The Cokin Z-Pro 027 81B filter controls the bluish coloration that affects daylight film. It can also remove excessive blue from the effects of electronic flash.
It's autumn. The clouds are low hanging and the sky is "patchy blue". You've gone for a ride upstate to the mountains and the light in open shade is bluish. A photo taken of your girlfriend leaning against the side of a barn with daylight balanced film will yield bluish tones that she might not find particularly flattering.
An 81B filter will replace even more yellow than an 81A and bring her skin tones back to a more appealing level. An 81B can also be used with electronic flash to reduce the amount of "blue" generated by the flash. The 81 series (81A, B, C, D and EF) allows you to choose how much warmth you wish to replace in the image.
Cokin makes the 81 series of filters in many sizes to accommodate the many uses this filter has. This filter can be used by the pro and amateur alike, weekend photo warrior or pro cinematographer and must be considered an essential addition to any photo arsenal.
The Z-PRO Series is recommended for focal lengths of 20mm onwards (in the 35mm format). This filter is made of CR39 Resin which is a reputable lightweight and unbreakable optical material also used for ophthalmic glasses.