Characterisations of ancient sheep breeds and wool types as well as theories about wool fibre processing have become integral parts of textile archaeology. The studies are based on statistical calculations of measurements of wool fibre diameters and reveal characteristics of the yarns that can be attributed to the available raw wool and to the production methods of the people creating the textiles. The types of microscopes used for the analyses have varied through the years and presently digital images from either scanning electron microscopy (SEM) or transmitted light microscopy (TLM) are the preferred methods for data collection. The advantage of SEM is the good depth of field at high magnification while TLM is simpler to use and more readily available. Several classification systems have been developed to facilitate the interpretation of the results. In this paper the comparability of the results from these two methods and from the use of different magnifications in general is examined based on the analyses of a large number of the Danish prehistoric textiles. The results do not indicate superiority of one microscope type in favour of another. Rather, they reveal differences in the calculations that can be ascribed to the diversity of the fibres in the individual yarns as well as to the methodology and the magnification level.