Some of the truths about lenses for “prism reflexes” by Herbert Keppler in his book, Keppler on the Eye-Level Reflex (1960).

1. No two lenses, even if they are the same aperture, the same focal length and the same make, are exactly alike in performance.
2. The best camera and lens makers, while producing the best lenses, also let a number of “dogs” get by (in error, we hope).
3. While no lens is perfect, some few lenses are capable of astounding performance.
4. A company which produces one astounding lens is quite capable of making another which is equally as bad as the first is good.
5. Almost no lens made for single-lens reflexes delivers its best performance at full opening. Almost all produce better results at f/5.6 and f/8.
6. The quality of lenses can only be discussed in comparison with other lenses since there is no practical-to-use standard of optical perfection.