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.

[Waddizzle]

When you're shooting with shutter priority and ISO Auto, then you're letting the camera set two legs of the exposure triangle, something which I personally try to avoid because I get the occasional outlier exposure.  

You can configure the camera to automatically control any one, any two, or all three legs of the exposure triangle.  I typically shoot in manual with Auto ISO under widely changing light conditions.  You can set an ISO upper limit in the menus.  I rarely use anything but the default setting of Evaluative Metering when shooting handheld.

I have also questioned whether or not the WB setting can impact the exposure metering.  Even though I shoot RAW, I have switched to shooting with a specific setting, instead of letting WB drift wherever it wants to go.  Again, the occasional bad exposure has become even more rare with this change.

[TTMartin]

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@SSipple wrote:

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.

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What was the maximum ISO you had set for 'Auto ISO'? You may have 'a very low ISO' set as the maximum. 

Also, make sure you don't have any exposure compensation dialed in. 

Accidentally setting exposure compensation is one of the leading causes of 'exposure problems'. 

[SSipple]

Max ISO was set at 25600.   There was no exposure comp dialed in.    

[TTMartin]

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@SSipple wrote:

Max ISO was set at 25600.   There was no exposure comp dialed in.    

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Could you post an example of the under exposure with the full EXIF data intact. 

[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?

[TTMartin]

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@SSipple wrote:

Max ISO was set at 25600.   There was no exposure comp dialed in.    

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Also just to be clear, Max Auto ISO is set to 25600? Not just Max ISO was set at 25600, those are two different settings.

[Waddizzle]

Tom's post reminds of another suspected issue that I had with the occasional over/under exposure.  I suspect that my thumb may have been hitting the exposure lock button, which would lock the automatically adjusted exposure settings. 

This becomes an issue when you point the camera in a different direction, such as recomposing.  Make sure your thumb is not acccidentally hitting AE lock.  I began paying attention to this possibiity, and the mystery over/under exposures have all but vanished.  I made few changes in how I did things all at the same time, so I cannot be certain which change may have cured the bad exposure problem.