Why does different lens glass give noticeable differences in results? Optics people?

All lenses have compromises in design, let's look at a perfect lens.

A perfect lenses projected image will have no distortion, with be evenly illuminated from corner the corner (no vignetting), will keep focus from corner to corner (flat field), have good colour correction (different colours focus on different planes), have good contrast both colour and Luminance, have no internal reflections to eliminate any tendency to flare (the last two are more to do with the coating than the glass), such a lens has never been made. But some lenses get closer to it.

The larger you make a lens the more accurately all the curves in the lens elements have to be ground, the tolerances get much tighter. The larger format lenses production costs go through the roof compared to lenses made for small sensors.

All you need for any prime lens to focus an image is just two elements (a doublet), but it will certainly not come near to our ideal. All the other elements in a lens are there to resolve some of the issues listed above and to correct for some of the things that correcting for one parameter will make another parameter worse, increase the aperture size and it all gets more difficult, and expensive, as I said there are compromises.

German manufacturers (by and large) made (make) lenses using parameters that can be easily measured such as the resolving power of the lens (lines per mm), some would say that they are 'sharper', but that's not the only thing that determines 'sharpness', sharpness can be defined as the way the lens defines edges, so contrast is probably as important if not more so than just resolution, Japanese lenses (again by and large) concentrate more on contrast than lines per mm, you pays your money and takes your choice.

Now we get down to the nitty gritty of current lens manufacture, by far the majority of lenses these days are designed and made by using CAD/CAM programs, the industry standard program for this is the Sigma program written by them originally to reverse engineer other manufacturers lenses, they made copies of good lenses in the early days so over the years they built up a vast reserve of knowledge about other peoples lenses, then when computers began to be used in manufacture they were ideally placed, they made their program available to all for a license fee which is used by all the major manufacturers today. The upshot of this is that Canon lenses are no better or worse than Nikon or Olympus, or Pentax.

What does differ is perceived value, the market leaders can price their products at a higher price, The so-called Canikon tax. You see people arguing that you should always use OEM products, if they actually tried the cheaper ones they would see little if any difference optically.

Chris

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