Blue Filter question

[RaglanSurf]

Shooting black and white film with a blue filter, what effect does this have on blue subjects, blue eyes, bluebells etc?

 

[Harlequin565]

If I've got this right, anything that's the colour of the filter is passed through and becomes brighter whilst the opposite colour (red/orange tones in this case) isn't passed through (i.e. it doesn't expose as well and thus becomes darker). Look up the colour wheel to see which is opposite to the shade of blue filter you have.

I would imagine the blue bits would get lighter and the red bits would get darker. So whilst blue eyes would lighten up, their skin tones would likely darken assuming a Caucasian skin.

 

[ancient_mariner]

Blue stuff becomes relatively bright compared to normal.

 

[woof woof]

Just out of interest and maybe those who know can tell me, is this what would happen with film?

Colour picture, a straight conversion to B&W in Silver Efex with nothing else done and then a Blue filter added.

 

[StephenM]

If I've got this right, anything that's the colour of the filter is passed through and becomes brighter whilst the opposite colour (red/orange tones in this case) isn't passed through (i.e. it doesn't expose as well and thus becomes darker). Look up the colour wheel to see which is opposite to the shade of blue filter you have.

I would imagine the blue bits would get lighter and the red bits would get darker. So whilst blue eyes would lighten up, their skin tones would likely darken assuming a Caucasian skin.

True as far as it goes, with a caveat...

Very few colours (none?) in nature are purely monochromatic, so you'll never get a "pure" effect. Not blue per se, but bluebells are notoriously difficult to photograph with colour film because they reflect fairly strongly in the deep red/infra red region, and tend to come out pink. This principle applies across the board. As an aside, that's why James Clerk Maxwell's tartan ribbon photo made in the 1860s using three coloured filters and film sensitive almost only to blue light worked - even red objects reflected some blue etc. (Pedantic edit - from memory, I think it was ultra violet (or "ultra violent" as I prefer to think of it) light, rather than blue, but it can be easily looked up somewhere I'm sure.)

A blue filter will effectively convert a panchromatic film to an ortho one, so expect lips to be darker (why the old silent film stars wore blue lipstick to make their lips look normal) and freckles to show up more. And, as everyone knows a red filter cuts through haze, blue has the opposite effect.

If you have a digital colour image, you can experiment with the channels in PhotoShop (if you have it) to examine exactly what happens with a single colour channel in terms of colour values. Those who were bored enough to download my (unreadable and boring) manuscript may examine a series of different effects from the same colour image.

 

[StephenM]

Just out of interest and maybe those who know can tell me, is this what would happen with film?

Colour picture, a straight conversion to B&W in Silver Efex with nothing else done and then a Blue filter added.

Yes

Good example.

 

[Harlequin565]

is this what would happen with film?

Possibly. Different films have different responses to different colours. You wouldn't get that exact effect with every film, but it's close enough to provide a good example.

For example, some films "see" deeper into the IR spectrum and can be classed as IR films. Put a red filter on HP5 (standard film), a red filter on SFX 200 (near IR film) and a red filter on something like Kodak HIE (true IR film) and you would get very different effects from the same scene. [Edit, having written this I realise that you would get very different effects without a filter so whilst the filter will do the same thing, each film has different sensitivity to different colours if that makes sense...]

That's my understanding anyway.