Effective sizes

Effective sizes are used in photography , especially macro photography , to expand some definitions to include close range.

Many definitions and formulas in photography are completely independent of the distance to the subject or refer tacitly to a very large distance to the subject compared to the focal length. Examples of this are: luminous intensity or f-number , image angle and parameters derived from this, such as the correct exposure time . At distances of the order of magnitude of the focal length, however, the image plane is no longer on the focal plane, so that in macro photography in the formulas the focal length f has to be replaced by the image distance b . If one refers to the image distance instead of the focal length, one speaks to differentiate between effective quantities such as. z. B. of effective light intensity and effective angle of view :

Distance ∞	Close range
Light intensity:   ${\displaystyle \displaystyle {\frac {D}{f}}}$	effective light intensity:   ${\displaystyle \displaystyle {\frac {D}{b}}={\frac {D}{f}}-{\frac {D}{g}}}$
Angle of view:   ${\displaystyle 2\cdot \arctan \left({\frac {d}{2\cdot f}}\right)}$	effective angle of view:   ${\displaystyle 2\cdot \arctan \left({\frac {d}{2\cdot b}}\right)=2\cdot \arctan \left({\frac {d}{2\cdot f}}-{\frac {d}{2\cdot g}}\right)}$

Variables derived from the effective values ​​- such as B. the exposure time required for correct exposure, which results from the effective light intensity and the subject brightness - are also given the addition "effective" , in this case effective exposure time . It results from the exposure time B (i.e. not the effective one ) ${\displaystyle B_{\text{eff}}=B\cdot {\frac {b^{2}}{f^{2}}}}$

• In photography and optics there is the term effective focal length . However, this does not refer to the object distance as described here, but simply refers to the focal length of a system composed of several refractive elements, e.g. B. a lens.

See also

• Macro photography