If you buy a Ferrari but never drive it over 60 mph, I question the purchase. If you buy a top of the line computer with maxed out RAM but use it only for email, I question the purchase. If you buy a home theater yet stay glued to a shopping network, I question the purchase. If you invest in a top of the line digital SLR and leave it in Program or Auto mode, I question the purchase. The Ferrari will be fun to drive, but I can also attain 60 mph in a used Corvair, I can receive and send email my iPhone and I can see what I buy from TV on a 10 inch B&W. In essence and to get to the topic at hand, if you have a digital SLR, use the features that allow you to determine what the picture will look like rather than let the camera make the decision.
Program and Auto both provide excellent exposures so they definitely serve a purpose. BUT, if the result you want is an intentional blur or an action stopping moment, using Program or Auto MAY result in obtaining the effect. If the result you want is an out of focus background to make the subject stand out or an image that's sharp from the foreground to the background, using Program or Auto MAY result in obtaining those effects. The reason I emphasize "MAY," is the above effects are dependent on the shutter speed or aperture the camera chooses and if it chooses the "wrong" ones, even though the exposure is spot on, the image falls short because the effect is not achieved. So when it comes time to take control of the picture making situation, get off Auto or Program and learn how to use Aperture or Shutter priority modes.
In talking with most professional and advanced amateurs, Aperture priority is the most popular mode. A big reason is you control the depth of field. A key way to do this is by adjusting the aperture. The more wide open the aperture, the less depth of field. The more wide open apertures are f2.8 / f4 / f5.6. As the values of the f numbers get higher, the amount of depth of field increases. Those that produce the greatest amount of depth of field are f16 and f22. So here's where it becomes real. For instance, let's go back to Auto mode and pretend you're photographing a landscape. The desired effect is to have everything in focus from an important subject that is close to the camera out to a distant one in the background. As stated above, an aperture of f16 or f22 is necessary to attain this. If Auto or Program chooses f8, even though the exposure will be perfect, the picture falls short because certain planes will be out of focus. But on the other hand if you switch to Aperture priority and physically set it to f16 or f22, the result will be what was envisioned. Conversely, if you're creating a portrait, the desired effect is to limit the plane of focus to just the person. As stated above, an aperture in the neighborhood of f4 is necessary to attain this. If Auto or program chooses f11 or f16, more of the background will appear in focus and be very distracting. Again, the exposure will be perfect, but the picture will fall short.
Equally as important as the chosen aperture is the resulting shutter speed it produces. If stopping down the lens to f16 creates a situation where the shutter is slow, you need to realize how it will impact the image. If the subject moves, it may produce a blur. If the shutter speed is slow and you hand hold the camera, the image will be soft.
It comes down to taking charge of the picture making experience so the desired effect is dictated by the photographer and not serendipitously by the camera. Make it a "Priority" to take control of the outcome of the image before you press the shutter. Setting a specific aperture and/or shutter speed to produce what you envision goes a long way in making you a better photographer.
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