F-stop Understanding Exposure

Lighting should be more of a look or a feel than a technical approach. You should start to develop your own approach or style when it comes to lighting. However, there are some technical aspects that will help you maintain a consistent look and that is what I am going to be discussing in this series of basic lighting concepts such as F-stop and Understanding Exposure.

One of the keys to forming an individual style is understanding the f-stop and in order to fully understand the f-stop we need to start with some basics of photography.

Understanding Exposure

Aperture

The lens aperture controls the diameter of the opening and is sometimes referred to as the “iris”. The Aperture is adjusted by a series of metal blades called the “diaphragm” which regulates the amount of light that pass through the lens to the sensor.

Diaphragm

image courtesy: Wikipedia

The aperture size is important because it controls the amount of light hitting the camera sensor and controls the depth of field.

F-stop and Aperture Size

F-stop or ƒ-number is the ratio of the lens’s focal length to the diameter the opening. It is a dimensionless number, but a very important concept to understand in photography.

The f-stop indicates the aperture size of a lens. f-stops can be confusing because the smaller number indicates a larger opening and a larger number indicates a smaller opening. The f-stop is normally listed on the lens barrel. (see image below)

lens-fstop

The ƒ-number on a lens will reflect the aperture sizes such as: ƒ/1.3, ƒ/2, ƒ/2.8, ƒ/4, ƒ/5.6, ƒ/8, ƒ/11, ƒ/16, ƒ/22

The smallest number (ƒ/1.3) will have the largest opening and will let more light into the camera. The larger number (ƒ/22) will have a smaller opening and will let less light into the camera. (see figure 1)lens-fstop-fig1

Each ƒ-number indicates an opening that will let in twice as much light at a given time, as the next smaller f-number.

Moving from one ƒ-number to the next largest opening increases the exposure by 100%

For example going from ƒ/5.6 to ƒ/4 you will increase your exposure by 100% or let in twice the amount of light that was being let in at ƒ/5.6, because the opening becomes twice as large.

Moving from one ƒ-number to the next smallest opening decreases the exposure by 50%

For example going from ƒ/5.6. to ƒ/8 you will decrease your exposure by 50% or let in half the amount of light that was being let in at ƒ/5.6, because the opening becomes twice as small. (see figure 2)lens-fstop-fig2

F-stop and Lighting

The concept of f-stop is important when selecting a light.

When selecting a light anytime you double the amount of light coming from the source, you are increasing the exposure by one stop. When you cut the amount of light coming from the source in half, you are decreasing the light by one stop.

For example: Let say you want to shoot at a ƒ/5.6. When you set up a 1k light you find out that you need to increase your exposure by 1 more stop of light to shoot with the proper exposure of ƒ/5.6. So you need to add another 1k light which gives you twice the amount of light at 2k and will increase your exposure by 1 stop and allows you to keep your exposure at ƒ/5.6. Remember that any time you double the light to your sensor you’re increasing the exposure by a stop, this is also true with lighting.

Light fixtures are designed to be configured in stops such as 150w, 300w, 650w, 1000w, 2000w, 5000w, 10,000w and etc.

F-stop and Filters

When filming outdoors in daylight you may want to reduce the amount of light entering the lens. A neutral density (ND) filter would be used in front of the lens to do this. When ever you place an ND Filter in front of your lens you need to know how many stops of light will be absorbed and how much light will be transmitted by that filter. This is referred to as the filter factor. ND filters have also been manufactured based on in stops. ND filters come in ND.3, ND.6, ND.9, ND1.2 and etc. (see figure 3) Each filter reduces the light entering the lens by 1 stop.nd-filters

For example, if you are shooting outside and want to shoot at a ƒ/5.6 but you have to set the lens aperture to ƒ/16 to get the proper exposure. So you would place an ND.9 filter in front of the lens to lower the amount of light entering by 3 stops, which would allow you to adjust the lens to ƒ/5.6 for the desired aperture.

F-stop and Shutter Speed

Since most cameras today have variable shutter speeds I wanted to take a second to discuss them. Shutter speed is normally discussed when you want to affect the motion blur of the subject when filming, but you also have to keep in mind that when go from one shutter speed to the next you are increasing or decreasing the exposure by 1 stops increments. Don’t forget that if you change your shutter speed you should change your aperture to compensate for the light loss. Common shutter speeds include 1/50th, 1/125, 1/250, 1/500, 1/1000, 1/2000 and etc. Notice that the shutters double as you go from one to the next so shutter speeds should also be thought of as stops.

Conclusion: F-stop Understanding Exposure

So as you can see when you talk about lighting the concept of the f-stop is important to understand. You should think of everything that effects exposure as f-stops. This not only includes lenses, filters, shutter speed and lighting, but also frame rate, camera gain, iso, scrims, diffusion and more.

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