If all you're looking for in a lens is wide-open sharpness (ie you don't care about size, weight, handling, filter thread size, DOF, bokeh, character, price etc), The recent Erwin Puts comment about lens reviews is particularly insightful.
I quote below:
"The case of the 75mm lenses
A recent review of the Leica and Cosina 75mm lenses may illustrate these points. Both lenses have been extensively tested and reviewed all over the internet and in many printed publications, and MTF graphs are available too. One may ask what is the added value of the constant repetition of known facts. In this context I am always reminded of one of the classical stories of Donald Duck: Duck is asking money from a rich foundation to discover America. The obvious reply is that America has already been discovered, and the answer by Duck is: yes, but not yet by me.
The general conclusion from the review: both lenses perform quite well and are of comparable quality is nothing new and the more impressionistic comments depend heavily on the particular choices in the imaging and viewing chain.
I have on my desk the measured results of the two lenses, Leica and Cosina. Here we have facts that have been collected for the lenses without any additional components from the imaging chain and are representative for the intrinsic quality of the lenses. The field of view corresponds to the normal 24x36mm frame and is not restricted to the smaller angle of view when one uses results that relate to the smaller M8 or Epson sensor. These results neglect the important outer parts of the capture area on the negative area as used in film-loading cameras like the M6 or M7 or MP. Comparing the MTF graphs we note that in the centre of the image and wide open both lenses perform equally well, but when looking at the outer zones there is a significant difference in correction philosophy. Leica has opted for a higher contrast, but allows more curvature of field and more astigmatism, where Cosina goes for lower contrast and less astigmatism. This correction philosophy has impact on the out of focus (bo keh) character of the lens. It is also intriguing to see that the Cosina designer has opted for a higher contrast in the sagittal direction, where the Leica designer has selected the tangential direction to be of higher contrast. The main differences between both lenses in this area of definition can be found in the 40 lp/mm of resolution. Most photographers have difficulties to find scenes where this level of definition can be captured. In itself this is interesting, but one needs to know that the higher resolutions are not only needed for the definition of fine detail, but also for better edge contrast at the lower frequencies.
The bigger differences between the two lenses are in the area of chromatic correction, where the Leica can show a much better apo-like correction. The performance of the Leica in the wavelength range of 400 to 500 micron is much better (by a factor of three). The upshot is that pictures taken with the Leica lens in scenes with a dominant blue light have a significantly higher contrast.
The illumination of the full image circle (radius 22mm) is quite even in the case of the Leica lens and worse in the case of the Cosina lens. You do not only note that in the amount of vignetting at the corners: the Leica at 2.8 is much better than the Cosina at 2.8 (difference is more than one stop). The illumination differences between Leica and Cosina at full aperture do present themselves in a hot spot in the centre of the image for the Cosina where the Leica has not such a spot. Transmission measurements also indicate that the Cosina is in effect closer to a true full aperture of 2.8 and not 2.5.
On a more practical level we may say that the actual comparison between both lenses is between a 2/75 and a 2.8/75. Then the performance advantages of the Leica lens are more impressive as a 2.8 lens is much simpler to correct to a high level of performance.
The Cosina lens exhibits a higher level of focus shift than the Leica lens. The focus shift for the Cosina lens is within the 0.03mm CoC tolerance area, but when one wants to have high magnification prints, the focus shift could become visible.
Upshot
At an aperture of f/4 both lenses perform visually equally well and that is what most reviewers will conclude. But such a conclusion can be made for most high quality lenses, especially in the realm of the M-bayonet lenses: lens design here has a long history and with modern design tools one can create good designs without much trouble. The step from a good to a superb design is not so easy, but then the naked eye might not be the best tool to detect the fine differences. The eye as an instrument for measurement is notoriously weak and one needs additional equipment to supplement and correct the conclusions of the eye. There is a dividing line between the seen and unseen and the eye is not good at detecting things one is not aware of. That is the basic but important conclusion of lens comparisons by the eye only: you see what you want to see and what you want to see is also influenced by the way one has configured the imaging and viewing chain.
You cannot rely on MTF measurements alone, and certainly not on one figure merit values (this lens has a score of 78.3 and that lens has a score of 80.1), but also not on personal impressions.
Lens testing such that reliable and representative conclusions can be drawn, is a complicated and time-consuming process. Without sophisticated equipment to back up the visual assessment of printed pictures (not screen analysis) lens reviews are of limited value when one wants to get a valid assessment of the intrinsic lens qualities. "
