Non Expert Photography for Non Expert Photographers

Questioner: My camera has Automatic mode. When I use it in that mode the pictures are ok, but I thought I would try to be more creative with my pictures so I tried to use one of the other modes but the pictures don't really look any different. I would like to learn how to control the camera but I have no idea what those other modes such as Aperture Priority and Shutter Priority. What do those modes do?.

Dan: Let's think of the pivot point of a seesaw. You do know what I'm talking about when I say "seesaw", don't you? Sometimes they're called "Teeter Totters". The center is fixed in place so that it cannot move other than rotate at the center point. The two ends, however, are free to move up and down. They cannot move independently of each other though. If one end goes up, the other must go down. If the other end goes up, this end must go down. Let's think of the exposure system of a camera working like a teeter totter. The fixed pivot point in the center is what we recognize as the proper exposure point.

To properly expose an image on a digital sensor (film works the same way) the metering system in the camera decides how much light is needed to make the proper exposure. There are two ways of controlling the amount of light the sensor receives. We open the aperture so that more light gets in. That is something like changing the angle of the slats of Venition blinds on a window to allow more light into a room. Remember the lesson on lens speed where we looked at the way a Camera Lens Aperture works. We open more to let in more light or close it down to let in less light. The second way is to leave the shutter open longer. Or we close it sooner. The analogy to a window falls apart here with that one but you get the point. The longer the camera shutter is left open, the more light is allowed to register on the sensor.

There are wo ways to control the exposure of the picture. One is to open the aperture (like opening the blinds more) and the second one is to leave the shutter open longer. Now the shutter and the "blinds" (the aperture) are not the same. The shutter is like a big curtain which opens and closes in front of the sensor to let light in or keep light out. Though the time of the opening can be controlled, (shutter speed) the amount of the opening cannot be controlled. So we can't use the shutter to control the amount of light coming through it. We can only control how long the shutter is open. The amount of time the sensor is exposed to light determines the amount of exposure. That's all we need to say about the shutter right now.

Remember the "pivot point" of the teeter totter. In the camera that is the determination by the metering system of how much light is needed to properly expose the picture. The following graph shows the teeter totter affect with the colored lines pivoting at the point we call "correct exposure." We can increase or decrease the values of either the shutter speed in the left column or the aperture opening in the right column. If we decrease the shutter speed (slow it down so that the shutter stays open longer) the metering system will increase the aperture number. Increasing the aperture number, closes down the lens opening so that less light gets through. Either side works the same. If we decrease the aperture number so that the lens is opened wider, the metering system will compensate by increasing the shutter speed so that the shutter is opened for less time.

The graph above shows the values for an ISO setting of 100. Later charts showing other ISO settings will be seen.

The questioner asked about the Automatic mode. In the Automatic mode, the camera is very much a point and shoot camera. Regardless of how sophisticated the SLR camera is with controls and features allowing all sorts of creative operations, if it is in the Automatic mode, the camera itself makes ALL the decisions on how to properly expose the picture. That is the same as the compact, simple and less expensive point and shoot models.

If a camera is set in the Automatic mode, the circuits within the camera determine both the aperture opening size and the shutter speed. The teeter totter affect is still there. The fixed pivot point is still the thing we call correct exposure. So if the camera decides to open the lens more it will compensate by shortening the shutter speed. If it decides to close the aperture down more, it will have to compensate by allowing the shutter to stay open longer. Likewise, if it controls the shutter by setting it to a slower shutter speed (again, that allows the shutter to stay open longer) it will have to compensate by closing down the aperture more to let in less light through it. And so it goes.

The problem with the automatic mode is that even though the camera does have a high degree of sophistication, it may not make the best judgment for the picture. That does not mean the picture will turn out under or over exposed or have any of those technical problems. It means that other things which I will talk about next may not turn out the way you want them to turn out.

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