The three elements of blur are linked to physics: why 50mm f/1.8 is not blurry enough for the whole body, while 85mm or 70-200mm can instantly melt the background

1. Geometric optical formula of blur intensity: calculation of circle of confusion size

The size of the blurry spot that the background is blurred into is essentially determined by the diameter of the Circle of Confusion (CoC) formed by the background points on the sensor:

1. Formula to determine the size of the bokeh circle of confusion:

• ( b \propto rac{f \cdot m}{N} \cdot \left( rac{d_{bg}}{d_{bg} + d_{subj}}
  ight) )
• Where ( f ) is the physical focal length, ( N ) is the aperture F value, ( m ) is the magnification, ( d_{bg} ) is the physical distance from the character to the background;

2. Why is the background still cluttered when taking full-body portraits at 50mm f/1.8? ：

• When taking a full-length portrait, in order to fit the 1.7-meter-tall figure into the frame, the photographer must step back 5 meters away from the person!
• Once the shooting distance is extended to 5 meters, the optical magnification ( m ) shrinks exponentially, causing the depth of field to become sharply deeper;
• Even at the maximum aperture of f/1.8, the messy streets and trash cans behind you are still clearly visible!

2. The real secret of "air cutting" of telephoto lenses: space compression and focal length doubling

Why can 70-200mm f/2.8 completely melt the background when shooting the whole body even at the 200mm end with an aperture of only f/2.8?

1. Focal length is a "square-level influence" in the blur formula:

• The clear aperture (physical diameter) of a lens is equal to the focal length divided by the F-number;
• The physical aperture diameter of 50mm f/1.8 is only ( 50 / 1.8 = 27.7 ext{mm} );
• The physical aperture diameter of 200mm f/2.8 is as high as ( 200 / 2.8 = 71.4 ext{mm} )!
• The huge light spot of 71.4mm can exaggerate the exaggeration of background debris;

2. The narrow viewing angle of the telephoto completely eliminates background clutter (Field of View Compression):

• The 50mm viewing angle is wider, and a large cluttered street scene of 10 meters wide will be included behind the character;
• The 200mm angle of view is extremely narrow. There is only a leaf or a small patch of color behind the character, and this small background is magnified dozens of times, forming an extremely pure and creamy silky bokeh.

3. Ultimate blur operation rule: increase the "distance between objects"

If you only have an entry-level 50mm f/1.8 lens at hand, how can you capture the strongest blur?

1. Extremely close to the subject: Take a half-length or close-up shot (the shooting distance is shortened to within 1 meter), and the magnification increases;
2. Keep the character away from the background: Let the character stand in the middle of an open space, at least 20 meters away from the building behind him; the farther away the subject is from the background, the background will fall into the out-of-focus super far area, and the bokeh will naturally disappear completely.

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