I must admit, reciprocity failure was a foreign concept to me when I started shooting film. "Recipro-what?"
Reciprocity Failure 101
Experts (and the Internet) say film is made of really tiny silver crystals that react to light, and I believe them. But those crystals can only react for a limited time while keeping their original properties.
This means that you don't have to worry if your exposure time is that of half a second or faster. Things start getting tricky the longer your exposure gets though - and most of my long exposures are somewhere between 2:30 and 20 minutes.
Not only film becomes less and less sensitive to light, but the way it "absorbs" it changes as well. Highlights get recorded much faster than shadows, creating a more contrasty image. I actually like this since my images have high contrast, but if you don't then you have to shorten your developing time.
Anyway, coming back to the time issue. In practice this means your exposure will be much longer than the time your meter indicates. How long? That's a good question, one that has been widely answered online so it should be fairly easy to find the adjusted exposure time for whatever film stock you are using (just note that some are better for long exposure than others). I don't remember where I got my times from, but they work.
I needed a different (and better) approach
Yesterday, I made a mistake. Somehow, I was reading the wrong values. Luckily, I underexposed the images by just 1 stop so it wasn't a big deal after all. But I decided to change my workflow to make it easier.
When I want to take a long exposure, I first think of a good time for the scene. For example, seascapes that include some fixed objects (be it a rock or the shore) usually look the better to me between 5 and 10 minutes. Cloudy skies vary a lot depending on how fast those clouds are moving. But you usually want shorter times here. And so on.
Adjusting my camera settings and choosing the right ND filter to get my desired exposure time isn't easy task. There are two conversions I have to make: first the N stops the ND filter takes from the scene and then the reciprocity failure. And that has led me to make some mistakes, like yesterday.
So I decided to make a little table to help me achieve what I'm looking for in the field much easier and faster. Instead of starting with the indicated time and converting it to the adjusted time, this table assumes you know the final exposure time and it tells you the settings to use. It's for HP5+ but if you get the idea you could easily make one for your favorite film stock.
Table: HP5+ using Lee ND filters
I hope this helps someone!