# Confused about thin-lens equation

Tags: confused, equation, thinlens
 P: 42 As you probably know, the thin-lens equation is given by: 1/s + 1/s' = 1/f where s = distance from object to lens, s' = distance from lens to image and f = focal length of lens Now, in a particular question, I needed to find s' given that s = f. From the equation, that means s' would become undefined! Is that even possible? If so, can you please explain what that means in terms of the image (i.e. real/virtual; upright/inverted)? Many thanks,
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