When you have to prepare your set and decide the distance between the perches and the camera you could face some tricky calculations.
This tool is aiming to help you, you should only insert the focal length (comprehensive of eventual sensor magnifying and extenders) and the measure (in centimetres) of the field you want to be framed
according to the longer side of the viewfinder.
Let's make an example: if you want to photograph a Bee-eater using a Canon EOS 7D + Canon EF 500mm first you have to type 800 as focal length (500 x 1,6, magnifying value of 7D sensor).
Then, you can add a few centimetres to the length of the subject in a percentage that you can vary according to the image you're going to compose.
In this case, as a Bee-eater is about 29 centimetres long, you can type 35 for example.
The system will show you the distance from the camera to the perch in order to frame the desired measure with the longer side of the viewfinder.
This tool is working properly with fogal lenghts greater than 50mm.
**Related to the longest side of the sensor