Wednesday, February 1, 2012

Understanding exposure - a precursor



One of the hardest concepts to understand in photography is exposure. Partly because it is an abstract concept rather than an apparently obvious and tangible action and partly because there are so many elements involved. One other problem that is seldom addressed is the fact that terms are thrown around with complete abandon. It's not just the novice photographer to blame for this, pros do it too.

In this article I will introduce you to the following acronyms and their meanings; Ev (exposure value), Lv (luminance value), Av (aperture value) and Tv (time value). Some of you may be familiar with the more common A and S for aperture and time (shutter) respectively.

Vocabulary is such a fluid and organic system that we tend to allude meaning to terms we use on a regular basis even though they are used in the wrong context. Because we understand the system behind it, we get lazy with language. So as a precursor to getting into understanding exposure let me go over some terminology.

A camera records an image by exposing film or digital sensor to light entering through a lens for a predetermined length of time. The size of the opening in a lens determines the quantitative amount of light that enters the tube. This opening is known as aperture or more commonly as the f-stop. The length of time the film or sensor is exposed to the light is known as the exposure time or length of exposure or more simply as shutter speed.

The trick is knowing how long to open the shutter to the light in order to record a properly exposed image. For that we have to do some math but before we can do the math we have to assign values to all our elements in order to make a calculation. I want to point out that from this point on we will assume that ISO is set to 100.

So what do we have to work with? First we have our lens opening then our shutter speed and finally that all elusive light value. For our calculation we are going to assign some algebraic numbers to these elements; Av is going to represent our lens opening or our aperture. Tv is going to represent the length of time the film/sensor gets exposed to the light or our shutter speed. Finally our light is going to be designated as Lv for luminance value.

While there is a whole science behind the term luminance, suffice it to say that it is a measure of light intensity (in candelas) that passes through, reflects from or is emitted by an object. Simply put it is how bright the surface of a subject is. Now I know what you're thinking, "light changes throughout the day. How do we know what values are what throughout the day?" That is where an exposure value table comes into play.

Ever hear of the "Sunny 16" rule? This rule states that in bright sun with a camera set to ISO 100 and an aperture of f/16 your shutter speed will be the closest speed to your ISO in full stops. In this case 1/125th of a second is the closest to ISO 100. There is a whole table built around this rule and it was something film guys had to memorize. It gives an exposure value to any given light situation from full sun to waning moon (more on that later). This value is going to be represented by Ev and is associated with several combinations of Av and Tv.

Now here is where the tossing around of terms came to be confusing. The value given to a specific amount of light reflected from a subject is called luminance value (Lv) but in our table you will see it is referred to as an exposure value (Ev) by photographers. Keep in mind that even though it is labeled Ev it is actually referencing a luminance value. Ev is a value as it relates to our aperture/shutter combinations. This was done to simplify the reference to the various aperture and shutter speed combinations for that given light value (Lv).


We now have all our elements; AvTvLv and Ev. At right is our equation for calculating an exposure value (Ev) from the camera's settings, the math problem I mentioned earlier. Looks foreboding doesn't it? I thought so too. Luckily we really don't have to know this but stick with me so you understand how the exposure value table calculates aperture/shutter combinations.

Now let's put some numbers to our values. ISO is film speed and the base ISO for most exposure value tables is ISO 100. For this formula to work you need to divide your working ISO into the base ISO of 100. As I mentioned above, we are working with the assumption that we are at ISO 100. NOTE: The second formula allows for adding an ISO factor into the formula.

Av refers to our lens opening and we know this to be our f-stop and it is represented by a fractional amount of one over our lens opening. For our example, let's say we have a 70-300mm lens with an aperture range of 5.6 to 22. One over 5.6 is our aperture of f/5.6 and one over 22 is our aperture of f/22. So the biggest our lens can open is f/5.6 and the smallest opening is f/22 ( 1/5 is bigger than 1/22).

We also have our camera, a nice dSLR with manual mode. The shutter speed is also a fractional amount and ranges from 'B' for bulb (which allows us to keep the shutter wide open) all the way up the range to a whopping 8000 or one over 8000th of a second. That's fast. This will give us our shutter speed or our Tv value.

To calculate an exposure value (Ev) you plug in the various numbers form your camera into the formula and you'll get a resulting Ev. If you want to try it yourself just plug in the formula into a math program or a cell in a spreadsheet program. Try it; here's the formula for Microsoft's Excel...
=ROUND(LOG(ISO/100,2)+LOG(AV^2/(1/TV),2),0)

The last element is our Lv value or luminance value. While the formula for this is way beyond the scope of this article (and my brain) suffice it to say that all we need is to go to our exposure value table. The table below does not list luminance values but rather a brief description of our lighting circumstance, from a bright sunny day all the way down to a dark night. This allows us to select the proper aperture (Av) and shutter speed (Tv) combination based on how bright or dark our subject is. The left hand column shows a short description to help us make an educated guess about our luminance value (Lv). The number along side it is the corresponding exposure value (Ev). This value is calculated using the formula above and allows us to have a selection of various aperture/shutter combinations for our given light situation.

How to use the table

As I mentioned above, you don't need to know or solve the formula above. However, the is the math behind an exposure value table or exposure calculator.

Across the top are our aperture values (Av) from which to choose based on the amount of depth of field we want (that will be covered in another post). The junction of these two values represent the appropriate shutter speed (Tv) in order to get a correct exposure. So if we take our "Sunny 16" rule (in orange) we look in the left column and find Bright Sunny Day (Hard Shadows) and its corresponding Ev of 15. We then scan the top and find our aperture value (Av) of f/16. The junction of these two brings us to a shutter speed (Tv) of 1/125th of a second.

Let's take a look at another example. Let's say we are in open shade (Ev 11), the side of someone's house. We're taking a portrait shot and looking for shallow depth of field. The largest aperture our example lens can do is f/5.6. If we follow our row and column we get a shutter speed of 1/60th of a second. This combination will give us proper exposure. "How," you ask? By doing the math. Let's break it down.

Another way to use the table is to start with your exposure value (Ev), move across the row to a shutter speed (Tv) you would like to use then climb up the column to find the correct aperture (Av). In our example we start with Lv 11, go across to a shutter speed of 1/60th of a second. For that combination we need to use an aperture of f/5.6 to attain proper exposure.

Hope this helps you understand exposure a bit more.

CALCULATING EV IN A SPREADSHEET PROGRAM: In your favorite spreadsheet program plug in the following values; in column A, 100; in column B, 16; column C, 125. Now in column D enter the following formula; ROUND(LOG(A1/100,2)+LOG(B1^2/(1/C1),2),0). It should return the value of 15.
Some programs have different formula formating. Double check with your particular program for exact format. For example, OpenOffice uses the following format; =ROUND(LOG(A1/100;2)+LOG(B1^2/(1/C1);2);0)

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