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Do you find studio lighting daunting? Understandable. When I got my first set of lights I played with it for half an hour. Intimidated and confused, I shoved it back in the box where it stayed for six months.
Don’t worry though, mastering exposure with studio strobes is easy; in fact it’s probably one of the easiest of the photographic skill sets. By following this tutorial, you can go from no experience with studio lighting, to getting a correct exposure on your first frame, without the aid of expensive and unnecessary light meters. This isn’t a crash course in complicated theories and physics; our goal is to get you using strobes and creating photographs as quickly as possible. You can return to the theories at your convenience. I am a firm believer in the idea that it’s easier to learn the why, when you’ve already figured out the how.
Before we start, you’ll need to do a few things in preparation:
Most of our modern cameras are capable of setting the aperture in increments of 1/3 or 1/2 of a stop. This is extremely useful for fine tuning the exposure in most situations; however, for the purpose of this exercise, it is much easier to ignore them for now and concentrate on the full stop values as indicated in the chart below.
The next step is to flip through your strobe’s manual and find where it lists the Guide Number (also check whether the guide number is listed in feet or meters; this is vital). This magic number is a rather complicated thing and steeped in the brain-wracking Inverse Square Law. As promised, you don’t yet need to know why it works; you only need to know how to use it at this point.
There are two main ways to use the guide number, that will be useful to you. You can choose your aperture based on the desired outcome of your photo and calculate where to put your strobe, or you can place your strobe for a desired lighting effect and calculate the correct aperture.
Some lighting effects require strobes to be in certain positions, and distances from your subject and other strobes. In other circumstances, your strobe might be in a fixed position and it cannot be moved.
In these events, to calculate the aperture for a correct exposure all you have to do is measure the distance between the light source and your subject in feet or meters (whichever your guide number is listed as). Now divide your guide number by that distance. For example a guide number of 66 with a distance of 6 feet between your subject and the light source would result in: 66 divided by 6 for a result of 11. Your answer is your aperture for a correct exposure: f/11.
Most guide numbers never divide so evenly into full stops. For example, if you get a result along the lines of f/9.2, just round it off to the nearest full-stop for the moment and we’ll learn how to fine-tune the exposure shortly.
Conversely, if you know what aperture you want to use, for a creative effect perhaps, simply divide the guide number by your desired aperture. My tendency for studio portraits is f/8, so if you divide our previous example of 66 by 8 (66/8) you get 8.25. To get a correct exposure for this setup, you would just place your light source 8.25 feet away from your subject. It really is that easy.
One caveat: if you have a high powered strobe in a small place, you’re not going to be able to set your camera to something ridiculous like f/64. If your power output is too high, just turn your strobe down to half power and divide the guide number by two.
As you learned, there are instances where you want your aperture to remain constant. You also don’t want to mess with the power settings on your strobe so much that they completely change your guide number and negate your ability to calculate your exposure with ease. Yet subjects move, or you might want to over or underexpose your image for creative effect. The way to do this is simple.
To underexpose or reduce the amount of light falling on your subject, you would move your strobe further away. If your light source is really close to the subject and you move it back about a foot, you will lose 1-2 stops of light (smaller f/number). If your source is farther away from the subject and you move it back one foot, you will lose less light. This is caused by the Inverse Square Law which states: In physics, an inverse-square law is any physical law stating that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity. (and link to the wiki definition). Just know that closer in you lose more light as you back it away, than if the light is situated farther from the subject to begin.
This is also why I asked you to consider thinking in full stops rather than fractional increments. Moving a strobe or a model one or two feet is much easier to judge on the fly than the four inches (10cm) a third of a stop increment would require.
The most common modifiers in studio photography will affect the output of the strobe in terms of exposure. You may need to increase or decrease your aperture, or the distance of your light source, depending on which modifier you choose.
Soft boxes and white or translucent umbrellas, which all produce a softer, more diffused light, will decrease the amount of light falling on your subject. To combat this, choose a larger aperture (smaller f/number) or move your light source closer to your subject.
Beauty dishes, silver umbrellas and reflectors, which all produce a harder, more defined light, will increase the amount of light falling on your subject requiring you to choose a smaller aperture (larger f/number) or to move your light source further from your subject.
If you choose to alter your aperture in these situations, feel free to revert to your 1/3 stop increments. A modifier will rarely alter the output of a strobe by a full stop.
You will quickly learn how your modifiers affect the output of your strobes and within a few sessions you should find yourself automatically compensating for them without thinking about it.
I set up a quick portrait session to help demonstrate how to employ these techniques. Gemma graciously volunteered for the task.
The only preparation was the setup of a paper background and fitting the strobe with a beauty dish. After she arrived, I asked Gemma to stand two feet from the background and I placed the light source straight in front of her and as high possible with the beauty dish pointed downward at her face.
All that was required to start shooting was the correct aperture. The Bowens GM400 I used has a guide number of 76. I measured how far away the light source was; which was 7.5 feet. Dividing 76 by 7.5 gives a result 10.133. Because I was using a silver beauty dish, we know that I need to stop down the aperture to get a precise exposure so I set the camera to f/11 and took a test shot. As you can see from the histogram, these simple calculations gave me the correct exposure and it took less than a minute to get there.
In this image you can see that from the first frame to the last, the exposure remained constant leaving me free to concentrate on other aspects of the images.
Finally, here you can see the end result.
I hope you’re still with me, and that I’ve convinced you that studio strobes are nothing to be afraid of. With a little practice, the techniques outlined in this article will quickly become second nature, allowing you to concentrate less on the technicalities of exposure, and more on aesthetic variables such as composition and establishing rapport with a client or model. As with most things, the key is practice. Snag a friend, a pet or even a bowl of fruit and run through the whole process again and again, until calculating the correct exposure becomes reflex, and you’ll find yourself spending more time and energy on the creative processes rather than the technical ones.
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