Using a candle as the object on an optical bench set up as shown in the diagram. What two measurements must be taken in order to find the focal length of the lens?

When you view the candle with a lens, located at a distance D of the candle, you will see an inverted image of the candle, and this image of the candle may be a little distorted. At this point you need to change the distance between the observer (you) and the lens.

When the image of the candle is nitid (this means that the image is focalized)

You need to measure the distance between the lens and the observer. suppose that this distance is d.

(this process is usually written as: If the actual height of the candle is H, you need to record the distance where the image of the candle also has a height H)

Now, the focal point of the lens will be obtained with the lens maker equation:

1/D + 1/d = 1/f

where f is the focal point of the lens.