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01-22-2018, 08:51 PM   #1
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Sigma 24mm f2.8 Superwide II PKA overexposure

I have the manual focus Sigma 24mm f2.8 on the A mount, and on both my K200D and K100 Super it overexposes quite a bit. The exact exposure seems to be on K100D Super
2.8 0.5 stop Underexposed
4.0 0.5 stop Overexposed
5.6 1.5 stop Overexposed

on the K200D it's about 0.5 overexposed vs the K100D. Is this normal? Need to try on my other Pentaxes, but haven't had the time yet.

It meters ok in stop down mode, but the change in fstop doesn't correlate exactly to what I expect the shutter speed to be.
e.g. f2.8 @ 1/60 should be 1/30 f4.0, but sometimes the camera meters at something faster than 1/30.

I tried to pull the aperture level and it snaps back instantly. Could there be something else wrong with the arm?

Thanks!

01-22-2018, 09:18 PM   #2
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QuoteOriginally posted by keos Quote
I have the manual focus Sigma 24mm f2.8 on the A mount, and on both my K200D and K100 Super it overexposes quite a bit. The exact exposure seems to be on K100D Super
2.8 0.5 stop Underexposed
4.0 0.5 stop Overexposed
5.6 1.5 stop Overexposed

on the K200D it's about 0.5 overexposed vs the K100D. Is this normal? Need to try on my other Pentaxes, but haven't had the time yet.

It meters ok in stop down mode, but the change in fstop doesn't correlate exactly to what I expect the shutter speed to be.
e.g. f2.8 @ 1/60 should be 1/30 f4.0, but sometimes the camera meters at something faster than 1/30.

I tried to pull the aperture level and it snaps back instantly. Could there be something else wrong with the arm?

Thanks!
Welcome to the Pentax Forums!

In M mode, how does manual aperture control using the aperture ring compare to the same setting using body aperture control? Are the exposures comparable?


Steve
01-22-2018, 09:29 PM   #3
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QuoteOriginally posted by stevebrot Quote
Welcome to the Pentax Forums!

In M mode, how does manual aperture control using the aperture ring compare to the same setting using body aperture control? Are the exposures comparable?


Steve
Step down metering in M mode results in an exposure +/- half a stop of what I would expect; the shutter speeds decided by step down metering and the body using A mode are pretty much in line, changes by half a stop here and there, but A mode causes the overexposure when stopped down. I should have added after F5.6 the overexposure stays at 1.5 stops.

---------- Post added 01-22-18 at 09:32 PM ----------

QuoteOriginally posted by keos Quote
Step down metering in M mode results in an exposure +/- half a stop of what I would expect; the shutter speeds decided by step down metering and the body using A mode are pretty much in line, changes by half a stop here and there, but A mode causes the overexposure when stopped down. I should have added after F5.6 the overexposure stays at 1.5 stops.
Ugh, I meant stop down metering. Also to add, stop down metering is close, but if it deviates, it is usually using a faster shutter than the calculated amount.
01-22-2018, 09:43 PM   #4
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QuoteOriginally posted by keos Quote
Step down metering in M mode results in an exposure +/- half a stop of what I would expect; the shutter speeds decided by step down metering and the body using A mode are pretty much in line, changes by half a stop here and there, but A mode causes the overexposure when stopped down. I should have added after F5.6 the overexposure stays at 1.5 stops.
Not a comparison of the metering. I was asking if body control of the aperture has the same affect as using the aperture ring. Steps:
  • M mode, fixed ISO
  • Aperture ring on the "A" position
  • Point the camera at a well-lit blank wall and adjust the exposure using the controls on the camera body. Write down the shutter speed and aperture.
  • Make an exposure
  • Using the aperture ring, set it to the same value you wrote down above
  • Make another exposure
Compare the two exposures. They should be the same brightness. If not, there is a problem with the aperture control by the body. This could be caused by a fault in the body or a fault in the lens actuator mechanism.


Steve

01-22-2018, 09:50 PM   #5
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QuoteOriginally posted by stevebrot Quote
Not a comparison of the metering. I was asking if body control of the aperture has the same affect as using the aperture ring. Steps:
  • M mode, fixed ISO
  • Aperture ring on the "A" position
  • Point the camera at a well-lit blank wall and adjust the exposure using the controls on the camera body. Write down the shutter speed and aperture.
  • Make an exposure
  • Using the aperture ring, set it to the same value you wrote down above
  • Make another exposure
Compare the two exposures. They should be the same brightness. If not, there is a problem with the aperture control by the body. This could be caused by a fault in the body or a fault in the lens actuator mechanism.


Steve
Sorry, misunderstood you.
Yes, if for example Av meters at F8 0.5s and I manually set the ring to F8 and shutter to 0.5s there is about a 1.5 stop difference between the two.
01-22-2018, 09:50 PM   #6
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QuoteOriginally posted by keos Quote
Ugh, I meant stop down metering.
I was hoping to avoid open-aperture vs. stop-down metering for now. There will likely be inconsistent stop-down metering with your cameras as with the K10D and K20D that is not related to problems with the lens.


Steve
01-22-2018, 10:12 PM   #7
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Mine does that too. I just live with it as otherwise it's an excellent lens.

01-22-2018, 10:32 PM   #8
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If I remember right, I had the same problem with another Sigma of the same vintage. I could never figure it out. I took it apart several times to look at the aperture arm mechanism. I checked the contacts inside and out. Eventually I sold it with that problem. My Superwide works fine. Sorry I can't be more help. If you can think of any tests I can run on mine, I'll give it a shot.
01-22-2018, 10:58 PM   #9
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I had bought one once that arrived with this issue, I contacted Gentec (Sigma's service in Canada) and due to the lack of any chance for getting new parts I sent it back to the person I bought it from. I think mine was either exposing correctly at F2.8 and horribly over exposing as you stop down or it was the other way around, horrible over exposure at F2.8 and better as you stop down... it was years ago so I just don't recall the exact issue.
01-27-2018, 10:11 AM   #10
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looks like I have found the problem. Using the optical preview, the aperture doesn't actually stop down until around f5.6; meaning it's about 2 stops before the aperture on the Av control actually does anything. Would explain why the overexposure maxes out after f5.6. Is this a loose aperture arm?
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