The f# estimate method applied

This is a typical example where the height of the camera above the plane corresponds to a person height at eye level (about 160mm) which means the relation becomes

$\Large f_{number} = f \times \frac{r}{2}$

With r:

• r: the part (quotient like 1/2, 1/3...) of the image behind or in front the focus plane. It should be measured in the image !
• f: the focal length

Let's take this example

We use as the ground as the straight plane, the horizontal line show the position of the different distant planes. These planes are vertical.

In red is the focus plane. I can focus anywhere in the focus plane, here for instance I can focus on the red cross.

In blue the far dof limit we want to have behind the focus plane. Please note that the far limit is behind the house.

The part between these 2 horizontal lines represent about r = 1/4 of the total height of the image.

I can use

f# > focal_length/8

In my case I had used focal_length = 23.3mm, this means I should use f#=3 at minimum. And that's it !!

The few assumptions made

• of course, you should not focus and recompose, otherwise you loose accuracy.
• I am standing up on the ground, the altitude of the camera is about 1,6m (1600mm) above the ground.
• The camera is not tilted down/up too much relatively to the plane. PThis can be neglected in most cases, otherwise there is a slight adjutement to make, see the goban example.

Just for information, F I had used f#=9 for this image.