6D Random underexposed image

[Z0mbieRecruiter]

Hello. I hope you can help.  I own a Canon 6D and twice in the past week I was taking photos where a random image would come up underexposed.  I was shooting in manual mode, each image was identical other than a slightly different angle and shot only a few seconds from each other.  Looking back at the settings for each image, the settings are exactly the same (ISO 1600 F/5.6 1/60 sec).  While taking the photo, the image in my view finder also darkened.  Both times this happened using the same lens, so I suppose it is possibly a lens issue?

Thanks in advance!

[kvbarkley]

If there is no difference in the EXIF data between good/bad shots (i.e., the ISO suddenly goes from 800 to 100), then it sounds like an aperture problem with the lens, especially if the viewfinder goes dark.

[Z0mbieRecruiter]

Thanks. This does make sense to me. What could cause this issue? Could the lens just need a good cleaning? I was using a Sigma 2.8 28-70MM lens.

[kvbarkley]

I don't know. It seems weird that it would stick at a *smaller* aperture. I could understand overexposing by the aperture not switching from wide-open-focus-exposure-mode to picture-taking-mode, but going *past* the set aperture to a smaller one is unexpected.

You might see if Sigma has a firmware update.

[BurnUnit]

Depending on your grip on the camera, is it possible that you might touching the DOF preview button on the camera body? I tend to forget that I even have that feature on my 60D.

Maybe cleaning the electrical contacts betwen the lens and body would help. Do you only have the problem with the one lens you mentioned? You're not using a flash for the shots in question, are you?

[Z0mbieRecruiter]

No, no flash being used. I could look into the contacts. I've been using this lens for the past 2-3 years and the camera for 1.5 years without problems until this past week. Thanks for the suggestions

[diverhank · Accepted solution]

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@Z0mbieRecruiter wrote:
Thanks. This does make sense to me. What could cause this issue? Could the lens just need a good cleaning? I was using a Sigma 2.8 28-70MM lens.

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I don't think cleaning will help.  It's probably some incompatibility between the 6D and this Sigma lens.  I'd do the following:

1. update the firmware for the lens

2. update the firmware for the camera (if not done already)

3. Turn off the camera's corrections like vignetting or peripheral illumination correction, etc.

[Z0mbieRecruiter]

I have not updated the firmware for some time, so I will look into this as solution. Thank you for your help!