Monday, October 5, 2020

DOFIS: Now supports Macro and Pano Photography

As promised in a previous post, DOFIS now uses an asymmetric, split lens model. This means that DOFIS gives both macro and pano photographers the best focus information they can get.

Before explaining why this is important, here is a reminder of the DOFIS model:

As was shown in a previous post, the near and far depths of field, ie relative to the in-focus object plane, are dependent on the exit pupil location. Also, the hyperfocal distance is measured from the exit pupil plane. Finally, the FoV is also calculated from the exit pupil plane, and the pano, no-parallax, pivot point is also located there. 

Clearly, knowing the location of the exit pupil is important to photographers, however, it is hardly ever stated by the lens manufacture. In fact, complicating things further, on many/most lenses, the position of the exit pupil, relative to the sensor, also changes with focus.

Most/all depth of field calculators ignore the pupil magnification, ie assume it is 1, and 'just' state the hyperfocal distance from the front principal of an assumed thin lens. This, to be fair, is OK for a lot of photography, eg landscapes etc; but fails when it comes to macro and pano photography,  when knowing the pupil magnification becomes important.

I have previously shown how to measure the pupil magnification, ie take a focused image of the exit and entrance pupils, with ruler positioned in the plane of each pupil, ie also in focus. Then, in Photoshop, align/scale the rulers and work of the ratio of the exit and entrance pupils, ie the pupil magnification.

The latest version of DOFIS comes with five pre registered lenses: a Sigma 12-24mm, a Canon 24-105mm, a Canon 24mm TSE, a Canon EFM 11-22mm and a Canon 100mm Macro. 

To add additional lenses, simply follow the prescription below, using DOFIS to extract the lens name:

-- Lens #1
mylens[1] = {}
mylens[1][1] = "12-24mm" -- lens name
mylens[1][2] = 0.154 -- Max Mag at longest FL
mylens[1][3] = 280 -- Minimum Focus at longest FL
mylens[1][4] = 24 -- Longest FL
mylens[1][5] = -1 -- move direction (towards infinity)
mylens[1][6] = 6.4 -- Pupil mag at widest FL
mylens[1][7] = 3.8 -- Pupil mag at longest FL
mylens[1][8] = 12 -- Widest FL
mylens[1][9] = 0.085 -- Max Mag at widest FL
mylens[1][10] = 280 -- Minimum Focus at widest FL

By the way, you can clearly see that, as one would expect of a wide angle lens, the Sigma 12-24mm is of a retro focus design, over its zoom range: that is the pupil magnification is greater than unity.

As can be seen, the DOFIS model is measured at the two focal length extremes on a Zoom or at the focal length of a prime lens. For a zoom DOFIS assumes a linear change in lens 'thickness' and pupil magnification between the two zoom extremes. A pragmatic assumption.

DOFIS info feedback now includes an estimate of the pano pivot point, relative to the sensor, and the horizontal, vertical and diagonal field of views, ie as they change because of lens focus breathing. You can also see the near/far DoF ratio as focus changes.

Hopefully those that have been following my DOFIS developments now appreciate that this Magic Lantern script provides the best focus information available, for all types of photography, including macro and pano, and even for videographers, eg ‘accurate’, relative DoFs. 

Finally, using DOFIS's auto modes allows the photographer to capture perfect deep focus, exposure bracket sets; or automatically position the lens at the hyperfocal.

As usual I welcome feedback on this post and DOFIS.


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