Forum: Photography


Subject: ISO Question

promiselamb opened this issue on May 11, 2006 · 24 posts


Valerie-Ducom posted Thu, 11 May 2006 at 11:48 PM

Attached Link: http://www.robert-barrett.com/photo/exposure_calculator.html

hi

well...

ISO speed is a standard for expressing the sensitivity of the film or sensor to light.

You'll also see this expressed as film speed (or sometimes ASA) The higher the number, the more sensitive or faster the film.

ISO stands for International Standards Organization

If you look at almost any basic photography book, you'll see some information on how exposure works.

The shutter speed a camera can use for proper exposure depends on the lighting, the film speed (a.k.a., ISO speed) and the Aperture (which is the ratio between the focal length and the diameter of the aperture iris opening).

A higher ISO speed setting, or higher ISO speed film, allows it to be exposed faster (shutter doesn't need to stay open as long for the same aperture).

With a digital camera, when you increase ISO speed, you amplify the signal from the sensor. Each time you double the ISO speed, the camera can use shutter speeds twice as fast for proper exposure in the same lighting, at the same aperture setting.

This amplification can cause noise (similar to film grain). When the light is low, not as many photons hit each photosite on the sensor (allowing the photosites for each pixel to generate a stronger signal before being read),

So, when you try to increase ISO speed, it can be like trying to turn up the volume on a weak radio station, only instead of hum, static and hiss, you get image noise.

But, you may need higher ISO speed to get shutter speeds fast enough to prevent blur from camera shake or subject movement in many conditions. So, it's often preferrable to have a little noise in the images, versus blur.

When light is good (and/or you don't need faster shutter speeds), it's preferrable to keep ISO speeds set lower for a cleaner image. Most subcompact digital cameras are limited to a maxiumum ISO speed of 400 (although some newer models can go higher now).

Most DSLR models can go to ISO 1600 or higher (their larger sensors can gather more light, since the photosites for each pixel are larger). But, you can't really go by sensor/photosite size alone, as each sensor may have slightly different characteristics.

Here is a handy online exposure calculator that can give you a better idea of how exposure works. Film speed is the same thing as ISO speed.

Note that most subcompact cameras have a largest available aperture of f/2.8 (at the widest zoom setting only), dropping off to a largest available aperture of around f/4.9 or so at their longest zoom setting.

The more optical zoom you use, the lower the light is getting through to the sensor with many models. With other models, you can maintain a larger aperture (smaller f/stop number) throughout their focal range.

So, your lens will also limit what you can do with a camera (you can't set the aperture larger than the lens permits).

http://www.robert-barrett.com/photo/exposure_calculator.html ..

   One way to look at it that helped me was to compare them to a glass of water. The light being the water.

50 ISO film is like a large glass of water it (say a quart). It takes so much time to fill it.

100 ISO would be half a quart so half the amount of time to fill

200 ISO would be half again smaller or 1/4 the amount of time to fill compared to the 50 ISO one. And so on.

To continue the analogy...

The apature is like the the size of the faucet. A small faucet (small apature say f22) takes longer to fill the glass than a large faucet (an apature of say f1.4) will fill it much more quickly.

In either case, when the glass is full that equates to when the film has recieved enough light for a proper exposure.

I hope that makes sense to you.  

Hugs