# Difference between revisions of "Basics"

## Nomography

Nomography may be considered as a graphical computation. By using straight edge (or ruler) and pen, one can solve how parameters relate to each other.

Nomography was invented in the end of 19th century and it was workhorse in engineering up to 1960s. In 1970s computers were more and more used for computations and nomography turned out to be a forgotten state of art. Today we use spread-sheet calculators with computers and compute with them how parameters relate to each other. However, by using this approach, we have difficult to understand relations intuitively. We may do not grasp the problem. It is also today very difficult to present graphically how more than three variables relate to each other. So we have almost lost a way to present laws (nomos) graphically (graphy). There is still room and need for nomography.

Below are two examples of the use of nomography in order show how nomographs may be used in practice.

### Example 1

This is a classic nomography example. The question is about body-mass-index that is a measure of over- or underweight. The index is calculated from height and weight, and if it turs out to be between 20 and 25, person has normal weight.

In the example we have person with weight 85 kg and 183 cm. By drawing a straing line between the points in the scales, one can read that body-mass-index is some 25.5. It means the person just crossed the limits of normal weight and has slight overweight. The red line for calculation is drawn in the nomograph below.

Body mass index
Generated portable document file (pdf): File:Ex BMI.pdf

This example is one of the more simple examples illustrated in Examples. One may see how it was constructed with PyNomo at the following page.

### Example 2

The second example represent use of a complex nomograph. The question to be asked is the following. A photographer wants to travel to Helsinki (Latitude 60) to take outdoor photographs around 2 pm First of May (Vappu). The sky is often dark gray (it's Helsinki, not Silicon Valley) and one wants to take photographs of people at open place. The photographer plans to use lenses with maximum aperture f/4 and wants to shoot with speed ISO100 in order to have low grain or noise in pictures. How does he find out what is the estimate for shutter time? He uses the following nomograph. See the nomograph in portable document format: File:Ex photo exposure.pdf

First he draws line from time 14.00 via grid point latitude 60 and month May. He finds out that solar zenith angle is around 50 degrees.

Next he draws horizontal line until it hits the line "Sky dark gray".

Next one he draws vertical line until it hits the line "Person at open place" and then horizontal line until he hits edge of the "contour plot"

He finds that lightning corresponds to EV100 value around 10.5. The he draws a line via point ISO100 to axis R. There he draws a second line to aperture point f/4. The crossing of the line in time axis or shutter speed tells that value is around 1/80s.

In order to determine if the shutter time is small enough for focal length of 30mm of his image stabilized (IS) zoom lens in his cropped sensor digital single-lens reflex (DSLR) cameran he plans to use, he draws line to the maximum focal length point vial DSLR IS point. Maximum is above 200mm, so it is safe to take handheld photographs in this focal length and shutter time.

This example uses many blocks of PyNomo and maybe considered such complex that it is a question, is it practical in real use. Maybe.

The theory, references, mathematics, nomography construction and PyNomo source code is elaborated at the following page.