I think I see what you are getting at, but unfortunately it isn't quite that simple.
The focal length of the lenses states at what distance a point light source would need to be, in order to convert the light into a parallel beam. The easiest way is usually to do it the other way around (and is usually covered in the basics at school [or at least used to be, don't know what rubbish they are teaching nowadays]).
The light from the sun is usually considered to be parallel, or as good as you are going to get.
If you have a focal length of 50mm, then the parallel light hitting the lens, will be focused to a point 50mm away from the focal plane of the lens (which may not be the back of a lens in the terms of camera use).
This means you hold the lens 5cm above the ant.
Now, there are two other things to take into consideration when you are using a camera.
1) You don't want to focus to a point, you have a 2 dimensional sensor
2) The light from the object is not parallel, each atom on the outside of the item you are imaging, is emitting light
So, in reality
You want to try and focus many different point sources, into points, on the back of a flat sensor. To determine how to do this, you need to know the distance from the focal point, and the size of the sensor and object being imaged, including the distance to that object.
Can we leave it at that?
