Need help understanding "exposure" and ISO

[snapsnap1973]

Hi.

Very new to photography and so far what I "think" I know is the following:

During a "shot" you of course first set the focal length and the aperture (f-stop) and then the exposure time.

What I'm wondering is HOW does having a longer exposure time affect the image. Also what is ISO exactly? I know that it makes things brighter and darker, but doesn't the aperture (f-stop) do that ?


Thanks

 

[cgipson1]

think of ISO as how sensitive the Sensor is to light....Low ISO (not very sensitive, but very clean) to High ISO (very sensitive, but very noisy)

Aperture is like a valve.... set to a Low F-Stop, it is wide open and lets in lots of light with a shallow Depth of Field, set it to a high F-Stop number, and it closes down and lets very little light through but has a large Depth of Field.

Shutter speed is like a timer.... set it a time and it opens for that time.... bring in light from the lens (aperture) to hit the sensor

All three of these have to be set to work together to allow a certain amount of light to hit the sensor (exposure)

Long Exposures? The sensor heats up in a long exposure... that can generate a lot of "noise" or random chatter in the image. You can also get color shifts in the image which can be interesting.

Does that help?

 

[debauch]

One of the best explanations I read...

Think of aperture as a window, the shutter speed as shutters on the window and ISO as you inside with sunglasses on.

The bigger the window, the more light the room gets. You can of course also control the amount of light the room gets by leaving the shutters open for a longer amount of time or less amount of time. These two work together. Your camera calculates the "correct" window size and amount of time the shutters need to be open to give you a good exposure. But, of course, you can change those. If you make the window bigger, you can leave the shutters open for less amount of time to get the same exposure. If you make the window smaller, you need to leave the shutters open for a longer amount of time to get the same exposure.

ISO is the sensitivity to light. If you have sunglasses on while you're inside the room, your eyes are less sensitive to the light. If you take them off, you eyes will be more sensitive to the light...as in, you can pick up more of the light that is coming into the room. Another good explanation I read for ISO is that it magnifies the signal from the camera's sensor. The more magnification (higher ISO), the more sensitivity to light but, also, the more noise in the photo.

Hope that wasn't confusing!

 

[snapsnap1973]

Yes, that helps ! Thanks A LOT!! I think I'm gonna go read about the basics of how a camera operates AND I mean WAY back to the first cameras! Wikipedia to the rescue!

 

[KmH]

The ISO setting biases an amplifier array that is on the image sensor chip.

Each one of those millions of pixels develop an analog voltage proportional to how much light falls on it. Actual a pixel is a group of transistors coupled to a photodetector - Active pixel sensor - Wikipedia, the free encyclopedia
The voltage a single pixel develops is pretty small even when a lot of light falls on the pixel.

So all those millions of pixel voltages get amplified (gain) the same amount based on the ISO setting. A low ISO causes a smaller gain in signal strength (amplification) than does a high ISO setting.
As the gain is increased, amplifier noise increases too. In technical terms it's called the Signal to Noise Ratio or SNR. Digital Camera Image Noise: Concept and Types