Hi Linda, It's difficult to say what the lowest useful magnification is, but in general, low magnifications are more useful then high magnifications for conchological work. I have a zoom stereomicroscope with a continuous range of magnifications from 7X to 30X. I most often work with magnifications from 7X to 10x. However, I do not specialize in microshells. If I did, then I would spend more time between 20X and 30X. Magnifications higher than that, in my opinion, are not very useful for shell work. After all, a magnification of 30X makes a 2 mm shell look like it is 60 mm, or almost two and a half inches! The single most important thing to keep in mind when looking for a microscope (or any optical instrument for that matter) is that the resolving power, or resolution, of the instrument is far more important than the magnification. You will see far more detail with a high resolution 20X system than you will with a low resolution 60X system. Paul M.