Quote Originally Posted by L Gebhardt View Post
I think you should really use Zone I as the exposure to gauge reciprocity failure since it's where you will first loose detail. So based on your numbers it should be exposures greater than 1 second. Then add 1 2/3 of a stop to the exposure. This will also get you closer to Ilford's number.
Thanks for your comment.

My logic was that I normally aim to place shadows where shadow detail is important not lower than Zone III. By targeting the Zone III density, the density difference between II and III will increase as the density of II will be reduced by reciprocity failure while the density of III will remain the same, so my shadow detail will be preserved. However there will be some compression between zones I and II, which I can live with in return for not making the highlights too dense (since they suffer less reciprocity failure); but of course this is a personal decision and your preference is equally valid.

I make 1 2/3 stops above 30 seconds to be about 95 seconds so although using Zone I as the reference brings the result closer to Ilford's recommendation of 155s it still falls well short.