Shaping Lights: The five variables of exposure

Last Sunday I taught another Shaping Light Workshop, my fourth. With the time my presentations techniques has changed and with the experience I can focus more on one aspect or another, depending by the photographers I teach to. I am still learning to teach and to help the others to get creative with the models and this is a very important and interesting part for me, a part that will probably never stop.
The Team Photographers: Fabiana and Carlo Nicora Make Up Artist: Nadine Elias Models: Lulabell Harron and Jojo
This time I will linger a little bit on the technical part of the workshop, writing down some of the things that I usually teach and focussing on the variables a photographer has to keep into consideration to achieve the perfect exposition

Jojo lying on a slightly reflective sheet of perspex and shot from a height of 3m.

The five variables of exposition When lighting a scene with a flash, there are five variable that concur to the correct exposition. They are: - Shutter speed - Aperture - ISO - Flash power - Flash to subject distance
When photographing a scene without the use of a flash, only Aperture, Shutter Speed and ISO are relevant, but in a complete different perspective as all of them concur to capture the same kind of light, therefore are interchangeable. Using the flash things change especially because there will be two two different types of light and they will have to be treated differently and controlled in different ways.
The two types of light The one coming from a flash is an instantaneous light. It generally goes off when the first curtains is completely open and it emanate all its power in a heartbeat. The flash take a single instant to push all the light on the film or on the sensor of the camera. The one coming from any source different from a flash is a continuous light. It may come from the sun outside the studio window, or from the bulbs switched on from the make up area. These lights irradiates their light constantly, therefore they hit the film or the camera sensor continuously, from the moment the first curtains starts to open to the moment the second curtains closes. These two types of light are working at the same time and concur to the correct exposure of every photo, but they are different, therefore they have to be controlled in a different way. F-Stops In photography the f-stops is a ratio that quantify the exposure. There is no number that defines the right exposure, but it is a number that defines the difference between one quantity of light and another.

Lulabell confidently posing with a veil wrapped around her face

Shutter speed The shutter speed defines the amount of time in which the light can reach the film or the camera sensor. If the camera is set to 1sec, the amount of light registered in the photo will be much more than the amount available with the camera set to 1/200sec. Keeping in mind that the flash light is instantaneous, it means that there will be no difference to the flash light if the shutter is open for 1/30sec or 1/160sec because the light coming from the flash will be exhausted in a heartbeat. A complete different logic applies to the continuous light sources: they are constant and the more time they have, the more they will reach the film or the camera sensor. It means that there will be a huge difference if the shutter remains open for 1sec or for 1/200sec, because in every moment the shutter is open, more light will come in. This means that the shutter speed does not have the power to control the light coming from the flash, but it can control the light coming from the continuous light sources (sun, bulbs, candles, etcetera...). "The shutter speed controls the exposure of the continuous light"

Jojo playing with a veil scarf, almost dancing in the light

Aperture The aperture defines the amount of light that is allowed to enter through the lens in every single instant. This means that limiting the amount of light that can hit the film or the camera sensor will limit both the flash light and the continuous light. We have told that the light coming from the flash is instantaneous, therefore in a single instant all its power reach the film or the camera sensor, independently by the time the shutter remains open. Using an aperture of f/22 or an aperture of f/1.8 will drastically change the quantity of flash light hitting the film or the camera sensor. Summarising, the aperture may control both flash and continuous light exposure, but having the shutter speed controlling exclusively the continuous light exposure, if there is the need to change the exposure of the light coming from a flash, the aperture is the key for doing it. "The aperture controls the exposure of the flash light"

A detail of a fantastic Make Up provided by Nadine for Lulabell

ISO The ISO (or ASA if you comes from a more "seasoned" photography background as I do) is the sensitivity of your film or of your camera sensor. This means that it works for every kind of light that hits the film or the camera sensor, independently from the type or duration. The ISO controls the exposure in a more broad sense, as it affects the exposure of every light. Generally I try to keep the ISO settings to 200, as in my camera those configurations produce crisper results, but if I need more light all over I can simply switch to ISO400 or even ISO800 without troubles and I will have one to two stops of light more.

Jojo walk is perfect for a catwalk. She knows how to move

Flash power I think that this is pretty simple to understand and it is probably the most direct of the five variables: the more powerful your flash is, the more light it will emanate, the more light will be captured by the film or the camera sensor. This will effect only the area actually hit by its light and it is controllable. One of the things you have to keep into consideration when you select the flash power is that the more power you need, the longer it will take to your flash to be fully recharged. Using every watt you have will drain all the power and the flash will take longer to be ready, but if you use just 1/16 of power, you will be able 16 shots before having spent the same amount of power. The other thing you have to consider is that if your flash is connected to a battery pack, having one stop less in the flash power will allow you to shoot twice the number of frames with a single battery charge.

Sometime simplicity is the best way to start, focussing on the details, as in this profile of Lulabell

Flash to subject distance If the flash power is possibly the easier of the five variable, the distance between the flash and the subject is the hardest to understand and to put to practice. The theory says that every time the distance between the flash and the subject is doubled, the light reaching the subject is 75% lower (or 2 f-stops of light less). The difficulty in the practice is that if my flash is 25cm from my subject and I double that distance, I will lose 2 f-stops in those added 25cm, but if I want to lose other 2 f-stops I will have to move other 50cm the flash away from my subject as the flash to subject distance is 50cm. The same logic applies if I want to lose other 2 f-stops: I will need to move the flash further 1m away. The tricky part is that the first two f-stops were lost in 25cm, while the last were lost in 1m. What does it mean in practice? It means if your subject is close to your flash, a small movement of the subject closer or further away from the flash may alter your exposure sensibly. It also mean that if your subject is far away form your flash, even if he or she will move, the exposure will be practically the same. In the same way you have to think that you can apply the same rule to both your subject and the background. If the flash is 4m from your subject and 4.25m from the background, they will be evenly lit. If your subject is 25cm from your flash and the background if 4m from the flash, the second will be 8 f-stops underexposed (25cm to 50cm - 2 f-stops/50cm to 1m - 4 f-stops/1m to 2m - 6 f-stops/2m to 4m - 8 f-stops). This means that if your subject is far from your background, you can lit perfectly the subject, letting the background go almost black, as 8 f-stops are a huge amount of less light, like passing from 1/15sec to 1/4000sec (1/15sec - 1/30sec - 1/60sec - 1/125sec - 1/250sec - 1/500sec - 1/1000sec - 1/2000sec - 1/4000sec) Try and see the difference: if your exposure is fine for 1/15sec, at 1/4000 your frame will be pitch black!

The ability to move with natural grace is something that Jojo seems to have in her blood

In conclusion When shooting with flash the five variables have to be assimilated. This means more than simply understood. This means that a photographer should know how to achieve a specific lighting settings without thinking too much, it is a process that takes time and efforts, but most of all it takes practice. As a photographer I want to go in front of a model knowing that to get more flash light on her face I can open the aperture and not the shutter speed. This may sound logic right now, but while on the shoot there are so many things to remember that these five information will be hard to remember, but harder will be combining them. Sometimes I want to remain conservative with the power coming out from my flash in order to be able to shoot more frame with a battery pack, therefore I can higher my ISO, open my aperture or reduce the distance between the flash and my subject. Other times I have different constraints, maybe I cannot variate the subject to flash distance and I need a narrower aperture to have a longer depth of field, therefore my options are limited. The thing that every photographer have to remember is that shooting with a flash is like playing with Lego: you can reach the same perfect exposure in many different ways, the only requirement is to know how to combine the various pieces together.

Lulabell on the reflective perspex, shot taken from a high of 3m