This guest post is by Olivier Lance, who with Pierre-Jean Quilleré launched Sylights in 2009, a wonderful web and mobile application for photographers that helps them document their lighting setups. Olivier, a full time IT engineer from Paris, France, develops MacOS X software for a living. He's also a passionate photographer. Please follow Olivier on Twitter.

Every now and then, when you post a photo to your blog or on a sharing platform, you get the common question: “how did you light it?”

That's a tricky one! How do you go explaining to somebody a full setup of light modifiers, gobos, flash settings, angles, etc.?

The short answer is: lighting diagrams.

Not everyone needs a lighting diagram to understand the lights behind a photography; experienced photographers can usually tell by looking at the shadows’ softness or shapes or at the catch light.

However, I'm not one of them. At least, not yet! So what do lighting diagrams have that can help me? Let's state the obvious first: lighting diagrams show the gear used to light a scene and how many lights were used. They show where the lights were set and how they were directed, relative to the subject.

“Relativity” is an important notion here. I think it's the key when working with lights. What I really see when I look at a lighting diagram is not “where” the main light is, but “how far” it is from the subject. And “how farther” (or closer, larger, etc.) the fill light has to be to do its job just as I see it in the resulting photo.

The same principle applies to flash power. This is a second set of information that's usually provided on or along with lighting diagrams. To correctly reproduce lighting, one has to know how powerful each light was. But it's really “the relationship between the different light levels that is important”, David Hobby tells us in his article Strobe/Ambient balance: A Shorthand Way of Thinking.

See, we're not used to describing amounts of light in an absolute manner. You'd rather say “this room is dark” – referring to what we usually consider a properly lit room – than “wow, it's barely 150 lux in here!”. Both are indicating a light level, but the former is relative whereas the latter is absolute. So a good lighting diagram could just use the main light as a reference and then say “this light is +1/3 of a stop, this other one -1 stop”.

That's why a lighting diagram is such a wonderful document to any photograph. No matter the kind of flash I use and no matter how powerful they are. By using that basic information – relative distances, angles, relative light levels – I can reproduce the lighting over and over again.

That's the main reason I see why photographers should use lighting diagrams: memory. Some photos are easy enough to remember; a lighting diagram will ensure the others are as easily reproducible. Besides the memory perspective, a document as complete as a lighting diagram is also a great way to share your knowledge … but more on that in a minute.

Another very simple reason is that “a picture is worth a thousand words.” If you’re working with one or several assistants, you’d rather be drawing what you have in mind than explaining it at great length several times! This leads me to the second main reason I think photographers should get used to lighting diagrams: teaching. There are so many people learning artificial lighting nowadays: amateur photographers, photography students and pros as well, who want to improve their technique.? To any of them, lighting diagrams are an incredible educational tool! By putting together a photo and its diagram, one can understand the role and effects of each light in a setup. So if you document your photos by using lighting diagrams, you can actually teach other photographers your lighting techniques and help them figure out very easily how you lit your shoots. Diagrams are also a perfect way to keep track of the setups that worked while testing and learning.

Finally, lighting diagrams allow any photographer to prepare a shoot beforehand. Just as I’m using a draft paper to prepare this post, a lighting diagram can easily be drawn to throw ideas, get the ball rolling and have some setups ready before a session. This is the third advantage of lighting diagrams: planning. You can of course use any other technique to prepare a shoot, but the completeness of a lighting diagram and its close representation of a real setup makes it a good choice to start with, before you can actually try your setups and make arrangements.

So, why should you use lighting diagrams as a photographer? Because it’s a great way to prepare your sessions, to document your photos afterwards and to keep track of your lighting setups as well as to share them with others and to teach beginners your techniques.

All those reasons pushed me to create Sylights, a service to photographers I launched a year ago with my friend Pierre-Jean Quilleré. Our idea is to make lighting diagrams easy to use and to base a complete sharing platform on them. Sylights’ goal is to accompany every photographers on their learning path and to store their lighting diagrams for them. We already offer a large range of tools for that purpose: an online diagram editor with a lighting diagram gallery accessible to anyone, an iPhone and an iPad app for when you’re on the go. Many features revolve around those three apps.

We’re still evolving and will gladly accept all your feedback to help us answer those three needs: memory, teaching and planning.

We’d also like to know: how do you use lighting diagrams, and why?