# Thin lens equation when the image distance is equal to the focal point of a convex lens

I'm not very knowledgeable in optics but i'm learning about lens and the thin lens equation:

$$\frac1f = \frac{1}{d_o} + \frac{1}{d_i}$$

It's asking me to determine the best focused object distance when a camera is at the same distance of the lens' focal length, 120mm.

$$\frac{1}{120} = \frac{1}{d_o} + \frac{1}{120}$$

The best focused image object distance would be infinity? What does that actually mean?

Many sites discuss that there are no images formed when the object distance is equal to the focal length, but there arent any I can find that discuss when the image length is equal to the focal lenght

• If the object is placed at the focal plane, then all rays that travel through the lens are parallel. Think about how far the image will form given the rays are parallel. Feb 7 at 0:31
• But what about trying to determine the object distance when the image is placed at the focal plane? Feb 7 at 1:12
• Then the same will be true. You will not be able to define the object distance given that the image forms exactly at the focal plane. If $v=f$ then $\frac1u +\frac1f=\frac1f$ means $\frac1u=0\therefore u\rightarrow \infty$ which is not defined. Feb 7 at 2:57
• Since infinity is not very nice to deal with, lets put the image plane almost at the focal plane. say, $d_i =f+\delta$ where $\delta$ is much, much smaller than $f$. Then calculate $d_o$ and see if you can make any sense out of the result. Feb 7 at 2:59