? "I was also wondering it's prerformed"
i am not sure i understand that statement, if you mean how , several people above me have given you some tips.
When you where a child did youi every make shadows on the wall in your bedroom while you were trying to stay awake? Well , it is the same thing. you create a shadow (i use my hands for 99% of the burning and dodging) between the lens and the paper.
take your hand and with it close to the enlarging lens see what kind of shadow it produces on the easel. bring your hand down and now see what occurs. You can create a cone of light by cuping you fingers and thumb and let the light shine beteen that, you can use just a finger , etc.
there are also wires, think a coat hanger cut into smaller pices , or flora wire which is thinner.
Someone mentioned cardboard, also construct paper will work, be sure the shape you cut out does not have straight edges, make then burry.
You always move your hand, or cardboard, or what ever else you may be using to change the amount of light; if you don't , you will leave a halo which is not very attractive.
If the question is when . then it is when yiou want to change the look of the print. I.E. There are high light arears that are printing pure white. You will need to add light to those areas, and a lot of light. If you want to see detail in the shadows you will need to block some light, with shadows it takes very little time.
you might try to find a book by Larry Bartlett Called Black White Photo graphic WOrkshop. HE has some great tips and examples they will be helpful for you. there are of course other books that can be helpful including the one's terri recommended.
just have fun and remember this all takes practice.
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