Stop! (F-stop, that is)

All disciplines have their own vocabulary, and photography is no different.

In an earlier entry, I defined ISO, shutter speed, and aperture. A concept related to all of these is the f-stop or simply “stop”. Stop is used in all areas of photography, and is a convenient means of denoting the relative change in the amount of light or the sensitivity to light. It is relative because it requires an arbitrary starting point, and then measures any change from that point. Every time the amount of light, or an object’s (film or sensor) sensitivity to it, changes by a factor of 2, a new stop is achieved. (Many modern digital cameras permit changes of ½ and/or ⅓ of a stop.)

Before we continue, let’s make certain that we understand what “a factor of 2” means.

Factor means to multiply or divide. The reason factor also means divide is that division is multiplying by a reciprocal. For example, to divide something by 2 means to multiply it by the reciprocal of 2 (i.e.: ½). Thus, dividing 8 by 2 is the same as multiplying 8 by ½. (Remember, in arithmetic, the word “of” means multiply.) So when we ask what is ½ of 8, we are asking what do you get when you multiply 8 by ½ which, in turn, is the same as asking what do you get when you divide 8 by 2. Thus, changing something by a factor of 2 means doubling it or halving it, depending upon whether more or less light (or light sensitivity) is involved. The number of stops tells you how many times to multiply or divide the original measure by 2.

Suppose the lighting conditions are such that the exposure you desire can be achieved by using an ISO of 200, a shutter speed of 1/250th of a second, and an aperture of f8.0. If the light were to become twice as bright, an increase of one stop, you would have to lower your ISO by one stop to 100, or increase your shutter speed one stop to 1/500th of a second, or reduce your aperture one stop to f11. Each of these would decrease the amount of light reaching your film or sensor by one stop. Realize that if you did all three, you would reduce your exposure by 3 stops.

Take the example above (ISO of 200; shutter speed 1/250th, and aperture of f8.0) and consider the effect of an increase in the light so that it were 4 times as bright. Many new photographers assume that this is an increase of 4 stops because the light became four times brighter. However, it is an increase of only 2 stops. Remember, an increase of 1 stop doubles the light. If that light is doubled again (2 times 2), it is now 4 times as bright as it was originally; hence 4 times the original brightness is an increase of 2 stops. Thus, your desired exposure can be achieved by lowering your ISO two stops to 50, or increasing your shutter speed two stops to 1/1000th of a second, or reducing your aperture two stops to f16. Keep in mind that doing all 3 would be a decrease of 6 stops, or a reduction in the amount of light reaching your film or sensor by a factor of 64!

Most modern cameras do a decent job of predicting a reasonable exposure under typical circumstances, and many people are satisfied with this. Just keep in mind that if you aim to exceed the ordinary, you cannot rely upon your camera’s light meter (or any other light meter for that matter) under all circumstances. You need to be able to adjust one or more of the three exposure variables in accord with conditions. To fully understand how to do this, a full understanding of stops is required and this involves the concept of exponents.

(If you are convinced that understanding exponents is beyond your ability, there is little I or anyone else can do to help you. However, if you are willing to follow a basic, step-by-step explanation, you can understand exponents. True, it may/should take some thought and, if your arithmetic skills are weak, more than one reading, but only you can decide if your photography is worth the effort.)

Just as multiplication is a short cut to addition (3 + 3 + 3 + 3 is the same a 3 x 4; both equal 12), exponents are a short cut to multiplication. For example, 2 x 2 x 2 x 2 is the same as 2 to the 4th power ( 2⁴); both equal 16. (2 x 2 = 4; 2 x 4 = 8; 2 x 8 = 16.)

So far so good, but how do you get ½, ¼, ⅛, etc., out of an exponent of 2 (2 to some power)?

The answer to this can be illustrated by considering what you get if you take 2²and divide it by 2⁴. Realize that all this means is 2 x 2 (4) divided by 2 x 2 x 2 x 2 (16) or 4/16 = ¼ (1 divided by 4). In arithmetic terms, this means 1 divided by 2²(1/2²= ¼).

Okay, so how does this relate to stops?

If, for example, you need to reduce your shutter speed of 1/500th of a second by 2 stops, you need to change it to 1/500 times 2²(4/500 = 1/125).  One-way of doing this is to divide 500 by 2 twice: 500/2 = 250; 250/2 = 125. Then express this as the fraction 1/125. Thus, a 2-stop decrease from 1/500th of a second is 1/125th of a second. Another way of doing this is to divide 500 by 2²(500/4) which, not surprisingly, also yields 125 which, in turn, equates to 1/125th of a second.

I explain the above because, as noted wildlife photographer John Shaw points out, a full understanding of exposure is essential for creating outstanding photos.

Again, if you are satisfied with images that are adequately exposed most of the time, your camera’s meter will work reasonably well. However, if you want your images to stand out from the mediocrity that seems to impress so many today, you need to take the time to understand exposure. While this may seem excessive to some, keep three points in mind:

1)    A year from now you will be a year older, whether you have taken the time to learn something new (such as exposure) or not;

2)    The histogram feature of digital photography allows you to quickly check your exposures and to easily modify them as needed; and

3)    Take photography classes (where you get to ask questions not just read text), but make certain that all of the basics are covered in detail!

If you need further incentive to improve your photography, take a look at the images of such wildlife photographers as Paul Denman (www.pmdphoto.com/PMDPhoto), Arthur Morris (www.birdsasart.org), Alan Murphy (www.alanmurphyphotography.com), Larry Ditto (www.larryditto.com), Kevin T Karlson (www.kevinkarlsonphotography.com), Kevin J Hurt (www.hurtphoto.com), and those who have shot at deep south Texas’s Dos Venadas Ranch (www.dosvenadas.com/gallery.htm), etc.

Remember, the only one holding you back is yourself!

In summary, f-stops (or simply stops) are factors of 2. If the brightness or sensitivity to light doubles, that is an increase of one stop (times 2); if the brightness or sensitivity to light is halved, that is a decrease of one stop (divided by two).