Why is AF active point disabled in preview when lens correction is enabled?

[johan162]

I finally realized why, when I use some lenses I can see the the active AF point used in the preview, and for other lenses it was not displayed on my 7DII

This might have been obvious to everyone but me but I discovered that if your lens is in the lens correction DB and you enabled the correction the AF point is not displayed in the preview on camera.

(It is however shown using the Canon SW)

I thought I knew the camera fairly well but I cannot come up with reasonable explanation for why enabling lens correction would prohibit showiing the active AF point at time of capture. I would have assued that the lens correction is applied as a post-processing step and should be independent of the AF point (and any slight warping would just adjust the AF point accordingly) but apparently I'm wrong.

Can anyone shed some light on my lack of understanding?

[Waddizzle]

Are you referring to viewing images saved in the camera, and seeing the “active AF” points?

I believe that when you view a preview in the camera, you are infact viewing a JPEG version of the RAW file, with whatever processing setting you have enabled in the camera.

Also, the AF points have a fixed location.  They do not move.  If lens correction is applied, then image may be distorted, or corrected, from what you viewed in the viewfinder.  In order to display the AF point on this “corrected image”, the AF point may have to be moved slightly from its’ default, fixed location. 

[kvbarkley · Accepted solution]

I don't that there is any official Canon explanation of this, but I feel it is because the distortion correction removes the 1-1 coreespondence between raw pixels and image pixel and for whatever reason, Canon has decided not to display it.

It could also be a Digic Horsepower issue, that the additional processing requred for correction prevents the display of AF points.

As I said, this is all guessing.

[Waddizzle]

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@kvbarkley wrote:

I don't that there is any official Canon explanation of this, but I feel it is because the distortion correction removes the 1-1 coreespondence between raw pixels and image pixel and for whatever reason, Canon has decided not to display it.

It could also be a Digic Horsepower issue, that the additional processing requred for correction prevents the display of AF points.

As I said, this is all guessing.

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That would be my conclusion, too.

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Also, the AF display is not necessarily showing you where the camera was focused.  You could have locked focus and recomposed the shot, and the camera would display the same AF point no matter how you recompose the shot.

Also, the AF display is not showing you which AF point locked focus, either.  Depending upon your focus mode, you may see multiple AF points lit up.  The AF display is showing which AF points were capable of locking focus. 

But, only one of them was used to actually lock focus when the shutter was activated.  The Canon display does not show you which AF point [was used when the camera] locked focus.  If you are using only one AF point, then it is a reasonable assumption to conclude that the lone AF point was used to lock focus, [but not where in the frame if you recomposed the shot.].  

Just keep in mind that the AF point display is always showing the AF point(s) that were capable of locking focus.

[johan162]

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@kvbarkley wrote:

I don't that there is any official Canon explanation of this, but I feel it is because the distortion correction removes the 1-1 coreespondence between raw pixels and image pixel and for whatever reason, Canon has decided not to display it.

It could also be a Digic Horsepower issue, that the additional processing requred for correction prevents the display of AF points.

As I said, this is all guessing.

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Fair enough guess, seems plausible. Although I very much doubt this is a horsepower issue since the heavy lifting is already done and the data to make the potential very small adjustment of the AF point(s) should already be available.

My guess would be that in cases of lenses with heavy correction the individual points will move different distances (due to warping) and would no longer show up as perfectly aligned and Canon might have thought that that would cause questions.

Sometimes It's nice to have a quick view if the possible points of focus lock was what you wanted. Would be nice with some official comment in the manual though. Took me way too long time to figure out the correspondance between lens correction enabled/disabled and the loss of the points in the camera preview.

[kvbarkley]

We get this question all the time. Another one for the faq.