DotTune: Video Tutorial for AF tuning without photographs

Started Feb 18, 2013 | Discussions thread
MOD TOF guy Forum Pro • Posts: 13,757

lock wrote:

Are hardware-measured far and close edges of the in-focus range range the same as the far and close edges of the range of perceived acceptable sharpness ?

"When the circle of confusion becomes perceptible to our eyes, this region is said to be outside the depth of field and thus no longer "acceptably sharp." Are we now saying that AF sensors are affected by the same effects as our eyes ?

DOF is not defined as a range of visually perceived acceptable sharpness, it is defined by the circle of confusion i.e. as much blur is allowed in the final print.

As far as vision is concerned it has to be linear for the most part because the picture is projected on the paper (or screen) plane. A feature which should be sharp, but is very slightly blurred, may still look adequately sharp if the blur is smaller than the circle of confusion. Regardless of whether in reality the feature is behind or in front of the focus plane we still look at it on the picture at the same eye to print distance, which would be by far the primary factor.

There are non-linear factors. The best known example is: the picture looks sharp if the eyes of the main subject are sharp. We may pay more sensitive to sharpness in the center of the image (or the immediate surroundings of the main subject) than away from it. And yes when looking at a picture we may instinctively more sensitive to detail in areas of the image that we guess are closer to the camera which took the picture. I don't think it's much of a factor.

In any it's totally irrelevant (!), since as stated before DOF used an an objective (not perception based) definition. In other words it's based on what the camera sees, not our eyes.

The objection remains: if the DOF does not extend evenly behind and in front of the focal plane then Horshak suggestion to use the center of the area delimited as in focus by the dot will induce an error. That is unless the camera corrects for that, which I very much doubt. I suppose (but not tested) that the way the dots work is simply by using the AF sensor, without correction. If the AF sensor detects a line of constrast as separated on 2 sensors then it's out of focus. If they match it's in focus. It's in fact looking at the circle of confusion and the camera calls it in focus if this circle is small enough that the AF sensor cannot make a difference whether it can be even smaller or not.

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Thierry - posted as regular forum member

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