Guide for the beginners - Understanding the reciprocity failure in film photography.

One of the challenges that you will encounter in film photography is known as reciprocity failure. I´m not an expert with long exposure photography with film by any means and I try to keep this as short and simple as possible since the internet is already filled with very long-winded overly technical explanations about reciprocity failure. I just want to give beginner film photographers some simple-to-understand instructions. You don´t need a degree in science to get this.

What´s it all about?

Reciprocity failure refers to the phenomenon where the sensitivity of film to light decreases as exposure time increases. This means that as the exposure time becomes longer, the film requires more light to properly expose the image. Understanding reciprocity failure is important to understand because it affects the way the shot is metered and exposed. At what point does reciprocity failure occur, depends on film stock and can vary between different brands and types of film. Usually, when exposing longer than 1-4 seconds you have to compensate for the reduced sensitivity by increasing the exposure time or using a wider aperture to keep the shutter speed high enough. We´ll focus on increasing the exposure time because that´s the relevant thing here.

How to calculate the needed exposure time?

Most film manufacturers include data sheets that will for one provide information with which you can calculate the compensation needed for reciprocity failure. I think many of those data sheets are not clear enough and easy to understand. If you have tried to search for help about calculating time to compensate for reciprocity failure on different forums and such you are usually just faced with a wall of math and technical jargon that can be overwhelming for a beginner or to be honest to anyone. Many manufacturers do luckily include a handy little multiplier number alongside with data sheet that you can easily calculate your new exposing time now including the needed compensation. This multiplier number is typically referred to as the "reciprocity failure factor." Keep in mind, that the reciprocity failure factor is specific to each film stock and varies between different brands and types of film.

Example:

Let's say you are shooting with a film that has a reciprocity failure factor of 1.30 and your metered exposure time is 8 seconds. You would take the 8 to the power of 1.30.

Like this: 8^1.30 = 14.92852786458892. This means you would need to expose the film for approximately 15 seconds to compensate for reciprocity failure. That is all.

Reciprocity failure factor on different film stocks:

Below, I have listed the reciprocity failure factors of several commonly used film stocks for you.

FACTORS FOR ILFORD FILMS

Film / Factor

SFX 1.43

Pan F+ 1.33

D100 1.26

D400 1.41

D3200 1.33

FP4+ 1.26

HP5+ 1.31

XP2 1.31

K100 1.26

K400 1.30

FACTORS FOR ROLLEI FILMS:

Film / Factor

Retro 400s 1.52

FOR COLOUR FILMS:

1.30 is a commonly used factor. It may vary but it´s a good starting point.

Happy shooting!