Tuesday, September 26, 2017

Friday Night Lights (with Crappy Lights) - Part 1

I know, I know, everyone thinks that the lights on their high school athletic fields are the worst but after a little over a year of shooting Friday night HS football games I can safely say that Algonquin high school has the worst lights around. I am an engineer at heart so I am trying to learn the technical aspects of how light impacts exposure and how the available light can be quantified and normalized for comparison. I understand how to utilize the exposure triangle to properly set up my camera and shoot in manual mode almost exclusively but since light is such an important aspect of photography I want to learn as much about it as possible. My primary goal of this blog post is to demonstrate the impact of lighting on picture quality, using high school football as an example, so that people understand why some photos look better than others. My secondary motivation is to justify to myself, and more importantly to my wife, the benefit my new gear provides. This post turned out to be longer than I expected so am going to break it into two parts. In the first part, which you are currently reading, I will present a very basic primer on exposure and try to explain the relationship between shutter speed, ISO speed and aperture and how they need to be balanced to achieve the best exposure settings for the subject matter you are shooting. Knowing the subject matter is critical to setting the exposure because, even in the same lighting conditions, the optimal settings for sports, landscape and portrait work can be very different. In the first part I will provide an objective comparison of available light by normalizing the exposure settings I use on different fields. In part two of the post I will attempt to demonstrate the same principles with a subjective comparison of photos taken with different exposure settings on different fields with very different levels of available light with a couple different cameras. I think that the second part should be far more intuitive and demonstrate how critical adequate lighting is to achieving photos that pop but I wanted to present the theory first.

Exposure Theory

Every photographer will eventually move of the automatic modes to one of the manual modes (for Canon these are M, Av and Tv) in the camera and have to learn how to properly set up their camera. The term exposure refers to exposing the sensor (or film back in the old days) to the correct amount of light for just the right amount of time with the sensor set to the proper sensitivity so that the photograph is not too dark or too bright. In order to quantify the parameters that affect exposure a system called the exposure triangle was developed which consists of three components. Shutter speed, ISO speed and aperture are the three knobs you have to control exposure and adjustments to each parameter have secondary effects that will impact the way a picture looks. We will take a look at how to adjust each of these parameters below and what these secondary effects are and how they impact the final product.

Shutter Speed

Shutter speed is pretty simple to understand - it determines how long the shutter is open and determines how long the sensor will be exposed to the light. The required shutter speed for proper exposure is proportional to the amount of available light so doubling the shutter speed (i.e. going from 1/500th second to 1/1000th second) will reduce the exposure by 2x, or 1 EV, assuming a fixed ISO, aperture and light intensity. Maintaining the same exposure with the higher shutter speed  requires twice the available light. Put another way, you will need to shoot at slower shutter speeds as it gets darker if ISO and aperture are fixed. The secondary effect of shutter speed is how well the camera freezes motion - the faster the object you are photographing moves, the higher the required shutter speed. This is demonstrated on the graphic below where shutter speed is listed in one stop increments and the resulting image blur is depicted in the stick figure.



ISO impacts the sensor's light sensitivity and the required setting for accurate exposure is inversely proportional to the amount of available light -higher ISO speeds correspond to higher sensitivity. For example, for a given aperture and shutter speed, the darker it is the higher you need to set your ISO speed and for a given shutter speed and aperture, doubling the ISO will raise the metered light by 2x or 1 EV. The downside of increased ISO speed is that above some baseline level, which varies from camera to camera, the noise or grain in the picture also increases. This effect is demonstrated in the figure below where the amount of background noise increases with increased ISO speed. Noise can be managed and reduced in post-processing but is generally bad and one of the main reasons that professional sports photographers spend the big bucks on "fast glass", which refers to the aperture of a lens, enabling them to shoot at lower ISO speeds. Aperture is the next subject that will be covered.


I think that aperture is the most difficult concept in photography for people to understand because its relationship to exposure is not linear like it is for ISO and shutter speed. Also, aperture is expressed in a funny notation, like f/1.4 or f/2.8, and does not seem to make sense. The notation simply refers to the ratio of the focal length of the lens (f is for focal length) to the aperture diameter (the number on the bottom, or denominator) and can be used to find the aperture surface area. The aperture surface area determines how much light reaches the sensor and the larger the aperture, the more light gets through. However, unlike ISO and shutter speed, the relationship is exponential because it is based on area (powers of 2). For example, if you have a 50mm f/2.8 lens, the aperture diameter is 50mm/2.8 = 17.9mm. Remembering back to geometry, the surface area of a circle (the aperture) is pi x (diameter/2)^2 = 250.4mm^2. If, on the other hand, you have a 50mm f/1.4 lens, which is a 'faster' lens, then the aperture diameter is 50mm/1.4 = 35.7mm and the aperture area is 1001.8mm^2 or 4 times the surface area of the 50mm f/2.8 lens. Therefore, smaller aperture numbers result in larger aperture opening and more available light. This means that the 50mm f/1.4 lens will allow 4 times as much light, or 2 stop (2 EV), to hit the sensor than the 50mm f/2.8 lens for a given ISO and shutter speed and it will cost at least twice as much. Therefore, aperture scales with the square root of 2 (~1.41) for each stop of light. An f/1.4 lens is 2 stops "faster" than an f/2.8 lens and 1 stop "faster" than an f/2.0 lens. As the available light decreases you want to move to a larger aperture (lower f-number) for a fixed shutter speed and ISO. One of the secondary effects of aperture is that the depth of field, or how much of the subject is in focus, front to back, changes. The wider the aperture, or the lower the f-number, the narrow the depth of field. This can be used as an artistic element or it can present challenges to sports shooters if the depth of field is too narrow.  The other secondary effect of aperture is cost - the price of lenses seem to scale exponentially with aperture size as it is much more expensive to manufacture large pieces of flourite glass which is required for sharp, fast lenses. The graphic below shows the trade-offs between aperture, depth of field and cost.

Exposure Example

Finally, to pull it all together, the following illustrations demonstrate how to change either aperture, shutter speed or ISO speed to maintain constant exposure. Each figure shows shutter speed, aperture and ISO in one stop (1 EV) steps and arrows indicate the impact to exposure for adjustments in each direction. All three parameters have been plotted so that shifts to the left reduce exposure and shifts to the right increase exposure. If any one parameter is changed then some combination of the other two parameter need to be adjusted an equal number of steps in the opposite direction to maintain constant exposure.

Consider the following example. Assume that we are shooting a night football game and initially set the shutter speed to 1/500 second, the aperture to f/2.8 and ISO to 6400 as shown in the first plot. After a few shots we find that we are seeing too much motion blur and want to increase the shutter speed.  What are the options?

One possible solution is shown below - if we increase the shutter speed to 1/1000 second to freeze the motion and maintain the same aperture (f/2.8) then the ISO must increase to 12,800. Therefore, the shutter speed moved 1 stop to the left and ISO moved one stop to the right. The sliders are analogous to a balance scale - any change in one direction needs to be balanced by an equal number of steps in the opposite direction.

Another possible solution would be to maintain the same ISO of 6400 and move the aperture to f/2.0 if we are lucky enough to have such a fast lens. Again, for the one stop to the left in shutter speed we moved ISO one stop to the right.

If the shutter speed moves one more stop to the left, to 1/2000 second, then the three most likely options would be:
  1. Move the aperture one stop to f/2.0 and the ISO one stop to 12,800
  2. Move the aperture two stops to f/1.4 and leave ISO at 6,400
  3. Leave the aperture at f/2.8 and move the ISO two stop to 25,600
All three choices produce the same exposure but they all have slightly different side effects. Choice 2 will (probably) produce the best quality photo but is the most expensive as an f/1.4 lens will be quite costly. Option 3 is the least expensive but will produce the noisiest photo since the ISO is so high. Choice 2 is the best compromise and should produce an acceptable photo. 

Comparing Available Light based On Exposure

I wish that I owned I light meter so that I could quantify the light level at each of the football fields I shoot at but I will have to settle for an online EV calculator I found for this comparison that estimates the available light from exposure settings (shutter speed, aperture and ISO). The calculator allows you to convert an in-camera exposure setting at a specific ISO to any other ISO for comparison. The calculator also computes the light intensity, expressed in terms of EV (Exposure Value), and normalizes this to ISO 100. Most cameras meter available light (usually reflected light for photography) and express it in term of +/- EV on the light meter. EV is in powers of 2 so each increment of one or decrement of one is a doubling or halving of light, respectively. In photography each 1 EV step is referred to as a stop. ISO, shutter speed and aperture will be adjusted based on the subject matter and the available light.

After half time, when the sun has completely set, I was setting exposure based on only the available reflected light from the field lights and I was shooting with a shutter speed of 1/1000 second to stop action as much as possible, aperture of f/2.8 and ISO 20,000-25,600. According to the EV calculator this is equivalent to 5 EV, or approximately 80 LUX, at ISO 100 with the same shutter speed and aperture. That is dark! During the most recent game that I shot at Wachusett I was also shooting at 1/1000 second and using an f/2.8 lens but my ISO speed was between 3200 and 6400 for most of the game. That means the field was at 7-8 EV and lights at Wachusett are between 4 and 8 times brighter than the lights at Algonquin. From what I could find on line most college and NFL stadiums are 8-9 EV, which is 3-4 stops more available light (that is 8-16 times more light) allowing me to drop from ISO 25,600 to at least 3200 if not 1600 in a good stadium. One can only dream.

I will post part two in the near future and show examples of photos taken under each of the lighting conditions described.

Gear used in this comparison:

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