them? Sounds like what you'd expect if you have exposure bracketing turned on. Or it you have spot metering set, it would be almost impossible to keep exposure constant unless you were using a tripod.
02-26-2010, 11:01 AM   #7
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I take a photo at 2.8, then one (of the same scene) at 4, then one at 5.6, etc.
The pic taken at 2.8 looks darker then those taken at 4 and 5.6. The change is significant (I can keep exposure even by compensating with -0.7)--even if I keep the camera on a table or tripod, the same thing happens.

I use spot metering in Av, because I know matrix metering doesn't work with A lenses. I don't think it has anything to do with the metering, since all lenses work perfectly in the fully manual (M) mode.
02-26-2010, 12:03 PM   #8
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I have done this with a few lenses: set the camera on a tripod with the lens pointed at an evenly lit white wall. Take some test shots to establish an exposure at ISO 200, let's say f8 at 1/250, with a histogram peak 3/4ths to the right side. Then go up a stop on aperture, down a stop on shutter speed and take another shot. I ignore metering in this test. That's how I'm reading your situation.

Most lenses will produce a set of very similar images. You may notice vignetting and lower contrast at wide apertures, but exposures are close enough. I only have tested two lenses that don't pass, both Sigma primes. The error is as much as 2 stops difference, and very large or very small apertures have the largest error. I have two other Sigma primes that pass just fine. I have looked for mechanical problems without success. I haven't tested every lens I have.

The older KA-mount system did not adjust well for variable aperture zooms. You can tell these lenses because although the lens is f5.6 at the long end of the zoom, the camera will allow you to set a wider aperture. On autofocus lenses like your Pentax-F, the maximum aperture is communicated differently and the variable aperture zooms work better. That's one possible issue, but it wouldn't explain problems with primes.

02-26-2010, 12:18 PM   #9
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Just a silly thought. You don't have Auto ISO turned on do you. I could see the ISO changing between those to apertures, and possibly causing a noticeable difference.
02-26-2010, 12:25 PM   #10
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QuoteOriginally posted by causey Quote
I take a photo at 2.8, then one (of the same scene) at 4, then one at 5.6, etc.
The pic taken at 2.8 looks darker then those taken at 4 and 5.6. The change is significant (I can keep exposure even by compensating with -0.7)--even if I keep the camera on a table or tripod, the same thing happens.

I use spot metering in Av, because I know matrix metering doesn't work with A lenses. I don't think it has anything to do with the metering, since all lenses work perfectly in the fully manual (M) mode.
Matrix does work with A-lenses, just not with older M42/K/M-lenses.

Spot metering is a helpful specialist mode, but can lead easily to severely off readings.

Ben
02-26-2010, 12:26 PM   #11
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I've spent the last two hours testing successive exposures of different lenses at different apertures: the only conclusion that I can draw is that the variations have to do with the construction of the lenses. My kit lens is amazingly consistent. I can't notice the slightest variation from one aperture to another. The biggest variation appears with the Sigma mini wide II, from 2.8 to 4. The other lenses, including the Pentax F 35-105, show much milder variations--nothing really bothersome. At first I was afraid there was oil on the blades of my Sigma, but they seem to move quickly.

What amazes me, however, is that in fully manual mode, all of them are very consistent. I might be wrong, but here is my guess. In A mode, the reading is done according to the mathematical formula: say, at 2.8 > 30 sec. shutter speed, at 4 > 60 sec., etc. But maybe not all lenses are made equal, and due to their optical construction some might need a bit more or less light at various apertures. In M mode, the camera 'reads' how much light a lens actually needs (so to speak) at each aperture.

QuoteOriginally posted by Just1MoreDave Quote
I have done this with a few lenses: set the camera on a tripod with the lens pointed at an evenly lit white wall. Take some test shots to establish an exposure at ISO 200, let's say f8 at 1/250, with a histogram peak 3/4ths to the right side. Then go up a stop on aperture, down a stop on shutter speed and take another shot. I ignore metering in this test. That's how I'm reading your situation.

Most lenses will produce a set of very similar images. You may notice vignetting and lower contrast at wide apertures, but exposures are close enough. I only have tested two lenses that don't pass, both Sigma primes. The error is as much as 2 stops difference, and very large or very small apertures have the largest error. I have two other Sigma primes that pass just fine. I have looked for mechanical problems without success. I haven't tested every lens I have.

The older KA-mount system did not adjust well for variable aperture zooms. You can tell these lenses because although the lens is f5.6 at the long end of the zoom, the camera will allow you to set a wider aperture. On autofocus lenses like your Pentax-F, the maximum aperture is communicated differently and the variable aperture zooms work better. That's one possible issue, but it wouldn't explain problems with primes.

Last edited by causey; 02-26-2010 at 12:36 PM.
02-26-2010, 12:27 PM   #12
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QuoteOriginally posted by little laker Quote
Just a silly thought. You don't have Auto ISO turned on do you. I could see the ISO changing between those to apertures, and possibly causing a noticeable difference.
No, no auto ISO
02-26-2010, 12:35 PM   #13
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QuoteOriginally posted by Ben_Edict Quote
Matrix does work with A-lenses, just not with older M42/K/M-lenses.

Spot metering is a helpful specialist mode, but can lead easily to severely off readings.

Ben

My K200d doesn't seem to work with matrix with the Sigma mini wide II. I select the multi-segment mode, but only the spot metering appears to be active on the top screen.

UPDATE: I was wrong. Multi-segment metering does work with A lenses...

Last edited by causey; 02-26-2010 at 01:07 PM.
02-26-2010, 02:11 PM   #14
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The exposure mode (M, P, Av, etc.) should not make any difference with your A contact lenses when the aperture ring is on the "A" setting. If you are seeing inconsistent exposure there may be a problem with the aperture mechanism. The first thing you need to do is to remove the meter from the equation. To test:
  1. Camera in M mode with ISO at set value (non-auto)
  2. Pointed at blank white wall with consistent lighting
  3. With lens wide open, determine base exposure using green button
  4. Make an exposure
  5. Close the aperture one stop and adjust the shutter speed one stop slower
  6. Make an exposure
  7. Repeat steps 5 and 6 until you reach the narrowest aperture. Do not re-meter the scene between exposures.
The exposure of each step in the series should be equivalent. There is a small chance that the first image will be somewhat darker than the rest. That is because many lens makers (including Pentax) exaggerate their lenses maximum apertures. Note that I said the exposures should be equivalent, not identical. That is because the aperture linkage is mechanical and there is manufacturing variance in both accuracy and precision.

Steve
02-26-2010, 02:19 PM   #15
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In M there is no problem whatsoever... And the exposure variations don't happen with only one lens, but with five. Perhaps I'm expecting too much consistency--and it's true that the first images (and max. apertures) seem to be the most problematic ones. Anyway, I'll try what you suggested tomorrow. Thank you and all the participants to the thread.
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