Reciprocity failure may sound complex, but it's an essential concept for photographers using film. In this blog post, I break down reciprocity failure in simple terms, explaining how it affects exposure and how you can overcome it to guarantee that you'll accurately expose to capture your stunning images.
Don't want to shoot long exposures on film?
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Overview of Reciprocity Failure
When taking photos, we rely on the reciprocity law: the relationship between the intensity of light and the duration of exposure. However, in certain conditions, such as very long or very short exposures, this law breaks down. This phenomenon is known as reciprocity failure.
How Reciprocity Failure Impacts Exposure
Reciprocity failure means that film doesn't respond to light in a linear way during extreme exposure conditions. In practical terms, it means that the exposure time needed for a proper image may not be what you'd expect. For example, a long exposure might need to be even longer than anticipated to achieve the correct brightness.
Photographers often encounter reciprocity failure in low light conditions or when using long exposure times, such as in night photography or when capturing star trails. Without compensating for reciprocity failure, images may well turn out underexposed or lacking in detail.
Adjusting exposure times to compensate for reciprocity failure is crucial for achieving accurate and well-exposed images when working with black and white film. This adjustment ensures that your images retain the desired level of brightness and detail, even in challenging lighting conditions.
Powering Up
Right, firstly if you hate maths then you can skip this bit. I'm AWFUL at maths - like properly terrible! It baffles me, however going through this next section really helped me out, weirdly.
I never really found any decent tutorials explaining this concept and I can't remember too much from learning maths school, hence why I'm writing this now! Fear not though, it's not rocket science! I'll break it down using a simple mathematical concept: the power of, using a special number called the "P factor." Here goes...
Crunching the Numbers
Here's the formula: Ta = Tm, to the power of P
Ta: Time, adjusted. The adjusted exposure time we're aiming for.
Tm: Time, metered. Your original metered exposure time.
P: Power Of The magic number, specific to your film type
So, by raising your original exposure time to the power of your film's special P factor, you're essentially giving it a boost to handle those tricky lighting situations like a pro.
What Are Powers?
Imagine you have a superpower, let's call it "multiplying power." This power allows you to make numbers bigger by multiplying them by themselves a certain number of times. The number of times you multiply is called the "power."
For example, let's say you have the number 2 and you want to use your multiplying power with a power of 2. You'd multiply 2 by itself twice:
2×2=4
So, using your multiplying power with a power of 2 turned the number 2 into 4. What about the power of 3?
2×2×2=8
How Do Powers Work?
Powers work by repeating multiplication. The base number (in our example, 2) gets multiplied by itself the number of times specified by the power.
Here's a breakdown:
Base Number: The number you're starting with (e.g., 2).
Power: The number of times you're multiplying the base number by itself (e.g., 3).
Result: The final answer after all the multiplication (e.g., 8).
So, when you raise a number to a power, you're essentially saying, "Take this number and multiply it by itself this many times!"
Nearly there, now back to photography
Let's connect the concept of powers back to Ilford HP5, making it relatable to our photography journey.
Applying Powers to Ilford HP5
Just like we've explored powers using simple numbers like 2, we can apply the same concept when working with Ilford HP5 film. Each type of film has its own unique "reciprocity factor," denoted as P, which helps us adjust exposure times to combat reciprocity failure.
For instance, let's say the reciprocitypower factor for Ilford HP5 is P=1.31. This means that when we raise our metered exposure time to the power of 1.31, we're essentially giving it a boost to overcome reciprocity failure and ensure our images turn out just right.
So, if our metered exposure time is 2 seconds, we'd calculate the adjusted exposure time using the formula:
Ta=TmP
Ta=2 to the power of 1.31 (written either with the 1.31 as superscripy above the line, or with a carrot [lol, great name!], which is one of these: ^. Therefore, it'd be written as Ta=2^1.31
Therefore Ta when calculated is 2.47 seconds.
In this example, by applying the power of 1.31 to our metered exposure time of 2 seconds, we calculate an adjusted exposure time of approximately 2.47 seconds.
Just like our powers examples with simple numbers, understanding the power factor of Ilford HP5 helps us manipulate exposure times to achieve optimal results, even in challenging lighting conditions. It's like having a secret weapon in our photography arsenal, allowing us to capture stunning images with confidence.
Here's another example , this time for a metered exposure time of 8 seconds:
Ta=8^1.31
which gives us an exposure time of 15.24 seconds. Personally, I'd just round this down to just be 15 seconds.
Using a phone calculator to work out your exposures
You can see me doing this in action in this video here.
If you use an iPhone, then go to the calculator app and turn your phone sideways to reveal the scientific calculator symbols. You're looking for this one:
Which is here:
And here's a short vid to show the 8 second example above:
Can I just Use an App?
Yup... You might be wondering if there's an easier way to handle exposure adjustments for reciprocity failure, perhaps by relying on a handy app. Well, the good news is, there are indeed a couple of apps available on app stores designed to help with exposure calculations for film photography, including reciprocity failure adjustments.
While these apps can certainly be convenient, there are some virtues to doing things yourself when it comes to exposure calculations. Here's why:
Understanding the Process: By manually calculating exposure adjustments, you gain a deeper understanding of how reciprocity failure affects your images and how to compensate for it. This knowledge can empower you to make informed decisions and adapt to various shooting conditions more effectively.
Flexibility and Adaptability: Relying on your own calculations allows for greater flexibility and adaptability in the field. You're not dependent on a digital tool that may or may not be available or accurate in every situation. Being able to adjust on the fly based on your own understanding and experience is invaluable.
Building Skills: Mastering exposure calculations is a fundamental skill for any film photographer. It's like learning to ride a bike without training wheels – once you've got the hang of it, you'll feel more confident and capable in your craft. Plus, it's a great way to flex your mental muscles and keep your brain sharp!
Personalisation: Every photographer's shooting style and preferences are unique. By doing your own exposure calculations, you can tailor your adjustments to suit your specific needs and creative vision. It's a more personalised approach that allows you to fine-tune your results to perfection.
It Slows You Down: This has ben shouted about for aeons now, (and if you're a film shooter (if you've got this far you almost definitely are!) but in today's fast-paced digital world, there's a certain charm in slowing down and taking the time to calculate exposure adjustments manually. It forces you to be present in the moment, to engage with your surroundings, and to truly think about each shot before pressing the shutter. This mindful approach can lead to more deliberate and thoughtful photography, resulting in images that resonate on a deeper level. So, while it may take a bit more time and effort, the process of doing things yourself can ultimately enrich your photographic experience and lead to more meaningful results.
So while smartphone apps can be handy tools, there's something to be said for rolling up your sleeves and crunching the numbers yourself. It's all part of the rewarding journey of mastering the art and science of film photography.
This post has gone on for a bit now, so rather than just rambling on further with examples of reciprocity charts and the like, I'll finish with the official Ilford compensation data, which you can find here in any case.
I hope you've found this post useful. If you did, please feel free to share it with others who might also find it useful, bookmark it, or whatevs. I'm going to chuck a couple of affiliate links below for film on Amazon, but really you should be supporting your local retailers. I get all of my film from Mathers of Lancashire in Bolton, so be sure to check them out. They don't pay me or sponsor me or anything, they don't even know I'm typing this bit now, but they're really friendly and their film is very well priced.
Finally, if you read all of this then happy days and thanks, you nutter! Hope you found it useful 🙂
Cheers,
Ol
Link to Analogue Stuff on Amazon*
Ilford HP5 film - https://amzn.to/3QQcJ8E
Ilford Delta 100 film - https://amzn.to/3R3unGi
RALENO LED panel for digitising at home - https://amzn.to/3ynJFz4
Sous Vide for developing at home - https://amzn.to/3WJhftl
*Hey! These links are affiliate links, and they'll take you through to Amazon if you wanted to check anything out. I'm sure you've seen messages like this before, but in case you've never seen an affiliate link or don't know what one is it is simply a link that earns me a small commission for referring you to Amazon for your purchases to help with the costs of running this site, and it won't cost you anything extra. Thanks!
Don't fancy buying anything off Amazon but still want to help me out a bit, then you can always buy me a brew 🙂
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