LensRentals looks inside the Canon 16-35 f/4 IS
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LensRentals looks inside the Canon 16-35 f/4 IS

This white element is an adjustment collar - great news for lens repairers, especially compared to the more costly 16-35mm F2.8:

'This is a thick, robust collar, similar to what we’ve seen in the 24-70 f/4 IS. Second, the rear group is adjusted with these three collars spaced every 120 degrees. Adjustable collars are nice and precise.  In the 16-35 f/2.8 lens there are no adjustable collars; to center the rear element you loosen three screws and slide it into position by hand, then tighten the screws back down.'