In this thread Policar jokingly suggested adding a close-up filter to his 180mm lens instead of buying a 240mm lens. I was curious about how that would work so I dug out my Canon 500D close-up filter and gave it a try.

I can't make sense of the results. The Canon 500D seems to shorten the focal lenght of the lens. When I add the Canon 500D to a lens I have to rack the bellows in which gives me a wider angle of view. I was quite surprised by that, I expected the opposite.

I tried this experiment with 90mm, 125mm, and 210mm lenses. The effect is more pronounced with longer focal lengths. I also tried different subject distances - one foot to about twenty feet - with similar results. I didn't measure the subject size on the ground glass, maybe I should.

I don't get it. Can someone explain this?