Friday, November 20, 2020

DOFIS: Plane of Sharp Focus

As DSLR tilt/shift (TS) lens users know, positioning the plane of sharp focus is inevitably an iterative process. However, with DOFIS, Canon Magic Lantern users can get a little help along the way.

Although there are hints that Canon are working on some exciting tilt/shift technology, eg a tilt/shift adapter, the current generation of Canon TS lenses are rather dumb, ie there is no link between the tilt angle and the camera.

To offset this lack of feedback, I've now added a Plane of Sharp Focus info field into DOFIS, in addition to the Hinge (J) calculator that I mentioned in a recent post.

The DOFIS menu feedback looks like this:

Here we see the Hinge calculator set to 4 degrees, showing a hinge height (parallel to the sensor plane), measured from the front principal (which DOFIS also tells us), of 34cm. 

Under DOFIS Additional Info, we also see the Plane of Sharp Focus estimate, at the current focus, is 37 degrees, ie down vertically from the Hinge plane, which is parallel to the sensor plane as shown below:

The J calculator and the PoSF feedback are in DOFIS to help you visualise how to control your TS focus: you will still need to carry out actual focusing using an iterative process. Whilst you are using the tilt calculator functionality in DOFIS, eg dialling in a tilt angle, you can also refocus and see how the plane of sharp focus angle changes. 

Finally, if tilting the camera as well, then it will be necessary to re-estimate J to your chosen hinge plane, as shown below, where the camera is tilted around the front principal of the tilted lens axis.

In the above sketch we see that the hinge needs to be reset to ‘connect’ it with the chosen ground plane, as the principal plane's node position hasn't changed in this example. 

As usual I welcome any feedback on this post.

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