Monday, March 2, 2009

Exposure in Digital Cameras (Part 2)

In the previous article we discussed relating photography exposure to bodily exposure to the elements. We also discussed f-stop, shutter speed, lens aperture, and camera ISO.

In this article we're going to discuss a scenario that anyone of us photographers might happen across on any given day out on a scenic drive.

Let's say it's a bright sunny day and we're shooting in full sunlight. We want a nice sharp photo of a scenic valley so we set the lens aperture to f16. The camera's ISO is set to 100. We look at the camera's LCD or LED readouts and the camera determines the shutter speed should be 1/125. (This by the way is the "sunny 16" rule, in bright sunlight at f16 the shutter speed is the inverse of the ISO). We release the shutter and capture our stunning scenic valley in all it's glorious splendor.

Now we want to get a little creative with our photography and shoot a close up flower in the foreground with vast valley behind it slightly out of focus. F16 has too much DoF and is rendering the background sharper than we want. So we open up the lens aperture to f11 (one click down from f16) allowing 1 stop more light to enter the camera through the lens. If we press the shutter now without compensating for the 1 stop increase in light we will over expose the image from the correct exposure by 1 stop. To balance this we have to reduce the light getting into the camera by 1 stop and we do this by increasing the shutter speed by 1 stop to 1/250.

A quick note here for those of you that noticed it took 2 clicks to change the shutter speed from 1/125 to 1/250. That is because modern digital cameras are set to half f-stops on the shutter speed. Some cameras (like the K10D) can even be set to 1/3 of an f-stop between clicks. This allows a more subtle change in exposure. ISO settings also can be fractions of the f-stop.

So to lessen the confusion here are the full(+) and half(-) stops on the Pentax K10D:
+ 1/4000
- 1/3000
- 1/1500
- 1/750
- 1/350
- 1/180
- 1/90
- 1/45
- 1/20
- 1/10
- 1/6
- 1/3
- 2/3
- 1.5"
- 3"
- 6"
- 10"
- 20"

Back to our flower. We now have the camera set to f11 and 1/250 with the ISO at 100. We press the shutter and look at the results in the LCD screen on the camera (chimping). We notice the background has gone out of focus a bit but not enough for our desired creative exposure. So we open the lens aperture one more stop to f8 and again compensate the shutter speed 1 stop to 1/500. A quick press on the shutter release give us the desired creative exposure in the camera's LCD screen while maintaining the correct exposure also.

All the above was an example and if you actually try this your exact settings will be different depending on the lens' focal length and the distance from the foreground to the background. What will be the same is for every lens aperture change you make you will have to make the reciprocal shutter speed change to maintain the same exposure.

In our next article we'll go over using shutter speed to tame motion blur.

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