[6-11] "Bokeh (ii)"

Bokeh refers to blur—image detail poorly resolved on a sensor due to non-convergence of the focused light.

But there is a difference in two types: DOF (depth of field) and background blur (BB). DOF refers to distance (depth) of an image reasonably in focus. BB refers to extent of blur in the background.

If bokeh is to be discussed, it matters which one we are referring to.

These two are dictated in separate equations.

This was a big surprise.

In 2020, I want to try a shallower portrait lens (less DOF). My best options were the 105mm/1.4 and 135mm/1.8.

For shallow DOF, which is better? I ran the test on the DOF simulator:

∆1: The 105mm 1.4 will have shallower DOF (3.2 vs 4.1cm)

∆2: The 105/1.4 and 135mm/1.8 will have identical BB.

∆3: 105mm sees a wider background. With the 105 I must stand closer to the subject.

∆4: Consider perspective distortion (distance between subject/camera).

Those were the major differences. I went with the 105/1.4.

If the subject size is "locked" (e.g. headshot), the DOF does not change no matter what focal length is used. A 14mm 1.4 will have the same shallow DOF as the 135mm 1.4.

Aperture + subject distance fully determine it.

BB depends on the physical aperture (mm). Because 135/1.8 and 104/1.4 have the same aperture diameter (135/1.8 == 104/1.4 == 75mm), they result in the same BB with a constant subject.

Shazam. I wanted shallower DOF, not BB, because I wanted the hair/cheeks to melt away in bokeh.