EOS 6D Mark II Conflicts between viewfinder focusing and Live View focusing

[Skip70]

Canon 6D Mark II
I am using the Tamron Tap-In console to do micro-focusing on what is now my all stable of Tamron lenses. I am finding a very obvious difference between focus points using the viewfinder and reference focus point using touch screen Live View. 

I will try to attach the photos to this post, but the first time using the regular viewfinder focus (spot focus and metering)the focus point) was front focus. I adjusted using the console forward, just as you would with there Canon micro-focus system. However, when I did the same shot zooming in on LiveView, the focus point was clearly back focus, reflecting the influence of my adjustments. 

The simplest way say this is that the camera is finding a different focus point on the viewfinder method than it is finding using LiveView. While there is always adjustment we can make on a lens, I'm concerned that there is an issue with the camera. 

Ideas?

[jrhoffman75]

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@Skip70 wrote:

Canon 6D Mark II
I am using the Tamron Tap-In console to do micro-focusing on what is now my all stable of Tamron lenses. I am finding a very obvious difference between focus points using the viewfinder and reference focus point using touch screen Live View. 

I will try to attach the photos to this post, but the first time using the regular viewfinder focus (spot focus and metering)the focus point) was front focus. I adjusted using the console forward, just as you would with there Canon micro-focus system. However, when I did the same shot zooming in on LiveView, the focus point was clearly back focus, reflecting the influence of my adjustments. 

The simplest way say this is that the camera is finding a different focus point on the viewfinder method than it is finding using LiveView. While there is always adjustment we can make on a lens, I'm concerned that there is an issue with the camera. 

Ideas?

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Camera focusing through the viewfinder uses a different focusing system in the camera than Live View.

Canon’s Free AutoFocus MicroAdjustment Guidebook

eos6d-mk2-im4-en.pdf See page 491.

[Skip70]

I am aware of that, but they should agree when pointed to exactly the same focus point at the same distance. 

[jrhoffman75]

Why should they? If the viewfinder focusing system is out of adjustment then it will, by definition, be out of focus compared to Live View.

The quick test is 1. focus using Live View. 2. switch of Live View and focus using viewfinder and shutter button. If the lens focus ring moves there is a calibration mismatch. 

[Skip70]

I'm perhaps not explaining well. I'm not talking about focus visible to the eye before shooting (that would be a diopter issue). I'm talking about the actual resultant image. Those should be the same, They are not. 

[Waddizzle · Accepted solution]

“ The simplest way say this is that the camera is finding a different focus point on the viewfinder method than it is finding using LiveView. While there is always adjustment we can make on a lens, I'm concerned that there is an issue with the camera. “

There is probably nothing wrong with the camera or your lenses.  As John pointed out, the camera has separate AF systems for OVF shooting and LV shooting.

This is a result of how DSLRs evolved from film cameras. 

 

The mirror (2) in a film camera redirects the light from the lens (1) upwards to the pentaprism (7), which reflects the light through the viewfinder and to your eye.

A focusing screen (5) served as a manual focusing aid.  When the shutter is fired, the mirror swings up to allow the light to reach the film (4).  

A DSLR works nearly the same way.  The film is replaced by a digital image sensor.  The mirror has been redesigned to redirect light both upwards and downwards.  

What is not shown in this diagram is the autofocus sensor at the bottom of the mirror box.  The AF sensor is precisely located so that the distance from the mirror is equal to the distance from the mirror to the image sensor.  An AFMA adjustment calibrates these two distances to be nearly identical.  The AF sensor is capable of moving itself to adjust its distance to the mirror. 

In Live View shooting mode, the mirror swings up to allow light to reach the image sensor.  In this mode of operation, the image sensor is used for auto focusing.

Notice how the distance between the imaging sensor and auto focus sensor will always be identical because they are the same sensor.  This also means that you don’t need to calibrate the auto focus, anymore.  

[Skip70]

I might be inviting far too much technical information on this. I appreciated your thorough response and th graphic but I am familiar with the two systems. My simple question remains: When focusing on a point with either the viewfinder or the LiveView the result should be the same Otherwise, we would never know which one to trust is correct. 

As far as whether the shared sensor obviates the need for autofocus calibration, I don't see that as true. I have done micro-adjustments on all my lenses, correcting for sometimes subtle, sometimes substantive issues with front or back focus. That's all I was trying to do today until I realized the differential between viewfinder and LiveView. One picture taken with the viewfinder would clearly show front focus; the Live View would show back focus. That suggests to me some issues within the camera. 

[jrhoffman75]

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@Skip70 wrote:

I'm perhaps not explaining well. I'm not talking about focus visible to the eye before shooting (that would be a diopter issue). I'm talking about the actual resultant image. Those should be the same, They are not. 

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https://www.reikanfocal.com/about-autofocus.html

Viewfinder focusing uses a sensor in the bottom of the mirror box. Live View focusing uses the actual sensor that records the image. The camera is calibrated to ensure that when the viewfinder sensor confirms focus the actual image sensor image will be in focus. Sometimes that calibration is off and that is why higher end cameras allow for AFMA (autofocus micro adjustment). 
Without you providing sample images it is not possible for us to diagnose your problem. 

[jrhoffman75]

“When focusing on a point with either the viewfinder or the LiveView the result should be the same Otherwise, we would never know which one to trust is correct. ”

This is absolutely true as long as there isn’t a calibration problem. Do you have the focus problem if you are using a Canon lens? You are injecting a second variable by adjusting a third party lens. 

[Skip70]

I am doing micro-adjustment (as I have done for years with all of my lenses) to correct back- or front-focus. In my latest case, when taking a test photo with viewfinder, it showed front focus. When I did exactly the sums photo using Live View, was generally at zero midpoint. (I did provide photos in my original post.)

My simple point is that the viewfinder focus and the Live View focus ought to be the same, otherwise we would never know which one to trust.