Computing seven lens variables from three inputs

**Jerry Fusselman** · 25-May-2006, 20:26

In a previous thread, "Portrait perspective: Quiz and two questions" I offered a quiz. The attached pdf contains my answers.

This is how the pdf starts:

This little paper discusses seven common photographic variables (focal length, magnification, angle of view, image size, subject size, and two focus-related distances), and it shows how a photographer armed with a few math skills from high school can compute all of these lens variables from limited information.

In most cases, knowing the value of three of these variables allows you to compute the other four. Table 1 on p. 3 gives the recipe—you choose a row based on which three variables you have, and it tells you if it is possible to deduce the others, and if so, exactly how to do it. On p. 4, I present two examples using table 1 to compute implied values.

As a part-time camera designer of modest skill, I have found that I use the table about ten times a week. When I have spacers, adapters, etc., made for me, I know ahead of time that the optical aspects of the design will work, which saves time and money.

I wrote an APL program to select and run whichever case of table 1 is needed. I can send the program (free) to anyone who uses APL. I might make a user-friendly version someday.

**Mik Wenger** · 26-May-2006, 14:47

I'm sure this is a fine work. But some people, including me, refuse to look at anything that mixes metric and English units. Would you consider creating a metric-only version?

**Jerry Fusselman** · 26-May-2006, 15:21

Actually, I agree with your thought that it is bad to mix units.

Are you referring to the two examples on the last page? If so, I was only trying to make clear that if you have numbers in two different units, you should covert them so that they are in the same units. I was only trying to say that 5 inches should be expressed as 5x25.4 mm so that you can use the formula properly. It sounds like you wished that the examples on the last page started with everything in metric. Is that it?