What kind of filters are these?

[Andrew O'Neill]

One thing that none has mentioned is that red light focus at a different plane than white light. Just try to focus with the red filter on and then check without it. You will see the difference. Personally, I focus without the red filter over a discarded paper sheet. I only use the red filter to practice the dodgings.

Ditto.

 

[AgX]

One thing that none has mentioned is that red light focus at a different plane than white light. Just try to focus with the red filter on and then check without it. You will see the difference.

A lens that focuses red apparently different than White light is not a good lens to use anyway, as that focus difference between colours remains in printing. (Just that by focusing on the red image you focus at one of the extremes.)

 

[MartinP]

Why would anyone want to focus in red light? I can't think of any conceivable benefit, unless the user is a masochist and enjoys eyestrain.

 

[AgX]

Maybe because he has paper around openly.
Either unexposed or in the developer, that might get fogged by diffuse reflection or direct hit with the head cranked up for cropping.

Think of most condensed darkrooms.



Or just no substitute paper at hand.