RA-4 negative/reversal process.

Hello everyone, hope finding you well.

I'm interested in starting to experiment with RA-4 process, specificaly photographing with a 4x5 camera instead of making prints out of the paper.

My question is about exposure. I know that many bw photo papers don't provide an ISO indicator. The same happens with the RA-4 paper.

I will use the ADOX RA-4 Printing Paper 10,2x12,7 CM (4x5 INCH) / 50 Sheets and don't know what ISO use for exposure.

Can Anyone give me a clue or advice?

Thank you all for your attencion.

Greetings from Portugal.

Welcome aboard @Mozzafiato !

There's no 'ISO' speed for paper because it is a typical apples & oranges situation. The curve of paper is so different (much, much steeper) than for film that an ISO speed rating would not be comparable to that of film. However, you can use ISO 10-25 as a starting point. You'll have to experiment to get a desirable result; this is in general always the case with direct paper positives. So in the end it doesn't matter that much; just do an experiment and adjust on the basis of the outcome.

Generally speaking, the film speed of RA-4 photo paper we test ranges between ISO 3 and ISO 5. Extending the development time for black and white can achieve around ISO 10. This is also related to the filter you choose, as some filters with strong light transmittance can slightly increase the ISO.