Focal length of Opteka 10x close-up lens?
 

I am interested in buying an Opteka 10 x close up lens if it is significantly stronger than my Canon 250 D.

The focal length of close-up lenses is often measured in diopters (the reciprocal of the focal length in meters). Common strengths are 1, 2, 3 & 4 diopters or 1m, 1/2m, 1/3m or 1/4 m respectively. Canon label theirs as 500D and 250D corresponding to 500 mm or 250 mm instead of 2 diopter and 4 diopter. Opteka market a 4 element lens they call the 10x but give no other indication of its strength. The focal length can readily be measured because a positive lens forms an image on a surface such as a piece of paper. A 4 diopter lens will form an image of a distant image 250 mm from a white surface. Another way of making the measurement is to measure the distance between the Opteka and subject when it is fitted to a camera set to infinity.