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)
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."!
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: