just got 7d, I am really frustrated right now.

Started Aug 27, 2012 | Discussions thread
Jerry-astro Forum Pro • Posts: 12,542
Re: Not so...

My assumption is that whomever is doing the testing is doing it correctly, i.e. with a flat, high contrast, perpendicular, well lit target and using a tripod. Under those circumstances LV should absolutely be a "gold standard" for accurate focus. If you're testing under more variable conditions, then all bets are off and your entire test is likely to be useless anyway.

riknash wrote:

This explanation is erroneous since it wrongly assumes that once focus is acquired with Liveview, that absolute accurate focus has been achieved or that there will be no possibility of a change in focus during the mechanical process of the shutter tripping. Observations are that the captured image can be significantly out of focus with liveview prior to tripping the shutter (the camera lights up a green square suggesting it has accurately acquired focus) or that the stored image is significantly less sharp than the one observed on the LCD prior to shutter-trip, even when a sufficiently fast shutter speed and rock-solid tripod are employed. This is due to the defective LV focus algorithm and also the shutter-mirror slap that is prevalent amongst copies of the 7D. No, one doesn't need to read the manual...if the LV LCD square lights up green, the image should be in focus, not at some high degree of no out-of-focus.

How could LV be considered a "gold standard" with such variability in performance amongst copies of the 7D?

Jerry-astro wrote:

caspianm wrote:

I am confused...
To test and MC a lens you would use LV.
Once it is done and/or LV shot is fine then it should do equally well in VF.

I just don't believe the lens is fine in LV but oof using VF unless the camera is defective not the lens.

In fact, the opposite is true. LV uses a completely different mechanism to focus, where it's iteratively done directly from the image sensor (contrast detect). Using conventional focus (phase detect), a completely separate subsystem and sensor is used. If there's any chance for a calibration issue, it would be differences between what the phase detect AF subsystem (not LV) thinks is the point of focus vs. where it actually is. MFA is used to close that gap via a compensation factor. A correctly calibrated lens should have exactly the same results regardless whether you're using LV or focusing via the VF.

BTW, where LV comes in handy is to provide a "gold standard" for focus. If you're consistently seeing different results between LV and VF focus, it might be an indication that MFA is required. At that point, attempting to calibrate the camera/lens combo and determine an appropriate MFA setting should fix the issue assuming the lens doesn't have other issues requiring it to be serviced.

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