Your aperture, shutter speed, and ISO each serve a very specific function during your dog photography sessions, but more important than their individual jobs is the relationship among them that allows you to take a properly exposed photo. Exposure is the amount of light it takes to develop your image.
You may be familiar with the terms underexposed and overexposed, but in reality, exposure is more of an artistic preference than a steadfast rule. What looks good to you may, in fact, look overexposed to another photographer and vice versa. Instead of spending time understanding “proper” exposure (which actually doesn’t even exist), familiarize yourself with the unique inverse relationship that aperture, shutter speed, and ISO possess.
Much like shutter speed and f-stop, exactly how you set your ISO depends on your individual camera, so check your manual if you’re unsure. Many digital SLR cameras have an easily accessible ISO button on the camera body, while most CDCs have this setting buried within the camera’s menus.
Exposure is measured in units called stops, and every time you adjust your exposure by a stop, you either double or halve the light that reaches your camera’s sensor. For example, if you’re shooting at 1/250 second at f/5.6 on a setting of ISO 200 and you change your ISO to 100, you effectively decrease your exposure by one stop.
For your image to remain at the same exposure, another setting needs to compensate (either aperture or shutter speed) in the opposite direction. In photography lingo, this is called reciprocity.
Now that your camera is set at ISO 100 and is essentially letting less light in, you can choose a larger aperture opening by changing your aperture setting to f/4, or you can choose a slower shutter speed by changing your shutter setting to 1/125 second.
Either of these compensations accomplishes the same thing — allowing in more light to compensate for the reduction of light that occurred when you “stopped down” your ISO.
This compensation is exactly what your camera automatically does for you in AV and TV modes. At this point you may be wondering why you should waste precious space in your brain for this information if your camera can take care of your settings automatically.
The answer? You have way more options when you choose your own settings!
Consider this situation: You’re outside on a sunny day and you’re trying to photograph Missy zipping back and forth through a large field. You set yourself up in TV mode, choose a shutter speed of 800, an ISO of 100 (it is sunny, after all), and begin to snap away.
Problem is, at these settings, your camera automatically chooses a very wide aperture, like f/4.5. Because Missy is so far away, your camera probably doesn’t have the depth of field that you need (despite how perfect your shot is, Missy still seems a tad fuzzy). You want to shoot at a smaller aperture — like f/8 — but your camera tells you that it’s not possible!
Consider the rule of reciprocity and do the following:
While remaining in TV mode, decrease your shutter speed one stop at a time as you count how many increments you have to go until your camera chooses f/8 for the aperture.
As an example, say you have to change your shutter speed five increments (to 1/250 second) before your camera automatically chooses f/8. Because 1/250 second is far too slow to capture Missy in action, you need to use the rule of reciprocity so that your camera is set at both the f-stop and shutter speed you want!
Bump up your ISO setting five increments.
In this example, that puts you at ISO 320.
While still in TV mode, change your shutter speed back to 1/800 second.
Now, when you look through your lens you see that your camera is magically choosing f/8, your desired f-stop!
Compose your photo and snap away!
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Source:http://www.dummies.com/how-to/content/exposure-settings-in-dog-photography.html
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