The stuff of Kryton

An eclectic selection of technology with some cookery

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Depth of Field

Working out what factors of a lens / camera body govern the depth of field of an image.

The two use cases:

  1. I want a shallow depth of field to use show the subject
  2. I want a deep depth of field to show the hole scine

Terms

Apatute (N)

Often know as a “F Number” in photograthry. Also the term “T number” is sometimes used in high end cinimatic lenses. To my understanding F numbers are calculated from the lenses design where “T” numbers are measured by sending light though a built lens. The latter being more accurite. For the purposes of the calcilation they will be used interchangeably.

Focal Lenght (f )

The focal length of the lens, normall stated in “mm”. Commonly a 50mm lense is used with a 35mm film stills or “full frame” camera.

f not to be confised with the “F number”.

Distance to Subject (u)

How far is the lense from the subject.

Circle of confusion diameter (c)

I’ll adopt the d/1500 calculation to simplify this.

Sensor Size CoC
1” 11μm
4/3” 15μm
APS-C 18μm
35mm 29μm

Formulas

Depth of field

DOC = (2 ( u 2) N c)/ ( f 2)

This suggests f focal length and Distance to subject have the largest effect as they are square factors

Conclusion

Doing some calculations on a spreadsheet agrees with the above statment, sensor size also has a noticable effect but not as much as you might think. Also having F Numbers on lenses and reffering to f for focal lenght is very confusing.