Problem with Canon rebel xsi- ISO exposure

Question

I need to know why my camera's ISO and a change in lighting makes the shutter slower? I don't understand why that is? This is my first camera and I don't know much about it, lol. It does long exposure when in dimly lit areas, but when in light its back to normal shutter speed?I just want to know why this happens? Thanks for any help on this...:)

Answer 1

How can I explain this without making a mess of things...

Your camera needs to take in a fixed amount of light to make a decent picture. No matter what the lighting conditions are, your camera always keeps the shutter open untill it collects the same amount of light for each shot.

When it's a sunny day, the light pours in like a waterfall and you can suffice with a very fast shutter speed. When the light is poor however, light trickles in slowly and it takes much longer to reach the required amount - this means a (much) slower shutter speed.

There are two different camera settings that you can use to influence the shutter speed.

One is the ISO setting. When you use a high ISO number, you can make the shutter speed faster at the expense of image quality. The higher the ISO number, the worse the overall image quality.

The other is the aperture setting. The lower the aperture number, the faster the shutter speed. This comes at the expense of how much you get in focus. The lower the aperture number, the less you get in focus.

This is also what you do to get blurry backgrounds with portraits - you use a low aperture number!

Read the manual and experiment. This is fun stuff and knowing how to use these options can improve your shots tremendously.

Answer 2

Correct exposure is a triangle consisting of ISO, aperture (f-stop) and shutter speed. Each affects the other and changing one changes the other two.

ISO is the measurement of the sensitivity to light of a light sensitive surface, whether film or digital sensor. A low ISO (50, 100) is very insensitive - it requires a lot of light. It also gives us the very best picture quality. A high ISO (400, 800, 1600) is very sensitive and needs less light. Unfortunately, higher ISOs give poorer quality.

The aperture is an opening formed by the diaphragm inside the lens. Its marked as 2, 2.8, 4, 5.6, etc. The smaller the number the bigger the opening. A lens at f2 would be "wide open" and admit all the light while at f16 it would be "stopped down" and admit very little light.

The shutter controls how long the light admitted by the f-stop chosen is allowed to expose our light sensitive surface. Admit more light (at any given ISO) and our shutter speed becomes faster. Admit less light and our shutter speed becomes slower. Shutter speed also determines whether or not motion in our scene will be blurred (slow shutter speed) orstopped (fast shutter speed).

Lets use the old "Sunny 16 Rule" to show the ISO/f-stop/shutter speed relationship. It states; "On a sunny day, set your aperture to f16 and your shutter speed to 1/ISO". So if we're using ISO 100 we'll be at f16 and 1/100sec.

f16 @ 1/100

f11 @ 1/200

f8 @ 1/400

f5.6 @ 1/800

f4 @ 1/1600

f2.8 @ 1/3200

f2 @ 1/6400

As you can easily see when we admit more light our shutter speed increases until we exceed the shutter speed capabilities of most cameras.

Now lets go indoors. Although our eyes may think of the room as well lit our camera may "see" it differently. Using ISO 100, at f2 our shutter speed may be 1/15 sec. At f2.8 it will be 1/8 sec. Obviously any motion in our scene will be a blur - and we may even need a tripod. So lets try ISO 800. Now at f2 our shutter speed is 1/125 sec. At f2.8 its 1/60 sec. At ISO 1600 our shutter speed becomes 1/250 sec. At f2.8, 1/125 sec. We pay a price for the higher ISO - increased digital noise. A program like Noise Ninja can help.

I realize this is probably more information than you wanted but ther simply is no other way to explain the ISO/f-stop/shutter speed relationship.