A Little Education on Light Ratios

Shooting family Portraits will always make you a better photographer as no two families are the same and each will present their own challenges to overcome.  Before taking the leap in to full time photography, I worked with Bill (the father of the family picutured in this post) on the fire department.  I have had the opportunity watch his family grow and shoot family's portraits a couple times now. 

These were all shot with a simple one light setup consisting of an Elinchrom Ranger Quadra with a 60" Umbrella.  Fortunatley it was an overcast day making locations easy as there were no hard or harsh shadows or areas competing with the minimal latitude of my camera's sensor.  Nonetheless, I did stick to more shaded areas to further cut ambient exposure to where I could tune my strobe to a lower power and keep the overall lighting ratios close for a more natural and inviting look. 

For many phototgraphers new to the world of off camera and artificial lighting, Light Ratios are often a confusing and an afterthought to their technical reasoning and worries of just trying to get decent exposure.  Light ratios should be a the forefront of each and every shoot as they will dictate the contrast and differences of quanity for each light source contributing to the exposure. 
To really understand Light Ratios and we need to understand the F-Stop and what it really is.  The F-Stop is not a measurement of quanity of light, but rather the relative difference between two light sources and their values.  Without going into the dungeons physics and rules of light, we basically know we have expodentially change the value of our light to double the output (See the Inverse Square Law). In a nut shell, if we double the output of a light source, then we have relatively increased our light by 1 Stop. If we decrease our lighting output in half, then we have decreased it by 1 stop.

When we set up our shots for a basic one light outdoor portrait, we are going to take a measurement of each light source and acquire our ratio by the looking at the difference in output from each source.  It's confusing when we state One Light Portrait as although we are only using one source of artificial or strobe light, we still have our second light or ambient exposure.  In the case where I was shooting these Fall Portraits, I used sun as one light source and my Elinchrom Ranger Strobe as the second source.  I wanted more natural feel and less edgy or dramatic look, hence I dialed in for a lower ratio than if I was going for sports, commerical campaigns, or nightlife imagery. 

I did this by simply measuring the output of sun or ambient exposure.  I then carefully walked up the power of my strobe until the output came close to that of the sun, thus giving a light ratio of near 2:1. If I were to match my strobe to the output as the sun, the image would appar flat and be at a 1:1 ratio.

What if I wanted a high ratio that would give me an edgy look?  I would simply adjust my exposure to decrease the output of the ambient light by increasing my shutter speed and perhaps stopping down my aperture.  I could then crank up the power on my strobe so its output was much higher than that of the ambient. 

Take the above image for example, which might I add was one of the largest and most challenging ad campaigns I shot as the images are still waiting to be licensed.  This was shot in an over the top mansion where I blocked nearly all the windows to leave barely enough light so my camera could accurately find focus.  What little ambient light leaked in to the frame was easily killed by shooting at a fast enough shutter speed so it wouldn't contribute to the exposure.  I then started sculpting my light by adding the key light (highest output) light frame left as a rim light.  After locking in that light's power setting and output to correlate where I set my aperture and depth of field, I then started adding fill lights to obtain the ratio I desired.
A second light was placed frame right with it's output set to nearly 2 stops less than the rim/key light, giving me effectively a 4:1 ratio.  I then added a small speed light on the counter and behind the models to fill in background.  I set this light's output to a power close to the main fill light's, giving me a lower ratio.

As I don't yet own a light meter, all these measurements were relative and shot from the hip so to speak.  I have enough experience to where I can generally use the best light meter I have access to- my eyes and brain! 
Here is a basic graph showing common light ratios and their potential use:

 More examples of setups with higher ratios nearing 4:1

One Light Setup with a 2:1 Ratio, where I balanced the strobe with the ambient light.

I challenge each of you to study images you see on the big screen, TV, and print and figure out the lighting ratios they used and why. How did it affect the mood?  How did it fit the theme or scene?  We will be better story tellers with our images if we simply take some time to learn, study and apply the technical side of the medium.  So much time is devoted to lighting modifiers and placement and not the very ratios that determine the exposure that directly correlates with the outcome of the image.  Spend as much time balancing your lighting as you do placing it and applying modifiers.

More to come. This new blog host has forced me to learn HTML and I am no coder.  

Happy Shooting, 
Ben.