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October 26, 2005

Film Speed and Your Digital Camera

Anyone who has ever had to buy film for a camera will know that film is rated by its sensitivity to light, or it’s speed. As the film speed (ASA or ISO) increases, two things happen, less time is needed to expose the film and the appearance of grain (noise) becomes greater. These principles also apply with digital photography.

Many digital cameras provide a mechanism for changing sensor sensitivity (film speed). Low end point and shoot cameras may only have one film speed, others may range from 100-200 or up to 400, while digital SLRs may have a selectable film speed of between 50 ISO and 3200 ISO or higher. So why would you want to change film speed?

With all other things being equal, a faster film speed allows you:

• The greater ability to stop motion
• To get a properly exposed shot in low light levels without camera shake
• Have more depth of field

So what are the advantages to a slower film speed?

• Slower film speed usually creates better quality images with less noise
• Show motion in an photo
• Can get less depth of field to isolate your subject.

You will notice that they seem to overlap. Why might you want to stop motion, in one shot but not another? Why would you sometimes want more depth of field, and other times not? There are creative reasons for changing your camera’s film speed. The most common reason though is for shooting in low light or indoors. Lighting indoors ay be significantly dimmer than outdoors, up to 50 times as dim, or even 500 times in extreme cases. Given that wide range of lighting levels, film or digital sensors cannot be made to accommodate all of them, so it is best to shoot with an appropriate film speed to ensure well exposed images without camera shake.

If you do want to achieve some creative effects, try slowing your film speed down which will force your camera to use longer shutter speeds and therefore give you motion in your shot. In a I shot I took this summer of a waterfall, motion is clearly visible in the water as it falls over the cliff (100 ISO) . If I had used a faster film speed, the shutter speed would have been slow enough that the motion of the water would not be clearly visible. Be aware that longer shutter speeds will need to be taken with a tripod.

To capture motion, use a faster shutter speed to create shorter shutter speeds. I used 800 ISO for this shot of a baseball game. You can see even at this film speed, the ball still has some motion to it. If I had used a slower shutter speed, the trail would have been much longer (8 times longer with 100 ISO).

There is a good interactive example that shows the differences between film speeds at http://www.photonhead.com/simcam/filmspeed.php

Next week I will discuss the relationship between shutter speed, aperture and how they relate to film speed.

Posted on October 26, 2005 12:20 PM | Permalink

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