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Lens calculator - Calculating the effective focal ratio / image brightness

The values of the focal ratio indicated on the lens are normally calculated for an image from the infinite. In case of images from a short distance, particularly macro-images, the effective focal ratio can be calculated which results in the actual image brightness:

keffective = k * (1 + ß)

Lenses calculation ofe  f-stop number

The image scale β is calculated from

ß = image size/ object size

Note: Even when using the drop-down lists own values can be entered. Please use the first entry "user def."!


Sensor size :
Set aperture of the lens:
Object size/ field of view:


Calculated effective aperture:
Effective image brightness in percent compared to selected aperture:


Further details on the function and influence of the aperture can be found in chapter "Aperture of the lens"

The calculated signal intensity must be seen in connection with the general reduction of the image brightness of the respective f-stops:
Reduction of image brightness resulting to a f-stop

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