It might seem strange that black and white photographers use coloured filters, but there is a reason for this. When you shoot in black and white, the camera or the film is converting different colours into shades of grey. When you use a coloured filter, you’re shifting and changing the brightness of the different colours in the scene, and this changes their shade of grey in the photograph. This is why they’re sometimes called ‘contrast’ filters too. For example, a red filter allows red light through but blocks light of other colours. Anything red in the scene becomes proportionally much brighter, anything opposite to red, like a blue sky, comes out a much darker shade of grey – nearly black, sometimes.
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