Monday, August 16, 2004

How to calculate Exposure Values (EV)


Mark M. Hancock / © The Dallas Morning News

McKamy Middle School 7th-grader William Dutcher counts donated pencils at the school in Flower Mound on Friday, October 24, 2003. Spanish class students in the school's Pan American Student Forum of Texas organization collected school supplies to share with students in Mexico and at Central Elementary in the Lewisville ISD.

I'll talk more about exposure, dynamic range and the Zone System in future blog entries. Today, we're going to change the capabilities and/or appearance of a known light source through Exposure Value (EV) calculations.

I'll keep this simple to reinforce the process. The common example uses a (figurative) bucket of water. Light is to a properly exposed image as water is to filling a bucket. We can change the diameter of the water hose (aperture) and thus the amount of time it takes to fill the bucket (shutter speed) or even the size of the bucket (film speed / ISO). All three ways fill a (figurative) bucket with the correct amount of water. Each approach has a use and trade-off. Each approach affects the other two variables.

An EV is an equivalency measurement of light. I'm trying to skip explaining the hard stuff, so this is one a situation where readers can either take my word or look it up at How Stuff Works.

Here are some examples of EVs:

1)   f/5.6   1/60   100iso   (0+0+0=0)
2)   f/2.8   1/250   100iso   (+2-2+0=0)
3)   f/5.6   1/250   400iso   (0-2+2=0)
4)   f/2.8   1/4000   1600iso   (+2-6+4=0)

All of the examples capture the same measurement of light. There are many more examples I could list to accomplish this same amount of light. The "f" number is the aperture setting. The divisional number is the shutter speed measured in fractions of a second. The ISO is the responsive speed of the film (or equivalent digital setting).

All three of these measurements can be divided into 1/3 increments. Again, it would be really complicated to explain how, but understand each full stop is three clicks away from the next full "stop" of light in each of these measurements on a pro camera and one click away from each other on an average manual SLR camera.

For anyone still reading, we're moving toward getting the camera on the lowest ISO possible for available light on a maximum length lens with the fastest aperture. The point is to show there are multiple ways to get the right amount of light. If PJs bracket exposures in constant light situations, they are only wasting valuable time and film or disk space.

Likewise, if PJs use auto exposure in a constant-light venue and the teams are wearing white or black, there may be a problem. Watch the camera settings for variations. If the settings change a stop or more, PJs need to manually control the exposure.

Yesterday we determined the minimum shutter speed for various lenses. Using a 200mm lens as an example, we need to shoot at a minimum speed of 1/250th of a second. With this one fixed variable, we can determine the other two settings required for a proper exposure of a constant light source.

For today's example, we'll be blessed with a stadium or gym with lots of light. The ambient light meter reads f/4 at 1/60 on 400ISO film. This is the constant light source.

We must adjust the EV to meet one fixed variable (1/250th). Using our example, we have the following whole-stop EV options:

1)   f/4   1/60   400iso   (0+0+0=0)
2)   f/4   1/125   800ISO   (0-1+1=0)
3)   f/4   1/250   1600ISO   (0-2+2=0)
4)   f/2.8   1/125   400ISO   (+1-1+0=0)
5)   f/2.8   1/250   800ISO   (+1-2+1=0)
6)   f/2.8   1/500   1600ISO   (+1-3+2=0)

Each setting yields the proper amount of light for the available light. The bold examples meet the minimum shutter speed requirement. For sports, I would choose option No. 5 as the preferred option. It allows the minimum speed while maintaining a relatively high-quality film speed. The depth of field is tight, but in sports it's best to shoot wide open with long lenses to blur out distracting background while directing viewers to the sharpest portion of the frame (where the PJ wants the viewer to look).

I'm going to save everyone the long list of numbers to do EV calculations for every possible setting. Instead, remember to adjust one full stop at a time and compensate for the movement with one of the other two variables. Then repeat as necessary until the optimal setting is achieved for the desired result.

Enough for now,

2 comments:

v012viper said...

I'm looking at the year posted (2004)... I'm just finding this info!
Thanks Mr Hancock for providing a very comprehensive guide to calculate exposure. Just what I've been looking for.

Mark M. Hancock said...

I'm glad it's still useful. :-)