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Rack Focus Distortion

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Dave Lewin
Rack Focus Distortion
on Jul 22, 2014 at 3:04:17 pm

I've noticed that the image quality of a rack focus shot can vary greatly from film to film. And I'm referring to big budget films that are likely shot with high-end digital cinema cameras or actual film cameras outfitted with cinema lenses.

In some shots the out-of-focus areas seem to distort as the lens begins to focus on them. Concurrently, as the in-focus area begins to fall out of focus it also seems to distort. But I have also seen plenty of films with seemingly smooth transitions.

What makes the difference and is there a name for this phenomenon? How do you minimize the possibility of those distorted results?


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Todd Terry
Re: Rack Focus Distortion
on Jul 22, 2014 at 3:21:36 pm

I don't know if there is really a name for that phenomenon.

My best guess is that the samples that you see which present this distortion are ones that were shot with anamorphic lenses, whereas the ones that seem clean were shot with spherical lenses.

If you have a particular example of an existing film, you can likely go and look it up on IMDB and the tech specs might tell you if it was shot anamorphic or not.

The other clue is bokeh, look at out-of-focus distant highlights and light sources in deep background.... if they are round, spherical lenses were used. If they are oval shapes, those were anamorphic lenses.

I don't know for sure if this issue was caused by anamorphic lenses in the particular cases you've seen, but it's a good guess. Anamorphic lenses do distort much more easily than spherical lenses.

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Emre Tufekcioglu
Re: Rack Focus Distortion
on Jul 22, 2014 at 4:38:25 pm

Its hard to identify exactly what is causing it without samples but let me try to simply a few of the elements that cause issues with lenses:

-Breathing: This is when performing a focus rack framing changes. More prominent with with zoom lenses and low end lenses. high end cinema lenses do not have this problem.

-MTF (Modulation transfer function) caused by physical limitations of glass, more prominent in low end glass, less in high end but everything has a MTF.(your eyes, your glasses, your car window...etc) Causes loss of sharpness, contrast.

-Bokeh style. Low end lenses will display bokeh in the shape of the iris where high end lenses will display the bokeh as perfect circles.

-Chromatic Aberration. Causes purple and yellow fringing in high contrast areas. Newer cameras have CAC correction software but still high end lenses will suffer less. More prominent in long end of the lenses (zoomed in)

-Barrel distortion with aberrations. Causes fringing/discoloration and vignetting around the circumference of the lens.

-Uncoated lenses (or intentionally stripped): Light will bounce around the lens cause flaring. See saving private ryan.

-High impedance air gap. You forgot to plug in your lights. Causes black frames. :)



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