Is there a linear correlation between focal length and enlargement size?

Either way, there is still a linear relationship between focal length and size on the sensor, i.e. crop factor. If you fill a 35mm frame with a subject using a 160mm lens, then a 100mm lens from the same vantage point on the same camera will cast an image that would fill an APS-C sized area in the middle of that sensor. The pixel dimensions will correspond identically to the linear proportion of the sensor that the subject covers. So it's all a direct relationship.

Treat the camera lens as a biconvex lens. It behaves the same way, just has a lot of aberrations reduced.

Read this
http://en.wikipedia.or​g/wiki/Lens_%28optics%​29

Ephemeral wrote in post #4276026
I was just curious to know whether there was a linear correlation between focal length and magnification. That is to say, if I look through a lens at a square at focal length of 24mm and it is a 100 pixels wide, if I look at the same square at a focal length of 240mm, would the square take up 1000 pixels?

My answer is much less detailed than most here, but, for all practical purposes......is yes, if the distance to the subject remains the same.