Originally posted by kiwi_jono but it did not occur to me that a coating could actually improve transmission (perhaps due to less reflection?).
That's the main purpose of lens coatings. Each lens-air or lens-lens transition in the lens incurs a little transmission loss, both through absorption & unwanted dispersion/reflection. With say 15 elements in a zoom lens, that's 16 transitions. Without coatings, both internal & external, the losses would be significant. Of equal importance is the effect of the dispersions on contrast. I have an old cheap lens with little/no lens coating, and I always have to boost the contrast in PP. However there's no way that a simple global PP contrast boost can reverse the image muddling created by all those aberrant reflections/dispersions ("ghosting") bouncing around inside the lens.
I suspect this ghosting, and the related veiling lens glare in high contrast situations (e.g. the area of unwanted glare around street light shots), are significant contributors to perceived image quality loss. The reduction of reflections/ghosting may be an important component of the "pixie dust" that separates the great lenses from the good ones.
http://www.imatest.com/support/image-quality/veiling-glare/ http://www.imatest.com/docs/veilingglare/
Since primes usually have less elements, I suspect this is one of the reasons they generally look better than zooms.
Another purpose of coatings is protection. Hence Pentax's SP coating is claimed to make it easier to clean the external surface of the front element. (I presume it's on the outside of the exit element, as well.)
Note Dr Nasse's mention of the unsophisticated coating on top of the DSLR sensor, creating some reflection from the top of the sensor (typically the IR filter coating) back to the lens.
Dan.
Last edited by dosdan; 09-27-2014 at 03:10 AM.