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08-27-2016, 05:35 PM   #1
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Pentax 16-50 2.8 DA Shutter speed issue

Recently purchased this lens and was comparing it to my sigma 18-55 2.8-4.5 The pentax at 16mm and 2.8 had a 20 sec shutter speed compared to the sigma at 18mm at 2.8 with a 10 sec shutter speed. I have no idea why this is the case, but it just about all the test shots I've taken, I've noticed the the pentax is shooting roughly half the shutter speed of the sigma. I'm using a pentax k-50.

08-27-2016, 06:14 PM   #2
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There is a difference in how the camera meters (especially in matrix) when you shoot at 16mm vs 18mm; I don't expect it to be the same, and IMO not a issue at all.
08-27-2016, 06:17 PM   #3
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Welcome to the Pentax Forums!

A full stop difference in metering between lenses of different design and the same maximum aperture is unusual, but not unheard of. That being said, it is possible that your new lens is not fully open when mounted. As an initial screen, compare the metering using center-weighted mode and an evenly-lit blank wall as a subject. Is there still a 1 stop difference in meter reading? If so, with the DA 16-50 off the camera:
  • Carefully inspect the mount and aperture actuator lever for evidence of damage
  • Using your finger, slowly move the aperture actuator from the fully stopped down position (resting state when off-camera) to fully open. The movement should be smooth with no roughness or catching. Confirm (looking from the front) that that aperture is fully open when the lever is full depressed.
  • Also using your finger, flick the actuator lever and note the blade action. The action should be snappy through its full range with no hesitation or slowness.
  • With the lens mounted on the camera and looking from the front, confirm that the aperture is fully open.
As a final test, put the camera in Av mode with the DA 16-50 mounted and take two photos of an evenly lit blank wall at both f/2.8 and f/4 (one stop difference in taking aperture). The brightness for both photos should be the same (similar, if not identical histograms). If the f/2.8 version is darker, there is a problem.


Steve

Last edited by stevebrot; 08-27-2016 at 06:22 PM.
08-27-2016, 09:31 PM   #4
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Set the camera on manual and use the same exposure o.k. the two lenses, does the result look the same or is the 16-50 darker? Do you have any lens corrections turned on? I wonder if that might impact exposure especially peripheral lighting corrections.

08-27-2016, 10:06 PM   #5
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Thanks so much for your suggestions! The issue was with the metering. I had it set to multi segment, and the pentax lens would take a picture with almost no vignetting, while the sigma had much more. Switched the setting to center weighted, and the Pentax lenses was faster than the Sigma.
08-27-2016, 10:08 PM   #6
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QuoteOriginally posted by UncleVanya Quote
Set the camera on manual and use the same exposure o.k. the two lenses, does the result look the same or is the 16-50 darker? Do you have any lens corrections turned on? I wonder if that might impact exposure especially peripheral lighting corrections.
Tried that. Turns out the pentax lens is much better at fully exposing with the multi segment metering with no vignetting, but needs a lower shutter speed to do so. At center weighted vignetting and shutter speed are comparable.
08-27-2016, 10:20 PM   #7
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QuoteOriginally posted by typi314 Quote
Thanks so much for your suggestions! The issue was with the metering. I had it set to multi segment, and the pentax lens would take a picture with almost no vignetting, while the sigma had much more. Switched the setting to center weighted, and the Pentax lenses was faster than the Sigma.
Cool! I forgot that image circle might be an issue with many lenses at the wide end.


Steve
08-28-2016, 05:03 AM   #8
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QuoteOriginally posted by typi314 Quote
Thanks so much for your suggestions! The issue was with the metering. I had it set to multi segment, and the pentax lens would take a picture with almost no vignetting, while the sigma had much more. Switched the setting to center weighted, and the Pentax lenses was faster than the Sigma.
There is another small point too in multi-segment metering or matrix metering (in Pentax), you can turn on/off the AE-lock with AF focus and get slightly different result. I usually keep AE unchanged regardless of where AF point is to avoid any surprises.

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