| The problem with this is that things are constantly changing – even if the light is not. There are two types of light in photography: incidence and reflective. Incidence light is what is falling on a subject. This is the ambient light. On an overcast day, a subject can run circles around you and incident light is not changing. It can go in front of a dark forest, but the incident light stays the same. It can then go stand in an open field of snow and the incident light remains the same. Reflective light is different. Reflective light is the light that is bouncing off your subject. A black bear bounces a different amount of light off its fur than a bobcat. And this is what aperture priority is trying to adjust for. Let me give you an example of what I mean in real photography terms. . . Let's take a bison because it's dark in color and let's put him in the snow because, well, bison look awesome in the snow. Now, assume you are shooting aperture priority and are most likely using matrix metering or evaluative metering on your camera as well – because that's what 99% of people using aperture priority are shooting in. Let's picture the bison standing out in a field of snow and only taking up about 10% of our composition. From the perspective of our camera, the bison is a minor part of the lighting equation. The dominant tone is the snow. So, the camera exposes for 18% grey snow. Yuck. Ugly. Not acceptable. Thus, we dial in a +2 on the exposure compensation to bring our snow back up to white. But what happens when the bison comes barreling through the snow toward you? What happens when suddenly the bison takes up 60% of the composition? 80% of the composition? Now, the reflective light on the snow is no longer the dominant tone of light. Now, the dark color of the bison dominates, and your camera adjusts accordingly. Actually, in aperture priority, it was adjusting constantly as that bison got closer and closer to you. Because you are in aperture priority and allowing the camera to make decisions for you, it now reads the dark tones of the bison and increases the base exposure to bring the dark up to 18% grey tonality. But you have a +2 dialed into your exposure compensation. +2 was fine originally because the computer was trying to reduce light to expose the snow as 18% grey. But now that it is increasing light to expose the dark fur of the bison, everything is going to be blown out around your bison. The photograph will most likely be disastrously overexposed at this point. That is unless you adjust your exposure compensation. But, as the bison was running toward you, you may have needed to adjust it two or three times in the process. Did you remember to do this? Did you take a test shot and check your histogram in the process to make sure you got it right? Did all of this take place in the excitement of photographing a bison running in your direction through deep snow with freshly fallen powder billowing out all around him? I sure hope so. Or there is a good chance at some point your exposure all went to shit and you will likely be throwing away most of those photographs. That's photography from the perspective of aperture priority and dependence on reflective lighting. Sound complicated? It is! Sure, aperture seems easy because you simply set your aperture and don't have to worry about the rest. But, then again, now you're worrying about a lot. Is the computer giving me a fast enough shutter speed? Do I need to adjust my ISO so the computer will bring up my shutter speed? Now that the darker tones of the bison dominate the frame, has my shutter slowed down too much? Am I now blowing out the snow? Do I need to adjust my exposure compensation or my ISO? I need a drink just from thinking about it all. Manual is different. Though your light meter is still reading everything for reflective light, once you have set your exposure, nothing changes. The bison is 10% of the frame, and then runs towards you and is now 60% of the frame. Throughout the entire experience, your exposure never changed in the camera. Your shutter speed stayed where it was. Your aperture stayed where it was. Your ISO stayed where it was. And you know what happened? You have a perfect exposure throughout the entire sequence. Why? Because though the scene changed, the incident light- the ambient light- never changed. If that isn't reason enough to give up aperture priority and go to manual, then I don't know what is. Still, confused? Don't worry, you will be seeing a whole lot more about the topic of manual exposure vs aperture priority this Spring in the Journal of Wildlife Photography. Photographing birds in flight and having a hard time with the exposure because they go from blue sky to a beautiful background with trees? Maybe you get it right in the sky, but then everything is blown out when they drop in front of the trees? Shoot in manual! If the whole thing is front-lit, to begin with, then the ambient light between the sky and the trees hasn't changed. So why let the computer change everything simply because there is a different color in the background? This is why manual exposure simplifies everything. It takes the guesswork out. It removes the variables that cause our cameras to make constant adjustments. But when I suggest people switch to manual exposure in the field, I swear I can almost hear the heart palpitations that occur at the mere thought. And yet, it's the simplest of things. Here is how I look at setting my exposure, and this is getting directly to the point of the original question. . . I see my subject and I make a judgment call as to what f/stop I want to use. I can make an assumption about how fast things will be unfolding and thus how fast I need to set my shutter speed. Here is my rule of thumb that at least gets me in the ballpark: If the subject is ¼ of the composition or less, I can shoot the situation wide open – which is f/4 for me. If the subject is larger than ¼ of the frame, I increase my aperture to f/8 for greater depth of field in the portrait. There are lots of reasons that you would set your f/stop differently than this. But, it's a general rule of thumb and will at least give you a starting point. And when action is happening fast, easy rules of thumb are lifesavers. If it's a large animal, I want 1/1000th of a second just in case. 1/500th is probably fine and if the light is lower and I could benefit from this extra stop of light, then I will adjust accordingly. If on the other hand, I am photographing something smaller like a weasel or a bird, then I automatically dial in 1/2000th of a second for my shutter speed. Again, many reasons exist that would make you want to change this or deviate. But, it's a rule of thumb that will give you a place to start with. So…. If I know what depth of field I want, and I know how fast I want my shutter speed (set to "just in case" speeds), what else is there? ISO. When I approach any given situation, the first question I ask is what depth of field I want. The second question I ask myself is what shutter speed will I need. For the most part, this has nothing to do with the light for me. It's about stopping or exaggerating the movement of the scene, and it's about depth of field. These are artistic decisions, plain and simple. All that I am left with then is ISO. And it's ISO that I use to adjust my exposure. If the light isn't changing, then like my bison example above, I can set it and forget it. If the light changes, all I have to do is adjust my ISO. I don't want to be worrying about what my camera is doing. I don't want to worry about technical settings. I want the simplest means of achieving an exposure because although getting a proper exposure is necessary, it's THE LEAST IMPORTANT part of all that is necessary for me. I can play Sudoku if I want a puzzle to work out, or go birding if I want an exercise in mental agility. ACTIONS When shooting wildlife in manual set the f/stop for the desired depth of field. Set your shutter speed for what you think you will need to stop the action. Then, adjust the ISO as necessary for the light. This simplifies the whole process. It gives you only one thing to worry about in terms of your exposure. It removes the variables. And it allows you to concentrate on what's most important – creating a compelling photograph that has impact! | 
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