![DSC07580](http://josevicentemartin.umh.es/files/2015/04/DSC075801-300x227.jpg)
We have built a camera obscura of black foam core cardboard with a base of 20 cm x 28 cm and a height according to the focal length of the lens, 20 cm.
![DSC07581](http://josevicentemartin.umh.es/files/2015/04/DSC07581-300x227.jpg)
So the height of the camera obscura must be 200 mm = 20 cm. When the sling part is closed, the lens will be focused to the infinity.
![DSC07601](http://josevicentemartin.umh.es/files/2015/04/DSC07601-300x115.jpg)
The scanner + camera obscura does not give any visual reference to focus the object, so we have to apply the formulas you can find in the paper: «How to calculate the focusing distance».
How to calculate the focusing distance
![formula2](http://josevicentemartin.umh.es/files/2015/04/formula2-300x132.jpg)
This is the formula, where f = focal length, s = the distance from lens to the object (focusing plane) and Δx = the sliding distance.
![If our globe sphere is placed 100 cm to the scanner and the focal lenght is 200 mm = 20cm, the result is 20 X 20 / 100 - 20 = 5 cm](http://josevicentemartin.umh.es/files/2015/04/DSC07605-300x197.jpg)
If our globe sphere is placed 100 cm to the scanner and the focal length is 200 mm = 20cm, the result is 20 x 20 / 100 – 20 = 5 cm. The sliding distance is 5 cm.
![Save0005--](http://josevicentemartin.umh.es/files/2015/04/Save0005-1-300x212.jpg)
With a Canonscan LiDE 35 scanner we got this image (a bright, sharp enough, white and black image).The scanner specifications are:
Scanning Element: Contact Image Sensor (CIS)
Light Source: Three-colour RGB LEDs.
![DSC07613](http://josevicentemartin.umh.es/files/2015/04/DSC076131-300x199.jpg)
Now, we are going to use the formula the other way round. We place the sliding part at 15 cm from the scanner and…
![formula1](http://josevicentemartin.umh.es/files/2015/04/formula1-300x114.jpg)
we apply this formula, where f= focal length= 200mm = 20 cm, and Δx (the sliding distance)= 15 cm. Then, 20 (20 + 15) / 15 = 46,666 cm. So the globe sphere has to be placed 46,6 cm from the scanner.
![Imagen 003-](http://josevicentemartin.umh.es/files/2015/04/Imagen-003--300x217.jpg)
With an Epson Perfection 1650 scanner we got this image (a dark but sharp colour image). The scanner specifications are:
Photoelectric device: Colour CCD line sensor
Light source: White cold cathode fluorescent lamp
Some further information: