Exposure issues with 5D MK IV and Tamron 150-600

[SSipple]

I seem to be experiencing e posture issues with my 5D MK IV when using my Tamron 150-600.    I did send the Tamron for the latest firmware update.   I took the camera and lens on a trip and was shooting with auto ISO and shutter priority.   All of a sudden, the exposures were dark.    I switched to my Canon 24-105, and then everything was dark.  Called Canon and they had me reset the camera settings.  Appears to have corrected the 24-105 issues.    For the most part, the 150-600 is working, but at the 150 range appears to randomly underexposed.   I looked and the auto ISO is being set to a very low ISO.    I was using evaluative metering.

called Canon and they basically told me I was using an inferior lens and that was the problem.   This has been working fine until now.  Not sure whether to send the camera to Canon, he lens to Tamron.   I used partial spot metering and the exposures seemed better.

[SSipple]

Yes, I am well versed with auto ISO. Both are set at 25600

[Waddizzle]

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@ebiggs1 wrote:

"...  but I know that wasn't the issue."

It would not be if you reset the camera like the Canon rep said.  That is the first thing to do when something isn't as it was.

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Really?  If you reset the camera to factory defaults, doesn't the AE button lock exposure settings?

[SSipple]

I did reset it and in mist circumstances it is working.   Prior to the reset, everything was being underexposed 

[SSipple]

Not sure of your question here.   I was not using AE lock.       Some of the photos are fine, others are not

[SSipple]

Here are 2 images shot seconds apart using auto ISO.    As you can see, one was shot at iso 20000 and the other at 8000.   Both were shot at 1/400 sec and f5.6 

[TTMartin]

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@SSipple wrote:

Here are 2 images shot seconds apart using auto ISO.    As you can see, one was shot at iso 20000 and the other at 8000.   Both were shot at 1/400 sec and f5.6 

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My guess is that Canon is correct it has to do with the lens's programing. Tamron reverse engineers how the EOS lenses are supposed to operate. They also have the lens 'lie' to the camera about its aperture so that an f/6.3 lens like the 150-600 can work with a camera that can only auto focus with a lens with a maximum aperture of f/5.6. Looks like somewhere there is a flaw in the Tamron's programming. 

FWIW, the Canon EF 100-400 L IS II is a superb lens. Even with a 1.4X TC III it is still sharper than the Tamron. 

[ebiggs1]

"Here are 2 images shot seconds apart using auto ISO."

The obvious is to turn off auto ISO and try it.  I bet it works without the exposure issues you are experiencing.  Neither of the two Tamron 150-600mm's I have do that. As a matter of fact none of the Tamron lenses I have do that either.  But I don't use, I don't even like, auto ISO.  And, like I said I have not tried it on a 5D Mk IV.

What do you have to lose?  Just give it a try.  BTW, are you perhaps using spot metering?

[SSipple]

Thing is, I have been using it since February and have not experienced this issue until the camera went out of whack and started underexposing everything. I used auto ISO because I usually shoot concerts where the lighting changes will and you don't have time to be adjusting. Sure, I can get around it, but shouldn't have to. I like to shoot manual and auto ISO

[ebiggs1]

"...until the camera went out of whack and started underexposing everything."

Whoa!  I missed this.  Are you saying it happens with more than just the big Tammy on your Mk IV?  Perhaps auto ISO is not working as it should.  Do try a few shots of known good conditions with out auto ISO turn on.  Fix it to a middle of the road 400 or so.  If it works, it works, and something else is a foot.  Also try your other lenses?

One thing about Canon support is, you get what you get.  I mean all techs are not created equal.  I have received great support, up and beyond with some, even most of them.  But, there is always a 'but' isn't there? But once in a while you get a dud.  In that case call back and you will likely get a different tech.

[TTMartin]

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@SSipple wrote:
Thing is, I have been using it since February and have not experienced this issue until the camera went out of whack and started underexposing everything. I used auto ISO because I usually shoot concerts where the lighting changes will and you don't have time to be adjusting. Sure, I can get around it, but shouldn't have to. I like to shoot manual and auto ISO

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Since a simple camera reset fixed the underexposure on everything, but, the Tamron lens, all things point to the Tamron lens as the issue.

If you are having issues with the Tamron lens you should be contacting TAMRON not Canon.