Zo’C » Mastering the Shutter speed

This is the 5th part of a six-part tutorial about photography basics. So far we’ve seen

By now, we have seen that, by changing the ISO value we can change the amount of light needed for a picture, in exchange for quality. We have also seen that, by changing the aperture value, we can change the amount of light actually entering the camera, in exchange for bigger and smaller depth of field.

But increasing the Aperture value is not the only way to increase the amount of light in a picture.

In a film camera, you need light to make the silver halides react, hence a clear solution would be just to increase the time the film is exposed. The more time the film is exposed, the more light it gets, the more it reacts, even with few light.

Curiously, this is another similarity on digital photography. The more a sensor is exposed, the more light the photo sensors accumulate and the brighter the picture becomes.

You might want to close the aperture to get more sharp pictures and you can compensate this by increasing exposure time, if you have a tripod and a still subject you can take a very slow picture with bad lighting conditions and closed aperture and the picture might look great.

The rule is simple. Camera apertures are measured in f/ stops. Each f/ stop multiplies or divides the amount for a certain factor. To compensate, divide or multiply the exposure by the same factor.

If you half the aperture you must double your exposure time. Simple as that. Most modern cameras can handle this by themselves, and even if yours don’t you don’t have to do the math all the time, the camera photometer will do this for you and signal when light is Ok.

Pictures taken with speeds of 1/90 and 1/500 of a second. The water in the slowest picture is blurred while in the fastest is sharper. Regard that the slowest picture had a closer aperture and hence a deeper depth of field. You can see the background is sharper in the slowest.

Altering the shutter speed has a trade-off, of course.

What happens in a long exposition is that the sensor collects more light to analize and create the image, but the problem is that this light has to be the same. If the intensity or color change during the exposure the results won’t be accurate.

Typically, if either the camera shakes or the subject moves, the light reaching the sensor will change and will interfere.

If a single pixel on the sensor receive light information from different sources, then multiple images will impress the same region and the same object will be imprinted on many regions of the sensor, as a result the image will be blurred.

Now, there are at least two ways you can take blur to your advantage. One is when part of the picture is in motion and you want this part to be blurred while the rest is sharp, one common use for this is with running water.

The second one is called panning and consist in photographing a moving subject by following with the camera while it moves, by doing this, the background will have a motion blur and the main subject will be focused. This is, of course, much difficult that the first one and requires some more experimentation.