Sort the raspberries if necessary, but avoid passing them under water to prevent them from losing their flavor.
Prepare a china cap or a strainer over a bowl, and line it with cheesecloth. Set aside.

Put the raspberries and water in a large pot and bring to a boil.
Cover and let the fruit simmer over low heat for 10 minutes.
Mash the fruit with a potato masher if necessary.

Pour the mixture into the cheesecloth.
Let it drip as much juice as possible, then gather the 4 corners of the cheesecloth and squeeze with your hands to extract the remaining juice.

Pour the raspberry juice with the sugar and lemon juice into a large pot and bring to a boil, stirring.
Maintain the cooking over high heat for about 10 minutes, or until the candy thermometer indicates 220 F (104 C).
Add a small piece of butter and mix. The foam will have disappeared.
You can check the consistency of your jelly by placing a few drops on a very cold plate: the drops should slightly gel.

Put the jelly into jars that have been previously washed and sterilized, then cover with a lid.