CanonFanBoy said:
wockawocka said:
CanonFanBoy said:
Again, no BR (Blue Goo). I have to now wonder why BR has just about disappeared. Then again, this lens might have been designed before BR was developed.
Not a wide enough lens to need it.
A 600mm DO with BR is rumored to be coming. I also thought that maybe the longer focal lengths wouldn't see a benefit. Since reading about the 600mm DO, I have no idea why.
Maybe it is only used with lenses that need help with CA?
That's true. BR is just another refractive material with another type of dispersion. Dispersion is a generic property of each optical material which measures how large is the refractive index (= capacity to alter the light path in optical elements) for different wavelengths (= colors). Dispersion is never a constant function, it has different refractive index values for different colors. A lens (e.g. a simple magnifying glass) has different focal lengths for different wavelengths / colors. That is the reason for CAs.
The art of lens design means that you have to combine two lenses with DIFFERENT DISPERSION to create a constant function for the dispersion of the lens system - same focal length for each wavelength / color.
My idea: Canon used the BR material just to add another type of "glass" which is not solid hence it is more expensive to integrate it into a lens: create sealings which show long term stability and maybe the material itself isn't that cheap.
As far as I know Canon telephoto lenses are very well corrected, so there is really no CA issue which needs to be compensated with BR material. But introducing a DO element and trying to make the lens light and compact might have been a reason to omit some lens elements and replace it e.g. with a BR group. Think about taking out 5 lenses and replace them by three flatter lenses to make things lighter and make more room for the movement of other lens groups: Then BR can be helpful while it is usually more important to wide angles.
The 1.4 85 hasn't got a BR element so maybe size / weight / pricing constraints lead to a non-BR design. And everything between 85 and 135mm for FF was always very good, just the great FD 2.5 135 S.C. which suffers only from LOCA wide open.
