56x0.5mm Filter - YC-18 for 28mm Konvas lens
This filter was purposed originally for 28mm lens of turret Konvas. But it can be used on other lenses that have 56x0.5mm filter thread - for example, on 35mm lens of turret Konvas. Take note that if will fit old lenses only (RO series made at KMZ). Later LOMO OKS lenses have different thread.
Used condition, needs some cleaning.
Yellow filter is used to shoot sky usually. It helps to distinguish clouds more clearly. Basically, it makes red, orange and yellow colors more bright, but makes blue and cyan color darker. So, it helps to render objects against clear blue sky with higher contrast.