Every lens is designed according to a certain set of parameters. As a rule of thumb, the "best" performance of an enlarger lens will be at the "middle" of the f/stop range. That is, if the f/stops range from f/4 to f/16, optimum performance will be at f/8.Originally Posted byJurajk

Another critical parameter is the optimum magnification ratio. Here, educated projections are made as to the use of the lens. 35mm lenses destined to be used for relatively small enlargements (as would be done by a beginner) will be designed with enlargement ratios of 1:6 or 1:8 in mind. These simple lenses (read: 3 element - inexpensive) will perform well in a range centered at those ratios. This is addressing "best" performance - they have some flexibilty, and do not fall off into the sunset at other ratios.

The more sophisticated designs (four, five, or more elements) will usually be designed for optimum performance at a greater ratios; 1:10 or 1:12, and have greater flexibility.

The large format lenses, for enlarging 8" x 10" or 11" x 14" formats are targeted to ratios of 1:1.5 or 1:2.